1Department of Astronomy and Astrophysics, Tata Institute of Fundamental
Research, Mumbai 400 005, India.

# Journey of X-ray astronomy: Indian perspectives

A. R. Rao 1,*

###### Abstract

X-ray astronomy is a mature area of observational astronomy. After the
discovery of the first non-solar X-ray source in 1962, X-ray astronomy
proliferated during the Apollo era’s space race. Then, it matured as an
established area of research during the period of Great Observatories, and now
it has become an indispensable tool to understand a wide variety of
astrophysical phenomena. Consequently, in recent times, niche observational
areas in X-ray astronomy have been explored, and attempts have been made to
expand the sensitivity of observations vastly. India was an active partner in
the growth of X-ray astronomy. In the initial years, India leveraged its
expertise in balloon technology to get significant results in the research
area of hard X-ray astronomy. During the rapid growth phase of X-ray
astronomy, India made divergent all-round efforts. Later on, however, the
technical expertise available in India was insufficient to compete with the
highly sophisticated satellite experiments from around the world. During this
phase, work in X-ray astronomy continued in a few low-key experiments,
eventually resulting in the launch of India’s first multi-wavelength
astronomical satellite, AstroSat, in 2015. In this article, I will trace the
journey of X-ray astronomy and the developments in the Indian context. I will
also explore the sociological aspects of the growth of X-ray astronomy, and,
in the end, I will present a speculative sketch of the future of X-ray
astronomy with an emphasis on the Indian contribution.

###### keywords:

X-ray astronomy—X-ray satellites—History of X-ray astronomy.

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##  1 Introduction

X-ray astronomy is one of the major branches of space astronomy, astronomy
that could only be done by sending instruments outside the protective blanket
of the Earth’s atmosphere (the textbook ‘The X-ray Universe’ , Tucker &
Giacconi, 1985, gives an excellent historical account of the birth and growth
of X-ray astronomy).

Even before acquiring the ability to send instruments above the Earth’s
atmosphere, it was surmised that the Sun emits X-rays. The optical spectral
characterisation of the solar corona indicated the existence of a hot, low-
density plasma as a potential emitter of X-rays. The disturbances in the
ionosphere were found to be related to solar activity, and it was surmised
that ionising radiations from the Sun (UV and X-rays) must be the cause for
such disturbances. Hence, when the captured German V2 rockets were made
available for scientific use in the USA, the first use was to measure X-rays
from the Sun using rudimentary X-ray detectors like Geiger counters. There
was, however, little expectation of detecting bright X-rays from any other
objects in the sky because the amount of solar X-ray radiation was negligibly
small compared to its optical emission, and an extrapolation of the then-
prevailing wisdom on stellar structure did not predict any measurable X-rays
from other sources in the sky.

The great space race of the sixties between the USA and the USSR saw the
establishment of the civilian NASA program in the USA, which started the famed
Apollo program. A vast amount of resources were channelled into the space
programs. Several groups started benefiting from this support, the prime among
them being the American Science & Engineering (AS&E), established by MIT as a
spin-off company. Riccardo Giacconi, who headed the Space Science Division at
AS&E, was a trained Cosmic Ray physicist, and he was fascinated by the
abundant possibilities of space technology for scientific use. He improved the
X-ray detection techniques and proposed to NASA that reflected X-rays from the
moon could be detected by making a rocket flight. This proposal was turned
down, ostensibly due to the dim possibility of detecting any X-rays from the
moon. Giacconi, however, re-proposed the same experiment to the Air Force,
which was approved. Though no X-rays were detected from the moon, a very
bright extra-solar X-ray source, Sco X-1, was discovered. This heralded the
birth of X-ray astronomy. Giacconi made further ambitious plans for X-ray
astronomy and was rewarded with a Nobel Prize in Physics in 2002 ‘for
pioneering contributions to astrophysics, which have led to the discovery of
cosmic X-ray sources’.

###  1.1 X-ray astronomy: new trends in doing science

The birth of X-ray astronomy, however, marked the beginning of two critical
trends in scientific research.

The twentieth century saw an explosion of our knowledge of the world around
us. Tremendous developments and insights were obtained in fundamental physics,
including relativity and quantum mechanics. These were duly applied to
understand the world around us, including the cosmos. Large optical telescopes
routinely collected high-quality spectral data, which were understood from
fundamental Physics, leading to very robust concepts of how stars formed,
evolved and radiated. Similarly, the rigorous application of the physical
principles to particle physics brought in the Standard Model of particle
physics. Thus, there was some element of confidence, or complacency, in the
way new phenomena are addressed: with a feeling that any new observation must
surely be able to be explained by the known laws of physics.

X-ray astronomy brought unexplained and new phenomena to the fore.
Chandrasekhar did theoretically demonstrate that White Dwarfs, the degenerate
state of the stars, will have their radius decreasing with their mass and will
be zero for a limiting mass called the Chandrasekar mass limit, thus hinting
at some state of zero size and large mass. Applying Hubble’s law showed
objects with huge luminosity and small sizes, known as Quasars. However, X-ray
astronomy made the romantic discovery of some unseen objects, more massive
than any degenerate star, emitting copious amounts of X-rays. The discovery of
such X-ray binaries nudged the community to accept the notion of ‘back holes’.
Detection of mysterious bright objects called the ‘gamma-ray bursts’, possibly
coming from cosmological distances, added to the general aura of seeing
something mysterious in the cosmos.

Another aspect brought about by X-ray astronomy is the full use of corporate
culture in doing science.

The extreme difficulties of sending something to space and making it work are
generally solved with the full use of planned and objective work with
attention to detail for every aspect of the work. Objectively looking at the
task at hand to optimize resources, time, and product quality can be dubbed as
extreme corporatization of scientific activities. X-ray astronomy hugely
benefited from this method by sending instruments of ever-increasing
complexity and sophistication to space and evolving the practice of optimally
using the data by disseminating it to a vast astrophysical community.

The dichotomy of corporate science and the sense of complacency in
understanding new phenomena by known theories is quite prevalent in X-ray
astronomy and perhaps in other branches of scientific research. I had pointed
out this earlier while reviewing a volume of Annual Review of Astronomy and
Astrophysics (Rao, 2015). Some trends in astronomy were noted: corporatization
of science and commodification of science products. One of the positive
aspects of this method is the ability to quickly realize well-defined problems
(like measuring the afterglows of Gamma-ray bursts by a quick-moving satellite
called Swift or detecting gamma-ray pulsars using a detector with superior
sensitivity). The negative aspect was ‘suppressing dissent and a lack of
progress in long term and deep thoughts’.

###  1.2 X-ray astronomy in India

When X-ray astronomy was born in 1962 due to the serendipitous discovery of an
extremely bright X-ray source called Sco X-1 by Riccardo Giacconi in a
legendary rocket experiment, India was in an excellent position to participate
in this exciting journey. After its independence, India allocated a good
amount of resources for doing fundamental research and space research. One of
the pioneering examples of fundamental research in India is the founding of
the Tata Institute of Fundamental Research (TIFR).

Tata Institute of Fundamental Research (TIFR) was founded by the visionary
Indian physicist Homi Bhabha in 1945, two years before India gained
independence. He wanted to establish ‘a vigorous school of research in
fundamental physics’ and grow it into ‘a school of physics comparable with the
best anywhere’. He could convince the political establishment of the newly
born relatively poor country that investing in the study of ‘abstract subjects
like advanced physics, mathematics, and astrophysics’ is worthwhile because
practical advances of a society in the modern times ‘have their origin in
fundamental research’. TIFR was liberally funded, and before his untimely
death in 1966, Homi Bhabha could grow several areas of research, like
mathematics, theoretical physics, and experimental cosmic-ray studies into
maturity and also venture into new areas like computer science, biological
science, and radio astronomy. Identifying talented youngsters from across the
county, rigorously training them, providing them total freedom to pursue their
dreams, and providing them full material support was the core method by which
Homi Bhabha built this research institute. This method was fully augmented and
complemented by a proactive approach of scouting for established scientists
from all across the globe and convincing them to join TIFR or at least have a
close interaction.

The spirit of nation-building and embarking on new areas was quite widespread.
Vikram Sarabhai, ten years younger than Homi Bhabha, was a multifaceted,
talented individual interested in industry, management, and science. He was a
trained cosmic ray physicist, and in 1947, he founded the Physical Research
Laboratory (PRL) in Ahmedabad, India. PRL is thought to be the cradle of space
science in India. Sarabhai, apart from his various other activities like
starting nuclear reactors in India (along with Homi Bhabha), was responsible
for establishing the India Space Research Organisation (ISRO) in 1969 with the
express purpose of making India ‘…second to none in the application of
advanced technology to meet the problems of man and society’.

Thus, there was enough expertise available in India in the sixties so that
India could be a part of the exciting journey of X-ray astronomy. In the
context of the development of space astronomy at TIFR, I had sketched the
exciting story of how TIFR used its know-how of balloon techniques to make
inroads in space astronomy, indulge in many diverse activities, eventually
culminating in the launch of India’s first multi-wavelength astronomical
satellite AstroSat in 2015, in which TIFR played a pivotal role (Rao, 2023,
hereafter referred to as Paper1).

###  1.3 Scope of the present paper

Paper1 was based on a talk at a TIFR conference organized by the TIFR Alumni
Association from December 17-18, 2022. Reflecting the ‘feel-good’ mood of the
meeting, Paper1 gave an extremely positive vibe to the events leading to the
launch of AstroSat.

The story of this exciting journey of a developing country to be a part of a
front-line research area has several lessons to offer.

Several streams of thoughts about doing science could be examined here.

  * •

Development of an area of science: individual or cultural?

  * •

How did the corporatization of science, brought about by X-ray astronomy,
influence the way of doing science?

  * •

Whether there is still a place for blue-sky research?

  * •

How can these insights be used to formulate a strategy to do X-ray astronomy
in the future, particularly in India?

With these questions in mind, I have attempted to examine the development of
X-ray astronomy in India. I have subjectively divided the past six decades of
research work in X-ray astronomy into three broad eras. The era from the birth
of X-ray astronomy in 1962 until 1980 saw a rapid expansion of X-ray
astronomy. It coincided with a period of ‘dreamers’ from India aggressively
trying to catch up with this exciting journey of new discoveries. The next
era, from 1980 to 2000, saw a consolidation of work in X-ray astronomy brought
about by the Great Observatories. During this period, India was struggling to
catch up. This millennium, however, brought about a maturity in X-ray
astronomy, the primary driver being the thinking of niche observational areas
of research that are topical and scientifically relevant. In India, this era
saw a renaissance in X-ray astronomy, culminating in the launch of AstroSat.
Finally, the future could be a balance between striving to leapfrog by vastly
expanding observational sensitivities and developing specific science-driven
observational campaigns. Here, I will dwell on some aspects of the style of
doing science and will give some personalized suggestions about advancing
X-ray astronomy as a collective creative art.

I must hasten to add that this article is not a rigorous and serious review of
X-ray astronomy. My understanding of X-ray astronomy is based on the practice
of this discipline for about four decades. I have gone through most of the
articles on the subject in the Annual Review of Astronomy and Astrophysics for
this article. I have referred to these articles at appropriate places but have
refrained from giving an extensive bibliography. This has enabled me to retain
an overall birds-eye view of the subject so that some general trends could be
identified and used to obtain a reasonable future strategy, particularly in
the Indian context.

Apart from the Annual Review articles, I benefited from the material available
from the HEASARC website and also some good review articles and books
(Agrawal, 2019; Santangelo $et~{}al$., 2023; Tucker & Giacconi, 1985;
Pulakkat, 2021).

##  2 1962 - 1980: rapid expansion; era of dreamers

Once it is known that bright X-ray sources exist in the sky, the scientific
community voraciously investigated the nature of these sources using all
available resources to send equipment above the Earth’s atmosphere. In the
initial years, however, rockets and balloons were available as the carriers of
the instruments. The short duration of exposure severely limited rocket-based
experiments. The balloon-borne instruments, however, could reach heights only
up to about 40 km above Earth, and the residual atmosphere at that height
blocked most of the X-ray radiation below about 20 keV, resulting in
relatively poor sensitivity for observations. Thus, by 1966, the total
exposure to the sky was about an hour from rockets and $\sim$ 150 hours from
balloons. About 20 X-ray sources were detected, and only about half were
confirmed by multiple observations. A few sources, like Crab and Sco X-1, were
securely identified. Most sources were found in the Galactic plane, mainly
near the Galactic centre. There were claimed, but unconfirmed, detection of
sources close to extragalactic objects like Coma cluster and M82. The
ubiquitous X-ray background was identified to be of cosmic origin.
Proportional counters in soft X-rays and scintillating crystal detectors in
hard X-rays were the detectors of choice, and various ways like scanning
collimators, modulation collimators and lunar occultation techniques were used
to localise sources. It was realised that grazing incidence mirrors are needed
to focus X-rays, and a small area X-ray focussing telescope could only be used
to image the Sun. It was concluded that X-ray astronomy was a ‘strong if
immature branch of science’ and it could be ‘..one of the most fruitful
channels of data about the largest scales of the universe’, or it could be
relegated ‘to the detailed study of certain unusual stars’ (Morrison, 1967).
In fact, at that time, very few precise measurements were available in X-ray
astronomy, and it was realised that the field was in the ‘last stages of
simple exploration’ and could enter the era of accurate measurements through
satellite-based observations (Giacconi $et~{}al$., 1968).

