∎

11institutetext: E. Kuulkers 22institutetext: Directorate of Science,
ESA/ESTEC
Noordwijk, The Netherlands.
22email<EMAIL_ADDRESS>33institutetext: C. Sánchez-Fernández
44institutetext: Directorate of Science, ESA/ESAC
Villanueva de la Cañada, Madrid, Spain.
55institutetext: A. Parmar 66institutetext: Former Head of the Science Support
Office
Directorate of Science, ESA/ESTEC
The Netherlands
_Present address:_
Department of Space and Climate Physics
MSSL/UCL
Dorking
UK

# ESA Science Programme Missions: Contributions and Exploitation – INTEGRAL
Observing Time Proposals

Erik Kuulkers  Celia Sánchez-Fernández  Arvind N. Parmar

(Received: date / Accepted: date)

###### Abstract

We examine the outcomes of the regular announcements of observing
opportunities for ESA’s gamma-ray observatory INTEGRAL issued between 2000 and
2021. We investigate how success rates vary with the lead proposer’s gender,
"academic age" and the country where the proposer’s institute is located. The
more than 20 years operational lifetime enable the evolution of the community
proposing for INTEGRAL to be probed. We determine proposal success rates for
high-priority and all proposals using both the numbers of accepted proposals
and the amounts of awarded observing time. We find that male lead proposers
are between 2–11% more successful than their female counterparts in obtaining
INTEGRAL observations. We investigate potential correlations between the
female-led proposal success rates and the amount of female participation in
the Time Allocation Committee.

††journal: ISSI Scientific Reports

##  1 Introduction

INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL) was launched from
the Baikonur cosmodrome on 17 October 2002 into a 72-hour highly elliptical
orbit by a Proton rocket. The nominal mission duration was 2 years with
resources available for a longer extended mission. In January 2015 the orbit
was changed to a 64 hour orbital period to ensure the safe disposal of the
mission in 2029. This ESA-led mission includes contributions from Russia
(launcher) and National Aeronautics and Space Administration (NASA) (Goldstone
ground station). INTEGRAL provides an unprecedented combination of celestial
imaging and spectroscopy over a wide range of X-ray and gamma-ray energies as
well as simultaneous optical monitoring 2003Winkler (1), 2021Kuulkers (2).
Many details about the INTEGRAL spacecraft, orbit, instruments, scientific
aims and first results can be found in volume 411 of A&A (2003), a special
Astronomy & Astrophysics issue dedicated to INTEGRAL. The INTEGRAL payload
consists of two gamma-ray instruments, one of which is optimised for 15 Kilo
Electron Volt (keV) to 10 Million Electron Volt (MeV) high-resolution imaging
( Imager On-Board the INTEGRAL Spacecraft (IBIS); 2003Ubertini (3)) and the
other for 20 keV to 8 MeV high-resolution spectroscopy ( Spectrometer on
INTEGRAL (SPI); 2003Vedrenne (4)). IBIS provides an angular resolution of 12′
Full-Width at Half Maximum (FWHM) over a 30$\times$30 degrees2 Field of View
(FOV) and an energy resolution, E/$\Delta$E, of $\sim$12 FWHM at 100 keV. SPI
provides an angular resolution of 2.7 degrees FWHM over a 30$\times$30
degrees2 FOV and an E/$\Delta$E of $\sim$500 FWHM at 1.3 MeV. Both instruments
provide millisecond time resolution. The extremely broad energy range of IBIS
is covered by two separate detector arrays, INTEGRAL Soft Gamma-Ray Imager
(ISGRI) (15–500 keV) and Pixellated Imaging Caesium iodide Telescope (PICsIT)
(0.2–10 MeV). The payload is completed by two Joint European X-ray Monitor
(JEM-X); 3–35 keV; 2003Lund (5)) and optical ( Optical Monitor Camera (OMC);
V-band; 2003MasHesse (6)) monitors. The instruments are co-aligned and are
normally operated simultaneously. By using the anti-coincidence shields of
IBIS and SPI as active detectors, an omni-directional sensitivity above 75 keV
is achieved. The highly eccentric orbit (period 2.7-days) allows the full sky
to be continuously monitored for about 85% of the time (when INTEGRAL is above
the Earth’s radiation belts). The fraction of sky occulted by the Earth is
much smaller than for satellites in low-Earth orbits.

The Mission Operations Centre (MOC) is located at European Space Operations
Centre (ESOC), in Darmstadt, Germany. The Science Operations Centre (SOC) was
originally in European Science and Technology Centre (ESTEC), Noordwijk, The
Netherlands but moved to European Space Astronomy Centre (ESAC), near Madrid,
Spain in February 2005. The SOC receives observation proposals and optimises
the accepted ones into an observation plan consisting of a time line of target
pointings together with the corresponding instrument configurations. The
INTEGRAL Science Data Centre (ISDC) receives the science telemetry plus the
relevant ancillary spacecraft data from the MOC which is responsible for the
operations of the spacecraft and payload. The ISDC Data Centre for
Astrophysics 2003Courvoiser (7) processes the received telemetry and generates
standard data products which are distributed and archived. The archive at the
ISDC receives about 300 independent visits per month, a rate that has been
stable over the past decade. INTEGRAL data can be processed using the Off-line
Scientific Analysis (OSA) software provided by the ISDC. This includes
pipelines for the reduction of INTEGRAL data from all four instruments. The
three high-energy instruments use coded masks to provide imaging information.
This means that photons from a source within the FOV are distributed over the
detector area in a pattern determined by the position of the source in the
FOV. Source positions and intensities are determined by matching the observed
distribution of counts with those produced by the mask modulation. The energy
range and use of coded masks means that observation lengths are usually longer
than for directly imaging instruments due to the higher background in the
source extraction regions.

##  2 Science Results

INTEGRAL has become part of the newly-established multi-messenger astrophysics
science network, measuring gamma-rays coincident with Gravitional Wave (GW),
ultra-high energy neutrino signals and Fast Radio Burst (FRB)s, so linking the
new non-electromagnetic domains with the hard X- and gamma-ray electromagnetic
radiation 2017NatAs…1E..83V (8), 2020ApJ…898L..29M (9), 2021NewAR..9201595F
(10). This synergy is achieved primarily by exploiting two of INTEGRAL’s key
capabilities – the highly efficient coverage of nearly the whole sky and the
rapid reaction capability for unexpected events. INTEGRAL can rapidly
(typically within half a day) re-point and conduct Target of Opportunity (ToO)
observations. Of all the ToO observations performed until the end of 2021, 4%
started within 8 hours, while 85% started 3 days or later after the alert.

A key part of INTEGRAL’s legacy is the continuing discovery of new high-energy
sources, now amounting to more than 700 2016ApJS..223…15B (11),
2022MNRAS.510.4796K (12). Using multi-wavelength follow-up observations, about