UHURU, launched in 1970, firmly established X-ray astronomy as a mature and
modern tool to understand the cosmos. Though modest by current standards (just
142 kg of instruments), the design was very carefully done (Table 1). The
detectors were two large area proportional counters with two different field-
of-view collimators. The detectors were continuously rotated to scan the sky.
The satellite was thoughtfully put in a near-equatorial orbit to achieve a low
and stable background and avoid the high background South Atlantic Anomaly
(SAA) region. The continuous observations possible in a satellite platform
were a vast improvement compared to barely an hour of observation possible
during the sum-total efforts of all previous rocket experiments in the low
energy ranges (1.7 - 18 keV). UHURU detected 161 sources in the first 125 days
of operation, and 40 were identified. Detection of X-ray pulsations in two
bright X-ray sources in the Galactic bulge (Cen X-3 and Her X-1) and measuring
their binary periods firmly put accreting compact objects as the sources of
bright Galactic sources. The precise X-ray position of Cyg X-1 led to radio
identification, which in turn helped the optical identification and, later, in
1975, its binary nature (Bolton, 1975). The simplest explanation was an
accreting black hole in Cyg X-1, though other possibilities could not be
unequivocally eliminated (Blumenthal & Tucker, 1974). The bright Galactic
sources like Sco X-1 and Cyg X-2, with no pulsations or clear evidence of
binarity, remained mysterious. UHURU also detected 40 high latitude sources,
20 of them firmly identified with extragalactic objects like AGN, QSOs, and
clusters of galaxies, thus making X-ray astronomy ‘..one of the most fruitful
channels of data about the largest scales of the universe’.

Table 1: Important X-ray astronomy missions during 1962-1980a

Name | Operation | Mass (kg) | Instruments
---|---|---|---
UHURU | 1970-73 | 142 | Collimated Proportional Counters
|  |  |  (0.5∘$\times$5∘) & (5∘$\times$5∘); 840 cm2; 1.7 - 18 keV
HEAO-A | 1977-79 | 2552 | A1: Scanning Sky Survey
|  |  |  2000 cm2; 0.25 - 25 keV
|  |  | A2: Collimated detectors (LED, MED, HED)
|  |  |  3000 cm2; 0.15 - 60 keV
|  |  | A3: Scanning modulation
|  |  |  450 cm2; 1 - 13 keV
|  |  | A4: Collimated detectors (LED, MED, HED)
|  |  |  220 cm2; 15 keV - 10 MeV
HEAO-B | 1978-81 | 3130 | High Resolution Imager (HRI) & grating
|  |  |  2”; 20 cm2; 0.15 - 3 keV
|  |  | Imaging Proportional Counter (IPC)
|  |  |  1’; 10 cm2; 0.4 - 4 keV
|  |  | Solid State Spectrometer (SSS)
|  |  |  200 cm2; 0.5 - 4.5 keV
|  |  | Focal Plane Crystal Spectrometer (FPCS)
|  |  |  1 cm2; 0.4 - 2.6 keV
|  |  | Monitor Proportional Counter (MPC)
|  |  |  1.5∘; 667 cm2; 1.5 - 20 keV

aAngular resolution, detector area and the energy range are given for each
instrument

The success of UHURU led to several small-scale experiments like Copernicus
(OAO-3), ANS, Ariel-5, Ariel-6, and SAS-3. Accretion onto compact objects
emerged as a mature field of study, and most of our current understandings of
accretion had their seeds firmly planted in the seventies. The leap of faith
to understand Active Galactic Nuclei as accreting supermassive black holes was
yet to mature. While discussing extragalactic X-ray sources Gursky & Schwartz
(1977) concluded that the unanswered questions are “of origin and evolution
which we are yet too timid to ask”. Weedman (1977), while discussing Seyfert
Galaxies, opined that they are similar to QSOs, but “…the reason for their
existence remains one of the most pressing astrophysical mysteries”. Inspired
by stellar black hole sources, they also discussed the possibilities of having
107 M⊙ black holes or neutron stars but concluded that “..how the required
compact objects get into the nuclei requires comprehensive knowledge of
galactic evolution…”

X-ray astronomy came into full maturity after the launch of HEAO-A in 1977 and
HEAO-B (Einstein Observatory) in 1978 (see Table 1). With about 3000 kgs of
instruments in each of these two satellites, they are on par with modern
large-scale experiments in scale and vastness. HEAO-A pushed to the limit the
capabilities of non-imaging X-ray telescopes with a collecting area close to a
square meter. The advantages of using focussing techniques in X-ray telescopes
are humongous. The ubiquitous X-ray background can be tamed. Firstly, the
‘seeing’ for a source is in arc seconds compared to the degree scale
achievable in collimated detectors, thus reducing the solid angle for the
isotropic background by a million times. Secondly, the geometric area of the
detectors is at least a couple of orders of magnitude lower than the
collecting area, thus reducing the internally generated background. Lastly,
the imaging properties of the detectors can be gainfully employed to measure
and correct for the residual background simultaneously. Further, the small
detector area required to measure X-rays from a large collecting area offered
by the X-ray mirrors afforded the luxury of developing innovative high-tech
X-ray detectors at the focal plane. Einstein Observatory used a whole gamut of
possible new X-ray detector concepts. These included arc-second imaging
capabilities, good resolution solid state detectors and high-resolution
spectrometers (see Table 1).

The results from these two observatories fully justified the expectations.
X-rays from clusters of galaxies were fully understood (Forman & Jones, 1982),
and X-ray emission was detected from all classes of astronomical objects
ranging from stellar coronae to distant quasars. Complementary X-ray
observations have become a necessity to understand any high-energy
astrophysical phenomena.

###  2.1 Meanwhile, in India…

The fascinating journey of X-ray explorations excited and enthused scientists
across the globe. The newly established scientific institutes in India, like
the Tata Institute of Fundamental Research (TIFR) and Physical Research
Laboratory (PRL), made it a point to expose young researchers to new
scientific activities by sending them to Western countries, particularly the
USA. These young researchers, whom I call the dreamers, with their unbridled
enthusiasm, were responsible for India’s foray into X-ray astronomy.

B. V. Sreekantan was one of the first students of Homi Bhabha, the founder of
TIFR. Sreekantan was a trained experimental Cosmic Ray physicist. He was at
MIT during the sixties and was fortunate to witness the fascinating story of
the birth and growth of X-ray astronomy. At that time, Homi Bhabha had
collected many young and enthusiastic scientists, and the mood and ambience at
TIFR were one of attacking any scientific problems with a sense of adventure.
TIFR had a vigorous experimental activity of studying cosmic rays using
balloon-borne instruments. Sreekantan used this infrastructure to start a
balloon-borne X-ray program at TIFR. He also played an essential role in
motivating and training many students. In the initial years, he proactively
participated in balloon-borne X-ray astronomy experiments to get exciting
results like the discovery of ‘Sudden Changes in the Intensity of High Energy
X-Rays from Sco X-1’ (Agrawal $et~{}al$., 1969) and the detection of ‘Rapid
Variations in the High Energy X-ray Flux from Cyg X-1’ (Agrawal $et~{}al$.,
1971). After he became the Director of TIFR in 1975, he was the perfect
benevolent fatherly figure to nurture and support all the new initiatives. He
also navigated and managed the socio-political ambience to support these new
activities adequately.

Similarly, U. R. Rao, one of the first students of Vikram Sarabhai, the
founder of PRL, was in the USA in the sixties studying Cosmic Rays. He, too,
was suitably impressed by the new developments in X-ray astronomy and made it
a point to start balloon-based and rocket-based X-ray astronomy experiments at
PRL. He was primarily responsible for the rocket-based X-ray astronomy
experiments in India, and several exciting results on the spectral and timing
studies of X-ray binaries like Sco X-1 and Cen X-2 were obtained (Rao
$et~{}al$., 1969b, a). PRL later morphed into an Institute under the umbrella
of the Indian Space Research Organisation (ISRO), and U R Rao moved to ISRO
and played a pivotal role in the early initiatives of ISRO in X-ray astronomy.

India enthusiastically developed several X-ray instruments during the post-
UHURU era when small-scale satellite experiments were attempted. Aryabhata,
the first indigenous Indian satellite launched in 1975, had an X-ray astronomy
experiment (though the satellite worked only for about a few days). Bhaskara
satellite, launched in 1979, was mainly a remote sensing and communication
satellite but had an X-ray sky monitor (lasting only a few days of operation).
Though the initial instruments were based on crystal scintillating detectors
for studying cosmic X-ray sources, new space X-ray experiments based on
proportional counters were initiated later. These included a rocket-based
investigation to explore the soft diffuse X-ray background and a large Xenon-
filled proportional counter for a balloon-borne hard X-ray experiment.

###  2.2 A critical summary of the first era of X-ray astronomy

I will first outline the theme of the two ways of doing science: ‘corporate’
style and ‘classical’ science.

In contrast to ‘corporate science’, where method, efficiency and end results
are essential and quantified, ‘classical science’ is the sort of blue-sky
research driven by individual brilliance where the process of doing science is
essential. The results in the former are estimated in advance, and the
efficiency quantified, whereas in ‘classical’ science, the results are only a
by-product of this way of doing science. Product is essential for the former,
whereas process is vital for the latter.

The corporatisation of science had its seeds in the organised efforts to do
particle physics using accelerators at CERN, founded in 1954. The growth of
X-ray astronomy clearly demonstrates corporate governance’s utility in large-
scale science activities. From the rudimentary detectors used in the discovery
rocket flight in 1962, growing as a mature field in less than two decades is
truly remarkable. Of course, the vast amount of resources available in that
era was also a tremendous positive incentive. Still, the emergence of a new
style of doing research was required to bring about this growth.

What are the negative aspects of corporate science?

For one thing, only problems and targets amenable to a well-defined
articulation would get precedence. One needs more prolonged exposure to make
better measurements in X-ray astronomy: good, build a satellite. One needs
grazing incidence X-ray focussing mirrors to improve sensitivity: excellent,
build one. In this style, subtle questions for a deeper understanding of
science, which need open-ended explorations, will lose out and, in a long-term
perspective, could be detrimental to the growth of science.

Are there any areas in X-ray astronomy which did not get sufficient support
due to the corporatisation of science?

Yes, I believe so. Let me articulate the concepts which will be developed
later on.

Astronomy is essentially measuring the Astros and making sense of them. The
vastness of the universe, in space and time, resulted in islands of matter
settled in equilibrium for long durations of time, like the stellar interiors.
The knowledge of physics and precise measurements in the optical band allowed
a detailed understanding of such steady-state objects in equilibrium - I will
call this ‘thermal’ astrophysics. Of course, the observation wavelength might
focus more on a specific temperature band (like the optical band being
sensitive to the temperature range of stellar surfaces). Observations in many
wavebands more or less covered all temperature ranges that could be
conceivably present in the universe in steady states, ranging from the
microwave background to the surfaces of hot white dwarfs.

In contrast to such steady state ‘thermal astrophysics’, observations during
the past several decades have shown that there exist certain ‘non-thermal’
astrophysical phenomena where energy is transferred from one form to another
in a non-equilibrium fashion, ranging from the magnetic reconnection events in
solar flares to the humongous release of the rest mass energy of one solar
mass in a few seconds during the birth of a compact object (or coalescing of
two compact objects) resulting in Gamma-ray Bursts (GRBs). Additionally, most
non-thermal phenomena we observe today are closely coupled to physical
phenomena near black holes (in GRBs, AGN, and X-ray Binaries). Hence, a
detailed understanding of such phenomena is expected to expand our knowledge
of matter in extreme conditions (high magnetic field, high gravity, etc.) and
possibly lead to new physical principles and understandings.

The manifestations of non-thermal phenomena have observables in different
wavelengths. The energy is mainly transferred to electrons; the immediate
manifestation is non-thermal hard X-ray emission. The protons and other nuclei
can be accelerated to high energies, giving gamma-ray emissions. However, the
cooling time scales are much longer than electrons; hence, they are a delayed
manifestation of the putative energy trigger. When the energy loss rate is
much lower, these accelerated electrons can remain energetic for a long time.
Any ambient magnetic field can make these electrons emit in radio wavelengths
- allowing the study of the remnants of non-thermal phenomena.

The X-ray range is in the cusp of the thermal and non-thermal emissions. A
detailed understanding of the non-thermal emission and the coupling of the
thermal and non-thermal emission is hampered by the lack of improvements in
the hard X-ray sensitivity. Low energy X-rays, up to 6.4 keV (the Fe K-alpha
line), are primarily thermal emission and improvement in the sensitivity of
X-ray telescopes, though difficult, is straightforward: build better and
larger mirrors. Hence, over the past six decades, we have seen almost a
million times improvements in the sensitivity of X-ray telescopes in soft
X-rays. The improvement in hard X-rays, particularly above 50 keV, is
marginal.

The importance of hard X-ray measurements has been known for a long time, but
the avenues to improve them are not very clear, nor are they straightforward.
The point I am making here is that the ‘corporate culture’ of doing science is
not amenable to attacking such open-ended problems.

I will expand on these ideas and try to quantify them later in this article.

####  2.2.1 The Indian context

If one wants to summarise the efforts in India during the first era of X-ray
astronomy, India benefited from its emphasis on the ‘classical style’ of doing
science to get into X-ray astronomy. However, it failed to make an impact
essentially due to her inability to get into the corporate style.

The foray into balloon-borne and rocket-borne X-ray astronomy in India was
done when it was realised that the field was in the ‘last stages of simple
exploration’ using balloons and rockets (Giacconi $et~{}al$., 1968). Though it
is commendable that significant results on the spectral and timing properties
of X-ray binaries were obtained, these results were of the type to find a
reference in a review of the individual sources but are of little value to
find mention in a review of a broad research area of X-ray astronomy.
Additionally, in the area of instrumentation in X-ray astronomy, there was not
a single instance of significant discoveries or improvements made in India
that merited an original contribution. I will examine some sociological
aspects that could have prevented India from substantially contributing to
X-ray astronomy in this era.

In this era, the primary centre for X-ray astronomy was TIFR (with some
contributions from PRL and, later, in satellite-borne X-ray astronomy, ISRO).
Hence, I will primarily concentrate on working with the X-ray astronomy group
at TIFR.

The style of working of many scientific Institutes in India in this era, like
TIFR and PRL, was driven by a sense of doing deep science; the icons of
science are the great scientists of the early twentieth century, exemplified
by Albert Einstein. The founding logic of these Institutes is quite simple:
science in the nineteenth and early twentieth centuries was driven by
brilliant individual scientists. Collect the best and brightest from all
across India, nurture and encourage them; surely, the result will be great
science.

There is an underlying assumption in the basic premise of the above method.
The assertions that science was driven by ‘brilliant individual scientists’
and if you collect the ‘best and the brightest’ you can nurture them to be
‘great scientists’ presupposes that there exists a separate class in human
talent, distinctly different from the average, destined to be great, only the
circumstances prohibiting them becoming great like the superlatively talented
individuals born during the birth of India ending up as wastrels in the poor
Indian conditions, as imagined in the novel ‘Midnights Children’ by Salman
Rushdie. Quite often, the marks of students appearing in the entrance test of
TIFR were carefully examined to look for two Gaussians in the distribution:
the broad lower one representing the vast dumb majority and a sharp short peak
on the right side representing the talented individuals.

However, alas, the distribution was mainly a power law.

Hence, I want to advance the following hypothesis to make sense of the
development of X-ray astronomy in India.

Instead, the inherent human talent in a population could be a Gaussian, like
many other measurable quantities like height, weight of the brain, etc. The
distribution can become a power law when talent growth requires multiple
social interactions. In a settled society, encouraging the high end of the
power-law distribution in talent may help develop a more mature community. In
a poor country like India, the education system was primitive in the fifties
and sixties. Typically, in a given class in an elementary school, only one or
two students, fortunate to have positive family support, managed to pass the
exam, and the rest dropped out of the education system. Hence, the obsessive
search for extreme talents naturally ended up selecting sort of ‘freaks’:
people who concentrated on their studies despite great odds, people who had
the willpower for extreme self-denial so that their studies would be
strengthened at the expense of other social interactions. TIFR of the sixties
and seventies was full of talented individuals, but most were socially
awkward, self-centred and had obsessive egos. I felt that the general ambience
was quite ‘caustic’. When I confided this to one of my senior colleagues, he
said that the ambience was definitely ‘not healthy for broad based
collaboraration’ (S N Tandon, private communications). The final effect of a
collection of talented but socially awkward individuals at TIFR was that there
indeed was individual brilliance but very little collective activity.

The development of X-ray astronomy in India, particularly in TIFR, can be
explained based on the above perspective. Selecting talented individuals from
across India and giving them training, facilities, exposure, and total freedom
resulted in many accomplished scientists. In a few years, India could produce
very competitive results in X-ray astronomy and later enthusiastically get
into satellite-based X-ray observations in the post-UHURU era. Nevertheless,
one cannot help noting that in TIFR of that era, the X-ray astronomy group had
the “best and the brightest from all across India” with good support, but,
sadly, the result was not great science.

I strongly feel that the negative social aspects of a bunch of talented
individuals prevented the nucleation of a strong group of people and, hence,
prevented the development of a sort of collective collaborative culture to
attack deeper problems at TIFR. Hence, new methods to make a mark in hard
X-ray astronomy could not be evolved. In the post-UHURU era, there was a
fertile scientific area of hard X-ray astronomy to be explored from the
homegrown balloon facility at TIFR. Indeed, once there were three parallel
hard X-ray balloon-borne experiments simultaneously carried out at TIFR in
this era. Hence, though there were foreign collaborations for hard X-ray
astronomy (with Japan, the USSR, and Canada), a strong group collaborating on
an equal footing was lacking. Therefore, a substantial homegrown area of
expertise was not developed.

##  3 1980 - 2000: consolidation; the era of strugglers

The post-Einstein Observatory era of X-ray astronomy saw the consolidation of
the gains made earlier. There were sustained efforts from Europe, the USSR,
Japan, and the USA. The developments were quite straightforward: make
collimated detectors based on proportional counters and scintillating crystals
and improve the focussing techniques at lower energies. Since the X-ray sky is
highly variable, all-sky monitoring was also one of these satellites’ key
ingredients.

###  3.1 Consolidation

From the European side, EXOSAT (launched in 1983; satellite weight 510 kg -
these two numbers would be given hereafter while introducing an X-ray mission)
focussed on making wide-band spectral observations. It used the concept of
having highly elliptical long satellite orbits to get uninterrupted long
observations (91-hour period). Focussing X-ray mirrors at low energies (a few
cm2 area) and a reasonably large area (1800 cm2) proportional counters (along
with the new concept of gas scintillation proportional counters to improve the
energy resolution) were the hallmarks of the detectors. ROSAT (1990; 2400 kg)
had a much larger area of X-ray mirrors (400 cm2), and with its large field of
view (2 degrees), it allowed a sensitive all-sky survey at low energies. The
BepoSAX satellite (1996; 1400 kg) had a modest X-ray mirror at low energies
(50 cm2 area), the widest band spectroscopic detectors and a Wide Field
Camera.

The USSR’s developments initially centred around space station modules, like
the Kvant module and the Roentgen X-ray Observatory. Though a gamma-ray
observatory, the Granat satellite (1989, 4400 kg) had proportional counter-
based X-ray detectors (2400 cm2) and a hard X-ray all-sky monitor.

Japan made a systematic effort to make bigger and better telescopes in the
eighties and the nineties. Starting with Hakucho (1979, 96 kg), which is a
modest UHURU class satellite with proportional counters ($<$ 100 cm2 area) and
modulation collimators, X-ray astronomy missions with increasing complexity
and sophistication were attempted every few years. Tenma (1983; 216 kg) had a
gas scintillation counter and a modest area X-ray mirror (15 cm2 area). Ginga
(1987, 400 kg) used very large area proportional counters (4500 cm2) and an
all-sky monitor (70 cm2) to make timing measurements of X-ray sources. ASCA
(1993, 420 kg) used the replicated X-ray mirror foil technique to get a large
area (1200 cm2) at the expense of seeing, and it was the first X-ray astronomy
mission to use X-ray CCDs.

The USA used the Space Station to demonstrate replicated X-ray mirrors to get
large areas (several hundred cm2) in a very wide bandwidth. It used Si(Li)
detectors to make good energy resolution spectral observations in the BBXRT
(1990) instrument. RXTE (1995, 3200 kg) and the Chandra Observatory (1999,
5000 kg), one of the Great Observatories of NASA, were significant
contributions from the USA in this era.

The superior sensitivity and longevity of Chandra, coupled with a similar set
of properties of XMM-Newton along with the socially impactful RXTE (see Table
2), made X-ray astronomy a complete science.

Table 2: Major X-ray astronomy missions during late ninties Name | Operation | Mass (kg) | Instruments
---|---|---|---
RXTE | 1995-2012 | 3200 | Proportional Counters Array - PCA (6500 cm2)
|  |  |  2 - 60 keV
|  |  | All Sky Monitor - ASM
|  |  | HEXTE (1600 cm2)
|  |  |  15 - 250 keV
Chandra | 1999- | 5000 | High Resolution Camera (HRC)
|  |  |  $<$0.”5; 200 cm2 at 1.0 keV
|  |  | Advanced CCD Imaging Spectrometer (ACIS)
|  |  |  1”; 600 cm2 at 1.5 keV
|  |  | Transmission Grating Spectrometers
XMM-Newton | 1999- | 3700 | European Photon Imaging Cameras (EPIC)
|  |  |  1”; 4425 cm2 at 1.5 keV
|  |  | Reflection Grating Spectrometers (RGS)
|  |  | Optical Monitor (OM)
|  |  |  30 cm dia telescope

###  3.2 The science context

As a mature branch of astronomy, X-ray astronomy contributed to all areas of
astrophysical observations. At this stage, let me subjectively divide the
research areas in astrophysics, primarily driven by X-ray observations.

  * •

X-ray Binaries: Bright Galactic X-ray binaries (XRB) were, traditionally, the
prime focus of study in X-ray astronomy. These accretion-powered compact
objects (neutron stars or black holes) primarily emit X-rays. A detailed study
of the spectral and the timing properties of these objects is expected to
yield answers to very fundamental astrophysical questions like a) measuring
the fundamental parameters of compact objects (mass and spin of black holes,
and, additionally, size and magnetic field of neutron stars and b) study of
Physics in extreme conditions near compact objects.

  * •

X-ray Spectroscopy: There are many classes of objects, which are copious X-ray
emitters, including stellar flares and coronae, cataclysmic variables, Active
Galactic Nuclei, clusters of galaxies, etc. A detailed spectral study of many
such bright (a few mCrab fluxes) X-ray sources was possible due to X-ray
focussing mirrors.

  * •

Deep X-ray imaging: Deep observations by high-quality X-ray imagers like
Chandra can go very deep and provide complementary observations for any
astrophysical object.

####  3.2.1 X-ray Binaries

Masses of compact objects were estimated in about six X-ray binaries before
the HEAO era (Bahcall, 1978).

Though neutron stars are known as the sources of radio pulsars, ‘relatively
little can be learned about neutron star masses or radii’ from the radio
observations (Joss & Rappaport, 1984). X-ray observations in the HEAO era
increased this number significantly. By 1990, there were 100 known binary
neutron star systems (81 of them being X-ray binaries), and several properties
of neutron stars were well documented (Canal $et~{}al$., 1990). One of the
very significant contributions of the EXOSAT satellite is the detection of
Quasi-Periodic Oscillations (QPO) and red noise in neutron star XRBs (van der
Klis, 1989). Ginga X-ray spectroscopic measurements detected the cyclotron
resonance features in several sources thus measuring their magnetic fields.
Tenma satellite identified the two component spectra in LMXBs and absorption
lines at 4.1 keV in X-ray bursts (Tanaka & Shibazaki, 1996).

Evidence for BH in XRBs was initially confirmed for only three sources (Cyg
X-1, LMC X-3, and A0620-00). The number significantly increased due to the
presence of all-sky monitors in the eighties and nineties. By 1996, there were
124 known LMXBs; 41 of these were transients, including 18 BH LMXBs (all BH
LMXBs are transients), and radio outbursts were seen in 8 of them, though
there was no talk about jet emission in X-ray binaries till the early nineties
(Tanaka & Shibazaki, 1996). QPOs were noticed in BH candidates in the EXOSAT
era, but ‘the full pattern, if there is one, has not yet emerged’ (van der
Klis, 1989).

The launch of the RXTE satellite in 1995 made a dramatic and impressive impact
on the study of XRBs. Though the prime detector, the Proportional Counter
Array (PCA), is similar to the proportional counter-based Large Area Counter
on the Ginga satellite launched in 1987 (about 20% better sensitivity), RXTE
was in the internet era. RXTE which included PCA, the All Sky Monitor (ASM),
and the hard X-ray detector HEXTE had a wide energy range, excellent timing
accuracy (1 $\mu$s), and the flexibility to target any desired object quickly.

Nevertheless, the most impressive aspect of RXTE is the vibrant use of the
internet to co-opt a large section of the astrophysical community to go deep
into the problems thrown open by the new observations. Though the USA had a
vibrant ‘guest observations’ and ‘archival data analysis’ program starting
from the HEAO satellites, the establishment of HEASARC in 1990 at NASA’s
Goddard Space Flight Center (GSFC) with an explicit charter to support high-
energy astronomy in all aspects of data analysis and the wide use of the
internet profoundly affected the use of the data from RXTE. Additionally,
observations of bright XRBs, some newly discovered transients, generated vast
spectral and timing data on these highly variable sources. The easy
availability of such highly variable (and potentially scientifically new) data
enabled the emergence of a considerable number of ‘entry’ level scientists
(‘plot a light curve and write a paper’), thus generating a vast but perhaps
quite elementary, bottom level in the pyramid of knowledge, based on whom,
quite conceivably, higher levels of scientific endeavours could be built. I
will say RXTE formalised the concept of writing scientific papers as an
educational tool.

Scientifically, RXTE proved to be highly rewarding. New phenomena at
millisecond time scales were discovered in X-ray binaries: millisecond
pulsations, burst oscillations, and kHz QPOs (van der Klis, 2000). Though
relativistic jets were discovered in the Galactic X-ray binary GRS 1915+105
(discovered by the Granat satellite) before the launch of RXTE, the all-sky
monitoring ability, quick observation sampling and easy availability of data
from RXTE enabled the discovery of many relativistic jet sources and their
detailed understanding (Mirabel & Rodríguez, 1999). GRS 1915+105 proved to be
an extremely fascinating object to study. It had several episodes of
superluminal jet emissions and multiple types of variability classes, and it
was a transient source that ‘forgot to fade away’. This source merited its own
separate article in the Annual Reviews (Fender & Belloni, 2004). There were 20
BH-XRBs with mass measurements, all monitored by RXTE, and almost half
discovered by RXTE (Remillard & McClintock, 2006). Such extensive studies
helped connect the accretion phenomena in XRBs and AGNs by discovering
fundamental relationships between X-ray and radio intensities and masses of
black holes across black hole mass ranging from stellar mass to supermassive.
The quick pointing ability of RXTE helped very long-term monitoring of bright
AGNs, enabling the discovery of the dependence of the break time scale in the
power spectral density to the mass and luminosity across mass scales (McHardy
$et~{}al$., 2006). It was established that the ‘physics of disc-jet coupling
in XRBs and AGN are closely linked’ (Fender & Belloni, 2004).

####  3.2.2 X-ray spectroscopy

Though X-ray spectroscopy using EXOSAT, Ginga and Tenma established several
characteristics of ‘faint’ (a few milliCrab) sources like the absorbed X-ray
spectra of Seyfert 2s, universal power-law in AGNs, along with a hint of
reprocessed reflection component above eight keV (Ginga), it was the superior
medium resolution X-ray spectroscopy using the XMM-Newton EPICpn instrument
and the high-resolution grating spectra (Chandra and XMM-Newton complementing
each other to get wider bandwidth) that X-ray spectroscopy finally fully
matured (Paerels & Kahn, 2003). In the coronae of late-type stars, the FIP
effect on abundance was inferred, and the Doppler effect on the spectral lines
indicated that emission is mainly near the poles. High-temperature gas was
detected in early-type stars. Although spatially resolved high-resolution
spectroscopy could not be done in SNR, high-resolution spectra coupled with
the spatially resolved medium-resolution spectra could provide insights into
these objects’ temperature and abundance distribution. The excellent quality
spectra on hot White Dwarves enabled very detailed modelling. P Cygni profiles
were detected in XRBs and CVs. A detailed study of clusters enabled the
understanding of the intricate dynamical evolution of these sources.

Figure 1: Number of refereed publications based on data from Chandra
Observatory and XMM-Newton (both launched in 1999).

####  3.2.3 Deep Imaging

The deep imaging provided by Chandra brought about a revolution in sensitivity
(McCammon & Sanders, 1990). Though the study of clusters of galaxies began
with the detection of a few clusters with UHURU and the detection of a few
dozen during the HEAO era, deep imaging by Chandra and medium-resolution
imaging spectroscopy by XMM-Newton enabled doing cosmology with X-ray clusters
by detecting them up to a redshift of 1 and beyond and discovering hierarchal
model of structure formation (Rosati $et~{}al$., 2002). Deep extragalactic
X-ray surveys using Chandra and XMM enabled going deep by one to two orders of
magnitude deeper than ROSAT and allowed the X-ray measurements of AGN
evolution and the growth of supermassive black holes, providing constraints on
the Physics of high-redshift (z $>$ 4) AGN (Brandt & Hasinger, 2005). The
fluctuation analysis in the X-ray images enabled going deep by another couple
of magnitudes fainter. Deep imaging also allowed the detection of X-ray
binaries up to the Virgo Cluster, the study of the population of X-ray sources
in galaxies, including the Ultra-luminous X-ray sources (Fabbiano, 2006), and
the detection of X-ray emission from extragalactic jets (Harris & Krawczynski,
2006).

###  3.3 In India, struggling to catch up …

X-ray astronomy was initially done at very few places in India: TIFR and PRL
were the only two places. After 1980, many X-ray astronomers from PRL moved to
ISRO and took up higher responsibilities like building Institutes for
satellite and rocket fabrication. Though some work on Gamma Ray Burst (GRB)
observations and optical observations of X-ray sources were carried out at PRL
and ISRO, mainstream X-ray astronomy in India in the eighties and the early
nineties was primarily done at TIFR. Hence, I will concentrate on the
sociological aspects of doing X-ray astronomy at TIFR.

Let me repeat: TIFR started with the philosophy of collecting the best and the
brightest and giving them ample resources and support to deliver great
science.

But, sadly, the result from TIFR was not great science.

Earlier in this article, I articulated the possibility that collecting ‘best
and the brightest’ and supporting them may not be sufficient conditions for
great science. I advanced the possibility that other ingredients, like the
ability to work collectively, may be required.

Since the founding philosophy of TIFR was attributed to Homi Bhabha, the great
founding director of TIFR, in the early eighties, the effort was to scrutinise
the lack of good quality science under the sanctity of the founding principle.
Hence, the diagnosis and the remedy veered on elementary premises. Like, since
the outcome is not good science, most likely, the collected individuals are
not bright enough for the task.

There were efforts to weed out the perceived laggards. There were attempts to
make the entry qualifications stricter. The practice of selecting the
available best and giving them jobs was abandoned: it was perceived to
encourage inbreeding.

The combined effect of such remedies on the working of scientific groups at
TIFR, particularly those requiring extensive collective efforts like X-ray
astronomy, was disastrous.

The number of X-ray astronomers working in TIFR was about ten when I joined
TIFR in the late seventies (including freshly appointed persons like me and
veterans like B V Sreekantan), and this number was reduced by half within a
decade. Additionally, there was minimal incentive for collective activity.
Everyone had to prove their mettle as science leaders. Hence, eventually, the
few individuals at TIFR ended up doing research in a few diverse areas with
little cohesion and coordination. There were a few independent balloon-borne
experiments for hard X-ray astronomy. There was also a collaboration with the
Soviet Union called the Gamma-ray Indo-Soviet Program (GRISP). Archival or
proposal-based X-ray data analysis was done using data from several
observatories like HEAO-A, Einstein Observatory and EXOSAT.

Finally, when the Polar Satellite Launch Vehicle (PSLV) capable of sending
2000 kg satellites to near-Earth orbits was developed and matured, X-ray
astronomers from TIFR collaborated with ISRO to make the Indian X-ray
astronomy experiment, IXAE.

####  3.3.1 The Indian X-ray astronomy experiment, IXAE

The Indian X-ray astronomy experiment (IXAE) was a small UHURU class satellite
which included a proportional counter-based instrument called Pointed
Proportional Counters (PPC) and one pinhole camera-based X-ray Sky Monitor
(XSM). IXAE was developed in India and launched (from India) in 1996 (Agrawal,
2019).

The story of how TIFR and ISRO collaborated to make this instrument quickly
and get exciting results on bright XRBs was told in Paper1. Here, however, I
will concentrate more on the sociological aspects of this effort. Several
exciting, positive aspects of doing X-ray astronomy fortuitously came together
for this effort, and I will try to expand on these.

  * •

Experienced talent in X-ray astronomy: Though I was lamenting earlier the
dwindling support for large-scale experiments like X-ray astronomy at TIFR,
the ethos of TIFR, namely, giving total freedom to individual scientists and
having an inherent expectation of getting ‘great’ science from them, though
had the effect of curtailing group efforts, encouraged individual scientists
to pursue their passion. There were efforts to use modern data analysis
techniques to understand topical astrophysical problems. There were efforts to
understand the results of homegrown experiments by coupling them to results
from other observatories and pitting them against the available current
models. There were efforts to take up new techniques in X-ray instrumentation.
There were continued efforts to persuade science management, particularly from
ISRO, to invest in large-scale X-ray astronomy experiments. Though there were
no ‘great’ scientific results, there was indeed a sufficient amount of ‘good’
science, and the scientists were experienced enough to take up the challenge
of making an X-ray astronomy instrument.

  * •

A pool of technical expertise: Science, in general, and X-ray astronomy, being
no exception, is a creative art. Good talent and continuous growth of this
talent are essential for doing good science. On the other hand, for
experimental science, particularly those needing large-scale experiments like
X-ray astronomy, making instruments work reliably and well requires a
different set of qualities, like perseverance, attention to detail for the
subtle outcomes from the experiments, following specific tasks, which could be
monotonous and repetitive, with a single-minded devotion to bring them to a
conclusion etc. TIFR did have a pool of technical people who had a lifetime of
experience making instruments work for X-ray astronomy. Though no patents were
in their name, they could make an amplifier work, for example.

  * •

Islands of technical excellence: TIFR concentrated on encouraging talent to
pursue good science. ISRO, on the other hand, had to get things delivered.
Particularly after the early setbacks for rocket development in the eighties,
there was a solid determination to concentrate on quality and make things
work. One of the very good offshoots of this is the emergence of pools of
technical excellence at ISRO in various space technology areas. A strong
collaboration between TIFR and ISRO for IXAE helped tap these resources and
make the instruments work.

  * •

Bits and bytes: The establishment of HEASARC in the early nineties, the launch
of RXTE in 1995, and the consequent easy availability of data and software
galvanised the use of IXAE data. TIFR scientists were already well versed in
using international data and implementing specialised software to homegrown
instruments. This enabled an easy adaptation of analysis techniques to IXAE
data.

  * •

Science and its growth: HEASRAC and RXTE made X-ray astronomy data more easily
accessible and made doing science in X-ray astronomy more democratic. This
helped X-ray astronomy grow in India beyond the traditional strongholds like
TIFR (and PRL/ ISRO). Many students across Indian universities and scientific
Institutes became interested in X-ray astronomy and started interacting with
TIFR.

The happy congruence of all these made IXAE an extremely positive impact on
the growth of X-ray astronomy in India. Though the data from IXAE were of
limited quality compared to that from RXTE, the combined effect of using home-
grown data in conjunction with the RXTE data made the science contribution
very meaningful. Further, RXTE had opened up the field of the study of bright
XRBs. While writing this article, I went through most of the related articles
in the Annual Reviews and kept an eye on the number of times Indian work on
X-ray astronomy was referred to by these articles. They were few and far
between (at best, one or two results in some articles). Compared to this, nine
IXAE and related results from India were mentioned in an Annual Review article
(Fender & Belloni 2004).

IXAE also had the effect of boosting the morale of Indian scientists: complex
space instruments could be made, results could be competitive, and, more
importantly, in my perspective, a large number of scientists could work
together.

###  3.4 A critical summary of the second era of X-ray astronomy

####  3.4.1 The science context

Space astronomy, in general, is very expensive. The primary funding source is
from the resources poured into space research, primarily driven by a sense of
national pride. It is a folklore that the perceived loss of face by the USA
due to the dramatic and stupendous foray into space research by the USSR in
the late fifties, sending an astronaut into space within a few years of the
first artificial satellite, resulted in the high resource-intensive Apollo
program. X-ray astronomy benefited hugely from this and advanced as a mature
branch of astronomy in the first era of X-ray astronomy, as detailed earlier.
During the second era, however, the power of the USSR was waning, and the USA
was recovering from the Challenger disaster, thus reducing the interest of
these superpowers to pour higher resources into space research. Though the USA
continued its commitment to developing the great observatory, Chandra
(launched in 1999), the primary resource allotment to X-ray astronomy came
from Japan and Europe, their national pride urging them to go ahead and be on
par with the achievements of the USA in the previous era.

Thus, the developments are incremental and based on specific improvements to
the HEAO legacies, like EXOSAT providing continuous observation, ROSAT giving
a good low-energy sky survey, and ASCA providing good spectral resolution.
Though these observations consolidated the knowledge provided by the HEAO
missions, there were no startling developments. Though it was known that hard
X-ray astronomy was important, apart from the incremental efforts to make
wide-band spectroscopic detectors like those in BeppoSAX, the number of known
hard X-ray sources above 80 keV did not go beyond what was learned from the
HEAO-A4 survey. As mentioned by Heckman & Best (2014), while discussing the
coevolution of galaxies and supermassive black holes, “X-ray data having the
depth and wide-field sky coverage to fully exploit the SDSS galaxy sample do
not exist”. Hence, before the launch of Chandra and XMM-Newton, X-ray
astronomy, by and large, was relegated ‘to the detailed study of certain
unusual objects’ like XRBs and AGN, rather than being ‘..one of the most
fruitful channels of data about the largest scales of the universe’.

####  3.4.2 And in India…

Research in X-ray astronomy in this era is primarily based on long
observations from satellite-borne instruments. India did not have any presence
in this area during this era. Of course, as I repeatedly hinted, science could
have benefited by concentrating on niche observing areas like new developments
in hard X-ray astronomy. However, the Indian science community was still in an
individualistic mode of work to make any effort in this direction.

##  4 2000 onwards: maturity; the era of renaissance

The launch of Chandra and XMM-Newton in 1999 and their continued operation
till today, along with RXTE, which lasted for 17 years, fundamentally impacted
X-ray astronomy. Even after two decades, Chandra and XMM-Newton are sustaining
an impact on X-ray astronomy (see the number of publications using data from
these two satellites in Fig 1). This was also the era of INTEGRAL (2002, 4000
kg), a gamma-ray observatory having significant X-ray sensitivity, operating
till now, and Suzaku (2005, 1700 kg), a suit of the most sensitive wide band
X-ray spectroscopic detectors, operating for a decade. With these advanced and
sophisticated instruments, X-ray astronomy became a mature area of research.
The impact of RXTE on the study of bright XRBs and bright AGNs is very
significant, and the combined effect of spectroscopy of bright objects using
Chandra, XMM-Newton and Suzaku was enormous. Of course, the deep imaging
capability of Chandra had a tremendous impact on almost all branches of
astrophysics: AGN feedback (Fabian, 2012) and outflows (King & Pounds, 2015),
Ultra-luminous X-ray sources (Kaaret $et~{}al$., 2017), dark matter haloes
(Wechsler & Tinker, 2018).

Here, I will subjectively divide the complexities of X-ray satellites (XS) as
improved sophisticated versions which made a quantum leap from the previous
versions, like XS v1.0 (UHURU type), XS v2.0 (HEAO-A/B type) and XS v3.0
(Chandra/ XMM-Newton type). Are there any attempts this century to go for the
next-generation satellites (XS v4.0)? Yes, attempts like the LOFT satellite
with an order of magnitude larger area than RXTE and Athena with an order of
magnitude larger area than Chandra were proposed, but we are still waiting to
see a launch. With its bolometer, ASTRO-H, or Hitomi (2016, 2700 kg), could be
considered as XS v4.0 for its spectroscopic capabilities, but unfortunately,
it lasted only a month (ASTRO-H2, or XRISM - 2023, 2300 kg - was launched
recently).

Table 3: Missions with specific goals in this century  Name | Operation | Mass (kg) | Instruments
---|---|---|---
Swift | 2004 - | 1470 | Burst Alert Telescope - BAT (5200 cm2)
|  |  |  15 - 150 keV
|  |  | X-ray Telescope- XRT (135 cm2)
|  |  |  4”; 0.2 - 10 keV
|  |  | UV Optical Telescope - UVOT
|  |  |  0.9”; 30 cm dia mirror;
NuSTAR | 2012- | 350 | Hard X-ray focusing telescope
|  |  |  1’; 60 cm2 at 78 keV; 3 - 78 keV
IXPE | 2021- | 330 | X-ray polarimeter (1 - 8 keV)
|  |  |  0.5”; 500 cm2 at 2.3 keV
Table 4: AstroSat and HMXT Name | Operation | Mass (kg) | Instruments
---|---|---|---
AstroSAT | 2015 - | 1513 | Large Area X-ray Proportional Counter - LAXPC (6500 cm2)
|  |  |  3 - 80 keV
|  |  | Scanning Sky Monitor - SSM (60 cm2)
|  |  |  2.5 - 10 keV
|  |  | Soft X-ray Telescope (SXT) (90 cm2 at 1.5 keV)
|  |  |  0.3 - 8 keV
|  |  | Cadmium Zinc Telluride Imager - CZTI (1000 cm2)
|  |  |  20 - 200 keV
|  |  | UV Imaging Telescope - UVIT
|  |  |  1.8”; 38 cm dia mirror
HXMT | 2017- | 2800 | Hard X-ray Modulation Telescope
|  |  | Low Energy Telescope (384 cm2)
|  |  |  0.7 - 15 keV
|  |  | Medium Energy Telescope (952 cm2)
|  |  |  5 - 30 keV
|  |  | High Energy Telescope (5100 cm2)
|  |  |  20 - 200 keV

On the other hand, this century can be considered the era of small-scale niche
experiments in X-ray astronomy. These could be XS v.2.0 type but concentrated
on specific topical issues. Three satellites of this class are noteworthy
(Table 3). Swift (2004, 1470 kg), though a satellite meant for GRB detection,
localisation, and afterglow studies, can swiftly move the satellite to any
desired direction. This enabled the focussing telescopes (XRT and UVOT) to be
used as X-ray and UV monitors. The hard X-ray detector, BAT, had a
considerable area (5200 cm2) and provided the deepest hard X-ray sky survey.
NuSTAR (2012, 350 kg) extended the focussing advantage to hard X-rays. The
recently launched IXPE (2021, 330 kg) finally realised the potential of X-ray
polarisation studies.

###  4.1 AstroSat: the major Indian contribution

Figure 2: Number of refereed publications based on data from currently
operating hard X-ray instruments NuSTAR (launched in 2012), AstroSat (2015)
and HXMT (2017), compared to those based on RXTE (1995) data.

The central story of Indian effort in this era is the successful launch and
operation of the AstroSat satellite.

AstroSat (2015, 1500 kg) can be considered a second generation X-ray satellite
(XS v.2.0) with two complementary science priorities: a wide-angle UV
telescope with excellent seeing (UVIT) and a suite of co-aligned X-ray
telescopes covering an extensive energy range of 0.3 to 190 keV. Along with
NuSTAR and HXMT (2017, 2800 kg), AstroSat data have the potential to make
inroads into the long-neglected hard X-ray astronomy (Table 4).

The story of the making of AstroSat is told in Paper1. Here, I will discuss
the sociological aspects. The fortuitous combination of multiple factors
positively reinforcing each other for IXAE development saw an improved version
for AstroSat. The same factors are discussed below at some length.

This century saw the emergence of many research centres in India practising
X-ray astronomy apart from the traditional strong bases at TIFR and ISRO:
Indian Center for Space Science (ICSP) at Kolkata, Inter-University Center for
Astronomy and Astrophysics (IUCAA) at Pune, Indian Institute of Astrophysics
(IIA) Bengaluru, Physical Research Laboratory (PRL) Ahmedabad, Raman Research
Institute (RRI) Bengaluru and many Universities and educational Institutes.
The easy availability of RXTE data, coupled with an effort to hold regular
training workshops, built up a reasonable number of X-ray data users and
interpreters. The effective number of core X-ray astronomers in India, that
is, X-ray astronomers who could build instruments, develop X-ray data analysis
codes, or manage X-ray missions, however, remained a handful (say, 5 to 10)
throughout the past six decades of X-ray astronomy. Though it is creditworthy
that a small number of X-ray astronomers could dedicate a lifetime to realise
AstroSat, several negative aspects of this approach prevented the healthy
growth of X-ray astronomy in India.

Compared to the IXAE effort, the technical support for AstroSat by the pool of
technical expertise from the various ISRO centres was of vastly superior
nature and content. While discussing the growth of creative talent for X-ray
astronomy, I emphasised that the growth of talent by collective, collaborative
efforts could be as significant, if not more, as the selection of good talent.
In hindsight, I can list several essential ingredients that could have been
responsible for developing collective creative talent that has emerged at ISRO
and extensively used by X-ray astronomers to realise AstroSat. I am pursuing
my core belief that X-ray astronomy is a creative art and, even more
importantly, a collective creative art. I am examining the events through the
prism of understanding the talent growth for such a creative art.

  * •

Long lead time for talent growth: TIFR and ISRO, though both traced the
development of Science and Technology in post-independent India, grew with
their distinctive styles. TIFR concentrated on making extra effort to identify
and encourage talents of high order. ISRO, on the other hand, though led by
talented individuals at various stages, required a considerable pool of
dedicated individuals, quite often for doing and developing mundane things
that were not highly challenging, intellectually, but nevertheless required
dedicated and continuous efforts. The fact that they were assured of
continuous appointments further helped develop pools of expertise at ISRO.

  * •

Specific projects: Since India was under a Western embargo for smooth
technical transfer of knowledge for various reasons during the past six
decades, several technical areas of expertise for space sciences had to be
developed in India from the first principles. Though most of these
technologies existed elsewhere, discovering new ways of doing things
independently has its creative charm. Additionally, these technologies had to
be made operational for specific purposes of launching or operating a
satellite, thus leading to a sense of purpose and completion for developing
the know-how.

  * •

Attitude of getting things done: Launching and operating a complex satellite
requires both a culture of science and a culture of making things work, that
is, engineering. The engineering challenges are very specific and product-
driven, which helps develop a culture of mutual help and collective work.
Coming from TIFR, which had a culture of extreme individualism, I found it
refreshing to see a culture of collective creative effort in ISRO.

  * •

Freedom with control: Unlike a private organisation where strict control of a
worker’s productivity and expertise can be monitored, nurtured or optimised,
ISRO essentially is a government organisation. Hence, extreme control of an
individual worker was not entirely possible. The team leader’s job was to
motivate and get the work done, leaving sufficient leeway for the workers to
innovate and develop workable solutions to the multiple tasks. It was quite
pleasing to see that ISRO had developed a culture of balanced control and
freedom to get the best out of a team for specific targeted projects. Though
this method did have certain negative connotations, like the tendency to
isolate highly talented individuals but having strong, uncompromising
individualistic stands, the overall effect was quite positive.

The above is a subjective list of the probable causes for the emergence of
significant talent pools at the various ISRO centres that were extensively
used to develop AstroSat. All instruments were successfully built and
operated. They performed (and are still performing) as designed. The
limitations were mainly two-fold. Firstly, the number of experienced
scientists available in India is limited. This led to a delay in the
realisation of AstroSat and, more importantly, in my opinion, made the
scientists indulge in diverse necessary activities, thus preventing the
emergence of deep expertise in any specific area of research. The culture of
ISRO, on the other hand, is excellent in tackling some specific tasks in a
project mode in a specified time but not entirely amenable to the realisation
of some long-standing open problems. The primary developmental activities of
AstroSat happened a few years before the launch of AstroSat in 2015: after
some visibility in the realisation of the instruments and after the Indian
science management decided to fully endorse AstroSat and work towards it. The
work before this intense project mode involved some individual scientists
toiling towards the instrument’s realisation and some help from specific ISRO
laboratories. After the launch, the work reverted to this mode of ‘few
scientists-and-few-centres’ style, severely affecting the effective use and
utilisation of AstroSat data.

###  4.2 A critical summary of the third era of X-ray astronomy

####  4.2.1 The science context

If I were to revisit the speculation from an early review that X-ray astronomy
could be ‘..one of the most fruitful channels of data about the largest scales
of the universe’, or it could be relegated ‘to the detailed study of certain
unusual stars’ (Morrison, 1967), what would be my take?

Mostly the latter.

Here, I highlight the factors limiting the impact of X-ray astronomy on our
broader understanding of the universe.

Among the three key research areas highlighted earlier, viz., the study of
X-ray binaries, X-ray spectroscopy, and deep imaging, the third one is
directly relevant to the quest for the ‘largest scales of the universe’. Deep
X-ray imaging is possible using large area focussing X-ray detectors like
Chandra. Due to the peculiar nature of grazing incidence mirror technology,
the effective area of the X-ray mirrors drops very sharply with energy; hence,
the imaging is done mainly at low energies (1 - 2 keV). Only the thermal part
of the X-ray spectra are probed at these energies. Though the temperature
ranges probed by X-ray observations are significantly higher than from
observations using lower frequencies, to trace the thermal universe in the
largest scales, the technical developments in other wavelengths (primarily
optical and infrared) ensured a deeper reach into the far universe
(particularly after the launch of JWST). These developments have relegated
X-ray imaging to a supporting role. There exist, though, some very niche
areas, like the study of the temperature distribution in clusters of galaxies,
where X-ray observations give a unique perspective and, in recent times,
eROSITA (2019, 810 kg) is making a mark in these areas. Mostly, however, the
restricted improvement of the sensitivity of X-ray telescopes has not made
X-ray astronomy the primary choice of wavelength to study the ‘largest scales
of the universe’.

The first two areas, though, pertain primarily to ‘the detailed study of
certain unusual stars’, a detailed characterisation of the exotic non-thermal
phenomena has far-reaching implications for our understanding of the largest
scales of the universe. If the X-ray detectors, particularly in the hard X-ray
range (more on this in the next section), can clearly decipher the source
emission mechanisms to unambiguously elicit pertinent information on the
emitting region like the source geometry, energy source and the details of the
emission mechanisms, it will have tremendous impact on our understanding of
the largest scales of the universe by, for example, using gamma-ray bursts as
distant indicators or Active Galactic Nuclei to understand the growth of black
holes. Though NuSTAR and Swift have provided some important inputs, and IXPE
is getting some exciting results on X-ray polarimetry, I would rather say that
there were no groundbreaking results in our understanding of, say, black hole
sources and their environments. Black hole masses are measured by velocity
measurements through optical spectroscopy, and the spin measurements by X-ray
observations are far from convincing (see Rao, 2024, for a detailed discussion
on the limitations of X-ray sensitivity for our understanding of the accretion
phenomenon near black holes).

In the next section, I will delve into the sensitivity improvements in X-ray
measurements. This will help us understand why X-ray astronomy has yet to make
deep inroads into the non-thermal phenomena in cosmic sources.

####  4.2.2 Sensitivity of X-ray detectors

Let us understand the spectroscopic capabilities of X-ray telescope/ detector
systems for the first two objectives: spectral and timing studies of bright
XRBs and continuum spectroscopy of a set of reasonably bright sources. The
former are bright objects up to the brightness of Crab, and the latter are
mostly a few milliCrab sources.

I have taken the data for Crab itself to examine the current state of the
X-ray spectral sensitivity for Crab-like sources, particularly at higher
energies ($>$ 50 keV). NuSTAR is the only operating hard X-ray focussing
instrument, and I have taken the Crab data based on an observation made on
2016 May 3 (see Table 5). Many hard X-ray collimated detectors are currently
operating, and they all have the effective area of the order of $\sim$ 1000
cm2 and hence are of similar sensitivity. Background estimation is an
intricate task at these energies, and it is imperative to make an accurate
background measurement to get good spectral sensitivity using collimated
detectors at higher energies. CZT Imager (CZTI) of AstroSat (Bhalerao
$et~{}al$., 2017) uses a box type Coded Aperture Mask (CAM) arrangement in a
collimator with a medium-sized field of view (4∘.6 $\times$ 4∘.6), enabling
simultaneous background measurement (Mithun, 2024). I have taken the Crab data
obtained from CZTI observations in the 20 – 200 keV range. The NuSTAR and CZTI
data are fit simultaneously with a power law, and the unfolded energy spectra
are shown in the top part of Fig 3.

As an example of a bright AGN, I have taken the simultaneous observations of
IC 4329A made using XMM-Newton and RXTE on 2003 Aug 3, and the spectral data
from RXTE-PCA and XMM-Newton EPIC-pn are simultaneously fit using a model
consisting of a power law and reflection component and Iron lines (model 3 in
Markowitz et al. 2006). The unfolded energy spectra are shown in the bottom
part of Figure 3. Note that a 25% area renormalisation is required for RXTE-
PCA compared to EPIC-pn. NuSTAR also observed IC 4329A on 2012 Aug 12. The
details of all these observations are given in Table 5. To have a uniform
spectral binning, all data are logarithmically re-binned to have 50 spectral
bins in a decade of energy.

The unfolded energy spectra as a fraction of Crab flux are given in Figure 4.
In the same figure, the sensitivity of each instrument, deemed as the errors
in the data for the uniform logarithmic binning of 50 data points per decade,
is also shown. It can be seen from Table 5 that the durations of observation
are vastly different for each instrument. Hence, the sensitivity numbers are
rescaled as the square root of time in the units of 10 ks, as shown in Figure
5. Also included in this figure are the data for IC 4329 A using NuSTAR
observations made in 2012. Note that the spectral sensitivity of NuSTAR for
bright sources like Crab is a few factors inferior compared to the AGN
observations because NuSTAR has the limitation of storing only about 300
photons per second, thus limiting the effective observation time for bright
sources (Harrison $et~{}al$., 2013).

Several aspects of the sensitivity of X-ray instruments can be discerned from
Figure 5.

  * •

The huge improvement in sensitivity by focussing techniques is mainly confined
to very low energies. At around 1 keV, XMM-Newton EPIC-pn has a few $\mu$Crab
spectral sensitivity.

  * •

The reflectivity of grazing incidence X-ray telescopes drops very sharply with
energy and beyond 6 keV, and in the energy region of 10 - 20 keV, the spectral
sensitivity of focussing X-ray instruments as well as the large area
collimated detectors are modest and comparable to each other. The
sensitivities are in the range of 0.1 to 1 milliCrab.

  * •

Above 20 keV, the sensitivity drops sharply with energy due to the sharply
decreasing effective area for X-ray mirror systems and sharply increasing
background for collimated detectors.

  * •

The sensitivity is quite poor in the hard X-ray regime of 50 - 200 keV.

It is doubtful that there will be drastic improvements in the spectral
sensitivity of X-ray instruments in the future. The area drops drastically
with energy for X-ray mirrors, and above 70 keV, it is very, very challenging
to build them. For the collimated detectors, a high particle-induced
background is a serious issue. Hence, we may have stupendous sensitivity below
6 keV for continuum X-ray spectroscopy but medium sensitivity in the 10 - 50
keV region and poor sensitivity above $\sim$ 50 keV.

Figure 3: Good quality X-ray spectra of representative X-ray sources: Crab (top part of the figure), as measured by NuSTAR and AstroSat-CZTI and a bright Seyfert galaxy IC 4329A (bottom part of the figure), as measured by XMM-Newton-EPIC-pn and RXTE-PCA.  Table 5: Data used for spectral fitting Mission | Instrument | Source | Date | Time | Duration (ks)
---|---|---|---|---|---
XMM-Newton | EPIC-pn | IC 4329A | 2003 Aug 3 | 06:56:27 | 136.0
RXTE | PCA | IC 4329A | 2003 Aug 3 | 09:32:16 | 19.3
NuSTAR | FPM A&B | IC 4329A | 2012 Aug 12 | 16:06:07 | 162.4
NuSTAR | FPM A&B | Crab | 2016 May 3 | 19:10:12 | 5.2
AstroSAT | CZTI | Crab | 2016 Mar 31 | 05:39:19 | 226.9
Figure 4: Unfolded spectra of Crab (data from NuSTAR and CZTI) and IC4329A
(data from XMM-Newton and PCA), in the units of milliCrab and shown in colors.
The errors in the data points are deemed spectral sensitivity and plotted in
the figure. CZTI appears more sensitive than NuSTAR due to the higher
exposure; see Fig. 5 for an exposure-corrected sensitivity plot. Figure 5: The
sensitivity values (from Figure 4) are plotted here after normalising them to
a uniform observation time of 10 ks. Also plotted is the sensitivity of NuSTAR
instrument for the observation of the AGN, IC4329A. An earlier version of this
figure is also given in Rao (2024).

####  4.2.3 Impact on science due to the poor sensitivity of X-ray detectors
above 50 keV

As mentioned earlier, X-ray astronomy straddles the thermal and non-thermal
astrophysical regimes.

Here, I want to highlight the phenomenal differences in how these two branches
of astrophysics are done.

When we peer into the cosmos, we observe a vast variety of exotic phenomena.
Most of these can generally be understood by the known laws of Physics. Hence,
it appears that the primary task of an astronomer is to carefully make
observations, note down possible boundary conditions, and diligently apply the
known laws of physics to explain the observations. If a new observation
indicates contradictions to the previously worked-out solutions, it is
generally surmised that either the assumed boundary conditions are wrong or
the previous observations had limitations. It is only a question of further
diligent work to iron out these contradictions.

The above approach is exceptionally well suited to study any astrophysical
phenomena that have had sufficient time to stabilise and thermalise,
particularly those objects that change in years or decades of time scales. For
one thing, the variable ‘time’ can be taken out of the considerations, and one
can go back and observe the object or phenomena with better equipment to
refine the measurables. Any slowly time-varying phenomena, too, can be
understood with the same approach by studying the systems as a series of time-
stamped pictures.

Hence, the crux of better understanding is better measurements.

However, we do observe many phenomena in the universe which can be called non-
thermal: the energy loss rate is comparable to or faster than the energy
generation rate. As a practical example, consider accretion phenomena near
stellar mass black holes. The typical time scale of energy generation is in
milliseconds. Since the energy source is gravitation and the gravitation
potential goes as the reciprocal of distance from the black hole, most energy
is released very close to the black hole, and most time scales (like the free-
fall time scale) are in the millisecond range. Further, the energy gain is up
to a significant fraction of the rest mass energy and thus, for neutral
materials, the electrons gain an enormous amount of energy (equivalent to
close to a thousand times their rest mass energy), and they copiously emit
radiation with energy peaking in hard X-rays, the very branch of
electromagnetic radiation where the observing efficiency is very very low.
Another intricacy is the sort of Physics governing these phenomena. To start
with, we are not entirely sure that event horizons exist. The intricacies of
the energetics in an environment never probed in the terrestrial environment
(high gravity, high magnetic field) add further layers of complications.

Hence, the methods commonly employed to understand the thermal phenomena in
the universe may not apply to understanding the non-thermal phenomena. The
time scales are too fast, observations are meagre, and the Physics principles
are uncertain.

The third era of X-ray astronomy failed to make inroads into this very
difficult but extremely important branch of astrophysics. In the next section,
I will dwell on the sociological aspects that could have influenced this lack
of progress in non-thermal astrophysics.

####  4.2.4 The way of doing science…

The way of doing science has changed during the past few decades.

One of the most significant changes in the way of doing science in the late
twentieth century, as compared to the nineteenth and early twentieth
centuries, is the dramatic increase in the number of people doing science.
Instead of being an exclusive club of talented individuals pursuing their
passion, science moved onto the universities which received massive funding,
particularly in the USA and Europe. Science was firmly coupled with education.

What are the consequences?

While discussing the effect of public science on corporate R&D, Arora
$et~{}al$. (2023) conclude that, as far as the effect on industry goes, the
university-led R&D effort did not significantly impact industrial
productivity. In the realm of astrophysics, concerns are expressed about the
way science is done. Antonucci (2013), while reviewing our understanding of
Quasars, points out that ‘Quasars still defy explanation’ and ‘…having given
up on understanding AGNs, the community now focuses on the more modest goal of
counting them’. I have also noted earlier that in recent times, there is ‘…a
lack of progress in the long term and deep thoughts’ in astronomy and
astrophysics (Rao, 2015). Following Antonucci (2013) I might also add that
‘having given up on understanding accretion onto black holes, the community
now focuses on the more modest goal of categorising QPOs’.

I want to examine the implications of the fact that the number of people doing
science has increased tremendously and offer a possible explanation for the
feeling that deeper science problems, in recent times, are not addressed.
Further, I would like to explore the possibility that the current mode of
doing science, which relies heavily on university teachers and corporate-style
funding, is not conducive to tackling complex, open-ended problems.

There are several obvious aspects of the current style of doing research that
are not very conducive to tackling complex problems. Since practising
scientists have many additional responsibilities (teaching, administration,
etc.), the incentive is always to do quick publishable research. Added to
this, the founding method is tuned to encourage predictable deliverables.
Hence, the incentive to take up complex problems with uncertain outcomes is
quite low.

There could be, however, one more additional prominent reason for the failure
of the scientific community to take up extremely challenging problems.

Science is a creative art and requires talent to deliver. Though various types
of screening are done to select the most talented people at the various stages
of entry levels in science, it is highly important that the practice of
science also emphasises talent growth. The obvious way of growing talent is
through the continuous and conscious practice of the trade, particularly for
collective creative talents like X-ray astronomy, and also to interact deeply
with other talented people. The current scenario of providing enormous
incentives for short-term products is possibly harming the requirement for
talent growth.

Hence, I want to advance the hypothesis that stunted talent growth is the
primary reason for the inability of the community to take up challenging
problems.

Let me re-articulate, re-emphasise and expand on this point.

The input stream of scientists was motivated by the individual romantic
fascination with astronomy and also as a profession. Since it was assumed that
science is important to society (as a unique style of thinking - the so-called
scientific temper - and also to deliver scientific products), significant
investments were made to make science a profession monetarily attractive. A
large number of ‘entry-level’ scientists were trained at graduate schools,
mostly coupled to the universities. Some are eventually selected from this
pool as professional astronomers, primarily in the university sector.

A professional X-ray astronomer is required to develop better and better
instruments to make good observations and then interpret these observations
based on the known laws of physics. Since the amount of work needed to
implement and operate a space science experiment successfully is quite huge,
several characteristics have evolved into the system. I will highlight some of
them which could be relevant to the theme of reduced talent growth in the
community. Firstly, a methodology was evolved to break down the task into
minute parts so that the work could be distributed to many people, including
those with very low training or capability. This, in effect, increased the
management burden on established X-ray astronomers. Secondly, a large number
of these foot soldiers are young students in the university setup, thus making
teaching and training these next-generation scientists a necessary activity of
the established scientists. This took away a substantial chunk of their time.
Lastly, since most activities are established and routine, the efforts that go
into actual creative activities are few and far between. Though some
individuals could retain their creative instincts till late in their career,
the deluge of work resulting in the explosive growth of the number of
publications (even the worst papers need to be refereed) generally stifles
good collective creative work.

The above is a cartoonish sketch of the current style of working in X-ray
astronomy, but, nevertheless, it provides a possible indicator of the lack of
deep thought in most of the recent scientific activities in X-ray astronomy.

In this perspective, the difficulties of doing hard X-ray astronomy and the
consequent limited advance in non-thermal astrophysics are symbolic of the
limitations of the way of doing science that is currently practised. The lack
of progress in hard X-ray astronomy and the understanding of the non-thermal
universe is merely, I venture to add, a byproduct of this style of doing
research.

####  4.2.5 The Indian scenario

In this century, the work on X-ray astronomy in India diversified to many
centres other than the traditional strongholds like TIFR and ISRO centres.
Instrumentation work was undertaken at PRL, RRI, and ICSP. ICSP also led a
very strong theoretical and observational effort in X-ray astronomy. RRI was
deeply involved in the development of a dedicated X-ray polarisation
satellite.

The significant advance in X-ray astronomy from India in this era, however, is
the launch and operation of the AstroSat satellite.

AstroSat, fortuitously, has all the ingredients to make a mark in the
difficult field of hard X-ray measurements.

The large area X-ray detectors of LAXPC can probe X-ray variabilities in the
millisecond range to understand the dynamics of accretion onto black holes for
bright Galactic X-ray binaries. LAXPC detectors have the highest effective
area among all X-ray detectors flown so far in the 30 – 70 keV range, thus
providing crucial timing information in this critical energy range. These data
are beautifully complemented at lower energies with SXT down to 0.3 keV, with
very good energy resolution. All attempts to use RXTE PCA data along with good
energy resolution low energy detectors like those in Chandra and XMM-Newton
had only limited success. For one thing, Galactic X-ray binaries are too
bright for these sensitive soft X-ray detectors. Secondly, coordinating
simultaneous observations using different satellites is quite difficult. SXT,
by design, can deal with the pulse pile-up effect from bright sources, and it
is in a platform meant for simultaneous observations with LAXPC.

Similarly, getting simultaneous hard X-ray measurements at higher energies was
challenging. CZTI of AstroSat is equipped with a box-type collimator that
enables simultaneous background measurement up to 200 keV, with an additional
input of polarisation measurements for bright sources. There are also other
features in AstroSat that are very useful for making wide-band X-ray spectral
measurements. The satellite is in a near-equatorial low earth orbit, just
skimming the dreaded high background South Atlantic Anomaly (SAA) region and
thus providing a stable background. All the instruments transmit time-tagged
data for individual photons. This is very useful for understanding possible
systematic uncertainties: one can examine the data at all time scales and use
the Poisson nature of individual photon interaction to address, understand,
and correct systematic errors in the data. AstroSat also has an X-ray all-sky
monitor, SSM, which followed closely the design of the highly successful ASM
of RXTE, but has several interesting additional features like having time-
tagged data for individual photons, low background due to the equatorial orbit
of AstroSat and the ability to stare at any interesting object for longer
durations.

AstroSat has been working for over eight years, and the instruments performed
as expected. The number of refereed publications compares well with other
similar observatories (see Figure 2).

A closer inspection of the publication record from AstroSat reveals the
following.

As of 2024 January, there are a total of 1435 publications with the word
‘AstroSat’ in the abstract, as per the ADS records (there indeed are some
papers based on AstroSat data without explicitly mentioning the word
‘AstroSat’ in their abstracts, but these are a few and should not dilute the
broad conclusions we are trying to draw). A significant fraction of these
publications are in GCN circulars (678). The number of refereed publications
is 413, 14 in high-impact journals like Nature, Nature Astronomy and ApJ
(Letters). Two of the prime instruments in AstroSat follow the legacy of
previously highly successful observatories: LAXPC is an improved version of
RXTE PCA, and UVIT is that of GALEX. The RXTE PCA heritage of using timing
data to examine the spectral and timing properties of bright X-ray binaries is
quite helpful for entry-level scientists to get into the subject. Similarly,
entry-level scientists can also use the wide FoV images from GALEX. Many
university students thus benefited from following these traditions in
analysing LAXPC and UVIT data.

Hence, to understand the trend of the most important results from AstroSat, I
have examined the highly cited papers from AstroSat. Among the 51 most cited
papers (citations more than 20) 23 are instrument papers or review papers. Out
of the remaining 28, 12 pertain to X-ray timing of bright sources (mainly
LAXPC driven) and 9 from UVIT (stars/ galaxies/ clusters) and 7 from CZTI
(GRBs/ polarimetry). Most of the results are authored by core instrument team
members. Further, there are very few results from multiple instruments in the
list of highly cited papers.

Thus, the promise of AstroSat to make substantial headway into the difficult
arena of non-thermal astrophysics is yet to be realised. A methodology to
estimate and correct the background using all background indicators from all
X-ray instruments is yet to emerge. A rigorous attempt to pit the individual
instrument responses with each other and instruments in other observatories
using simultaneous and quasi-simultaneous observations of multiple sources and
arriving at an authoritative understanding of the systematics in the responses
is currently a work in progress. The promise of SSM as an improved monitor
compared to ASM (of RXTE) is indeed very far from realisation. One can say
that a huge amount of rigorous work remains to be done, for each of the
instruments individually as well as jointly, to carefully mine the extremely
important wide band X-ray data from AstroSat. Also, the operation and analysis
are not nimble enough and the analysis method has not yet reached the
sophistication of the many of the established observatories, leading to an
under-utilisation of data.

Though it is very credible that a second generation X-ray satellite (XS V2.0)
was conceived and launched from India, and much effort has gone into the
operation of the satellite and smoothening data analysis, the under-
utilisation of the precious AstroSat data perhaps points towards a larger
malaise. Now, I want to tie this up with my observations about how science is
done worldwide. The negative effects I outlined earlier had an exaggerated
impact in the Indian context. The quality of entry-level scientists is
relatively lower, the number of established scientists is few, and the burden
of multiple tasks like management, administration and teaching is amplified. I
am advocating that the style of doing science does not encourage talent
growth; this can have an exaggerated impact in the context of Indian science.

Looking back at the development of X-ray astronomy in India over the past six
decades, we can treat these developments as a sort of experiment at talent
growth. The idea of TIFR, to identify potential talents and give them full
encouragement, resulted in a set of experts in X-ray astronomy, able to do
meaningful experiments but fell short of doing truly impactful research, my
interpretation of this being their inability to work together and in cohesion.
The spread of X-ray astronomy in this century mostly followed the Western
model of coupling research with education. Hence, the multitude of tasks
required to be done is quite enormous. Further, the X-ray astronomy community
is still tiny, thus enhancing the necessary mundane work for each researcher,
further limiting the growth of talent.

As per my observations, the most worrying aspect is the lack of growth as
researchers. Hence, many of the routine tasks of AstroSat got done, leaving
much to be desired in the intricate tasks of going into depth the finer
aspects of the work like cross-calibration, response building, etc.

##  5 Summary and conclusions

This article started with an ambitious and vast canvas: understanding the
evolution of X-ray astronomy during the past six decades, juxtapositioning it
with Indian developments and deriving some meaningful lessons. By the very
nature of the topic, in several places, the article’s tone was more general
and perhaps lacking in rigour, but I have tried to distinguish clearly between
information and surmises.

X-ray astronomy is a branch of space astronomy, an area of science requiring
very high resource investment. The national pride of being the first to
achieve some extraordinary feat was the driving force behind the huge
investments, particularly in the Apollo era of the sixties. This led to very
rapid developments in X-ray astronomy, opening up this new window in the
electromagnetic spectrum to observe and understand the universe. X-ray
astronomy enriched the wisdom already gained by the use of the explosive
growth in physics coupled with optical and radio observations in the early and
mid-twentieth century, and it also enabled us to peep into some exotic
observations like emissions from close to black holes. The peace and
accelerated growth of wealth in the developed countries in the post-Second
World War era ensured further ploughing of resources into space sciences,
including X-ray astronomy, enabling it to grow as a mature field.

Though X-ray astronomy is contributing meaningfully to our quest to
understanding the cosmos, it is argued that there are niche areas where X-ray
observations, particularly in the higher energies, can, in principle, help
obtain very insightful answers to several critical questions in astrophysics
like the nature of black holes, growth of black holes, and disk-jet connection
in accreting objects. Two obstacles are identified. It is demonstrated that
the improvement in the sensitivity of observations in the higher energy X-rays
is not very significant, and it is unlikely to improve in the near future,
thus limiting the effectiveness of a critical observational area. The second
obstacle is more sociological. The nature of X-ray astronomy demands the
execution of complex projects in a corporate style requiring clear-cut targets
and deliverables, thus sidelining any area which requires a nebulous and
exploratory style of research. As an off-shoot of this style (along with the
effect of coupling research with education), it is proposed that continuous
talent growth in researchers is hindered, thus preventing a collective and
deep investigation of complex and subtle problems.

The developments in India grew in parallel with the trends of X-ray astronomy
worldwide. It is creditworthy that with limited resources, India could
initially replicate most of the developments in X-ray astronomy, particularly
when the work was limited to balloon and rocket-borne experiments. It was,
however, pointed out that in this game of catching up, no original or
noteworthy results were obtained. Though lack of resources (material as well
as human resources) could have played a major role, the inability to
collectively address some deep problems was noticed. In recent times, an
observatory class satellite AstroSat was launched and made operational, but
extracting the most meaningful results from the AstroSat data, which could
have made a deeper impact on the subject of non-thermal astrophysics,
particularly in areas related to accretion onto stellar mass black holes, is a
work that largely remained undone.

It is hinted that the deluge of work that a professional X-ray astronomer has
to face due to the job’s complexity and the necessity of involving a huge
number of entry-level scientists in the education sector could have inhibited
robust and collective talent growth. This could be one of the core reasons for
the lack of growth in hard X-ray astronomy (resulting in a lack of growth in
our understanding of the non-thermal astrophysical phenomena) and the lack of
more impactful results from AstroSat.

Based on these insights, I will give a personalised answer to some of the
questions posed in the first section (Introduction).

  * •

Development of an area of science: individual or cultural?

Based on the experiences of TIFR, where sustained efforts were made to support
talented individuals, I would say that the development in a scientific
research area is primarily cultural, though individuals with extra drive and
talent play a critical role. The individual’s push and the societal fertility
need to work symbiotically for a scientific area to flourish. From this
perspective, the idea of TIFR, establishing a centre of excellence in
isolation, unsurprisingly did not yield any great scientific results (in the
context of X-ray astronomy). Still, nevertheless, it enabled the development
of a fertile ground through which new emerging opportunities could be
exploited (like the launch and operation of AstroSat).

  * •

How did the corporatization of science, brought about by X-ray astronomy,
influence the way of doing science?

The most obvious positive effect is the quick realization of solutions to
‘known’ problems. Once it was known that bright X-ray sources existed, it took
less than a decade to send an X-ray satellite to study them in detail. Once it
was known that the subsequent improvement in sensitivity necessarily required
the development of X-ray mirrors, Einstein Observatory, with a good quality
X-ray mirror, was up and working within the next decade. Once it was known
that GRBs are cosmological and have X-ray afterglows, a swift-moving
satellite, Swift, was launched in a few years, and redshifts of many GRBs were
measured. The style of funding the expensive satellite programs ensured an
adequate use of the data, or rather, the data from these instruments became
the prime focus of the community’s attention.

Such a sledgehammer approach to problem-solving has the possible side effect
of ignoring alternate ideas, dissents, open-minded questioning, and attention
to subtle signals and thoughts. Though GRBs were detected in the sixties,
their cosmological origin was identified three decades later with an almost
serendipitous detection of the X-ray afterglows. The reasons could be that the
methods to solve the problem of the origin of GRBs were not obvious, and the
community’s working style was not very conducive to exploring subtle and
obscure signals.

  * •

Whether there is still a place for blue-sky research?

There are two aspects to this question. Whether there should be societal
support for blue-sky research and whether individuals should devote a
disproportionately large amount of their time to doing blue-sky research. I am
providing extremely speculative answers to these critical questions.

The intense levels of the articulation of the human mind, embodied in arts and
science, have fascinated humanity for a long time, and there has always been
societal support. I want to highlight some subtle differences over the period
of time in this societal support to these individualistic activities.

The world has seen explosive growth in wealth during the past century, and in
recent times, a significant fraction of humanity has a comfortable life and
needs not to worry about day-to-day survival; they can, in principle, indulge
in their individual passions. This very significant change can have, and
perhaps has, a subtle effect on the way society supports extreme talents. In
the earlier era, where society could not afford to support a large number of
people indulging in specialised activities, unique talents could have been
supported by a ‘queen-bee’ style of grooming (that is, someone is selected
quite early and given extra special treatment). Some signs of this style are a
little arbitrariness in selecting individuals (luck too playing an important
part), eulogising the selected ones, ascribing innate qualities and special
reasons for their brilliance.

If, on the other hand, we accept my earlier hypothesis that innate talent is
essentially a Gaussian and the different conditions for their growth turn the
talent distribution in society into a power law with a long tail (nurture
being more important than nature), the current changed circumstances (of
increased life span and wealth) can tilt the balance more in favour of
nurture. Many people can afford to grow to their satisfaction in their chosen
area without recourse to huge social incentives and support.

Hence, over time, societal support might diminish. But, doing science from a
classical perspective (that is, getting pleasure in satiating a curious mind)
is quite enjoyable to the human mind, and there will always be people willing
to spend a significant amount of resources to pursue their passion.

A scientifically trained mind is always helpful to society. There could be a
situation in the future where a substantial number of people hit the right
balance of segregating their time into satisfying their passion (indulging in
a sort of blue-sky research) and monetising their talent without too much of a
compromise on their core activity of improving their talent. Fortunately, in
X-ray astronomy, the avenues for revenue generation are relatively easy and
straightforward: education (space is always fascinating to a young mind) and
industry (many of the techniques used in X-ray detections have good industry
applications) are two of the easy options. But, smoothly balancing the
requirements of talent growth (needing a relaxed frame of mind in an unhurried
setting) and talent monetisation (requiring alertness in managing time, effort
and user requirements) could be a difficult task.

  * •

How can these insights be used to formulate a strategy to do X-ray astronomy
in the future, particularly in India?

I look at this problem with the central idea that new insightful results in
X-ray astronomy can be obtained by applying a collective and refined talent.
The question of talent growth also needs to be addressed.

Experiments in X-ray astronomy are not actual science experiments: one cannot
alter the objects of experiments in a controlled manner. However, many objects
are in the sky at different settings and conditions, providing a fertile
sample to select objects to validate any models and assumptions. Hence,
instead of controlling objects for experimentation, there is a possibility of
subtly controlling the observations to falsify models or validate the
assumptions. For this approach to succeed, the number of such ‘experiments’
needs to be large, and the questions posed need to be subtle and deep.

Since space science experiments are expensive, only a few opportunities are
currently available, and these opportunities are cornered mainly by projects
claiming to make larger and larger telescopes. ‘Make a big enough telescope,
and then anybody can get a breakthrough result’ seems the motto. There are
several drawbacks in this scheme: it corners all the resources to a few
projects; it encourages routine work (of getting results from the current big
telescope) and probably influences the lack of talent growth that I am harping
about throughout this article; since fund givers (who are also influenced by
popular sentiments) may not be specialists in a subject, this method also
‘dumbs down’ and trivialises the results (‘detecting the farthest black hole
or the closest planet’). Also, such a ‘bigger and better’ mode closely
resembles Ponzi schemes and hence cannot last long.

For a healthy and meaningful growth of X-ray astronomy, an alternate scheme of
a large number of thoughtful experiments proposed and executed by a group of
very talented people may be necessary. The opportunities to send more niche
experiments undoubtedly exist. The world is getting more prosperous, and doing
a space experiment has become, relatively speaking, cheaper. It is well within
the means of even small universities to be able to finance a good space
experiment. The second aspect, a group of talented people coming together to
think of some critical observations, might require purposeful efforts to
evolve. The current style of doing research where immediate goals are
emphasised and encouraged has probably led to the negative vicious cycle of
people getting fully occupied with mundane tasks of teaching, writing
proposals, and converting the project reports of the students into research
papers, resulting in a lower emphasis on creative work thus progressively
lowering their individual expertise and ending up doing things of even lower
significance.

It is quite possible to evolve a positive virtuous cycle of improving
expertise. If a researcher consciously decides to devote a significant amount
of time to only core science activities, foregoing the immediate benefits like
number of publications, grants, students (and promotion), it is possible to
grow the core expertise and produce better results in the long term. It
requires the firm conviction that enhanced expertise in a few years will not
only be personally very satisfying, but it will also eventually fetch better
publications, grants, and students (and even faster promotion). To
substantially change the style of functioning of X-ray astronomy, a good
fraction (say, 20%) of researchers need to follow this style. Creative work is
inherently satisfying to the human mind, and it can act as a driver for a
change such that in future, when the futility of running after short-term
goals becomes apparent, such a shift in culture can really happen.

In the Indian context, opportunities to make and launch X-ray astronomy
instruments have increased significantly in recent times. The number of people
active in X-ray astronomy has also increased. However, the ability to
consistently grow scientific expertise is somewhat lacking, mainly because of
the deluge of allied works. Though the number of X-ray astronomers in India
has increased, the total number is still sub-critical, thus providing only a
limited opportunity to interact with experts in one’s own or allied fields.

From my personal experience, I can conclude that the inherent urge to do
creative work is still intact among a large number of X-ray astronomers in
India. The bottlenecks to growth are high, as are the demands on their time
from day-to-day (mostly trivial) activities. It is, however, possible for a
few people to come together and create their own virtual creative bubble and
concentrate on some core science activities. If the long-term benefits of such
a collaboration are demonstrated at least by a few such groups, it has the
potential to accelerate and create the sort of virtuous cycle that I talked
about earlier. In the Appendix, I have given words to my flights of fancy on
how any individual can grow as a scientist, even in adverse conditions.

In conclusion, X-ray astronomy is a fascinating window to the universe, having
an unrealised potential to provide insightful information to solve several
critical astrophysical problems pertaining to non-thermal astrophysics. It is
suggested that a concentrated collective creative work culture will enable a
very meaningful growth in X-ray astronomy.

.

##  Appendix A X-ray astronomer’s art of work

X-ray astronomy is a creative art, in particular, a collective creative art.
For the healthy growth of X-ray astronomy, it is essential that practising
X-ray astronomers proactively concentrate on enhancing their creative
abilities, even when the demands on their time for several mundane non-
creative, but necessary activities are very high. I am fascinated by the set
of tongue-in-cheek advice to practice the creative art of mathematics given by
John E. Littlewood, the British mathematician of the early twentieth century
(‘Mathematician’s art of work’, Bollobas, 1986). These should perhaps apply to
X-ray astronomy, too.

Littlewood gives examples of different creative work styles and suggests that
there are no set rules for creative work, but it would be wise to find out the
usual methods and try them out. Then, he goes on to give examples of extreme
levels of creativity and posits that ‘between the extremes [of exceptionally
brilliant people and dull people], there is the army of people very gifted but
short of genius’ who possess ‘an intense conscious curiosity about the
subject, with a craving to exercise the mind on it’. Littlewood, ‘with a good
deal of diffidence’, proceeds to give ‘some practical advice about research
and the strategy it calls for’, which includes, among other things, ‘4 (or 5
at most) hours per day with breaks every hour’, ‘either work all out or rest
completely’, ‘5 and a half days a week’ and ‘3 weeks of holiday’ in a year. A
clear headed distinction is made to define what work is.

In the context of practising X-ray astronomy in a world full of essential
activities that are not necessarily creative work, it would be wise to
distinguish between creative work and other necessary work. One easy thumb
rule is whatever demands your undivided concentrated effort (writing a paper,
critically reading a paper, making a summary of results, writing a code,
designing equipment) is creative work, and the rest (attending seminars,
attending meetings, giving sermons to students) are not.

The first task is to snatch from your busy schedule a few hours (3 - 4) daily,
at least four to five times a week. It is essential to have this ‘creative’
time contiguously; at least it should not be shorter than an hour. Only
planned jobs should be taken up without any multi-tasking. Concentrate on only
one topic in a single unit of an hour, without any interruptions.

Once you have such intense doses of creative work, it will help you deal with
other tasks with less effort. Mostly, it is not the non-availability of time
that reduces your efficiency; instead, it is the multiple events and issues
that compete for your mental space which influence your productivity. After a
draining effort to write an inspiring introduction to your forthcoming paper,
the ‘moronic suggestions from your dumb colleagues’ in a meeting will not
agitate you and snatch away your mental space; you will ignore them and will
still be thinking about that clever sentence in the Introduction that you just
wrote.

The next effort is to ensure a more significant fraction of creative work in
most of your other activities. You need to be quite creative about it; some
specific examples are given below.

  * •

Collaboration
Collaborative work could be a bane to creative work if you are passively
trading your expertise to get nth authorship in a 100-author publication, but
it is a great boon if you collaborate with like-minded experts such that after
the collaboration, your expertise matures and distinctly sharpens. You can
progressively reduce your involvement in ‘mass-produced’ papers and
concentrate on meaningful collaborations instead. You may ensure that the
person you are collaborating with has good professionalism and decision-making
abilities and has sufficient time to invest in the project under
consideration. The project needs to be well-defined and should fascinate you.
If you are alert when selecting your collaborations, it is likely that over a
period of time, you will have a small number of very meaningful
collaborations, at least one of which you lead. Though most collaborations can
be done remotely, it is a good practice to occasionally have personal meetings
with the collaborators, say once in three or six months.

  * •

Students and post-docs
Many of the extra efforts required to handle students and post-docs can be
reduced considerably by a subtle shift in the idea of guiding students.
Instead of thinking that it is your responsibility to groom them and ‘impart’
the knowledge you have gained over time, it may be beneficial to treat them as
adults and deem them responsible for their own learning. In today’s connected
world, information and knowledge are freely available, and hence, you may
confine your role of guiding and mentoring only to review their work and
suggest corrections, if any, periodically; the period of review can
progressively increase from a week to a few months once the style of review is
established. Instead of pushing them to good research work, try pulling them
to your style of work. Initially, try giving them a distinct part of your
current research work and make them a co-worker. Instead of motivating them by
the lure of seeing their name as the first author of ‘a’ research paper,
imparting the sense of fun in doing good research could be more motivational.
Students and post-docs should be good collaborators, that is it.

  * •

Teaching
Teaching, taken as a separate activity, could be a big drain on your time and
energy. Here, it would be rewarding to you and the students to introduce your
own research activity as a part of the teaching. It may be relatively easy for
graduate courses, but there could be ways to cleverly introduce your research
as a part of the course, even in undergraduate courses. For example, if your
research interest is Gamma-ray Bursts (GRBs) and if you are teaching
‘Introductory astronomy’ for under-grads, the GRB motif can be introduced at
various stages like the studies of supernovae, endpoints of stellar evolution,
cosmic distance ladder, and cosmology. One of the lectures could be a summary
of your own research: it will be an exercise for you to communicate your
research to the uninitiated. Also, it can be highly motivating to the
students. Again, treating the students as adults and emphasising their need to
own up to the responsibility of learning early in their career would be very
positive learning.

  * •

Reviewing
Reviewing and refereeing could drain your time, mainly if you are quite
efficient about it. A bit of pragmatic thinking will help. Take up only those
works for review if you feel there could be interesting learning. You can be
shamelessly ruthless about refusing to referee papers which you think won’t
add anything to your learning. So long as, on average, the number of papers
that you referee equals the number of papers that you submit (not the number
of papers you co-author), the law of averages should catch up, and your
conscience can be clear.

  * •

Management and administartion
Managing a project or administering the running of a department requires a lot
of effort: management and administration is a totally different profession
compared to doing science. It is very difficult to practice two professions
simultaneously: at best, you can have one as a profession and another as a
hobby. If you decide to become a professional manager, you have to painfully
accept that you no longer wish to be a scientist. Practising science as a
hobby and returning to it as a full profession later is extremely difficult.
On the other hand, you may do the management as a hobby for a few years and
then fully revert back to science. You may have to hire professional managers
or secretaries to assist you. It is quite a difficult balancing game, but if
you love doing science, there are always ways to stick to it as a profession.
The bottom line is it is up to you to make the painful choice, but you can’t
ride both horses.

The above suggestions are a set of fanciful subjective expansions of the basic
tenet that if you have ‘intense conscious curiosity about the subject, with a
craving to exercise the mind on it’, as Littlewood puts it, then it is
beneficial both to you and the society at large to find ways to satiate that
curiosity, even while practising the worldly needs of ‘using’ your expertise
for an economically beneficial activity like teaching.

## Acknowledgements

I thank Gulab Dewangan and N.P.S. Mithun for providing the data for Crab and
IC 4329A and helping analyse these data for the sensitivity calculations.

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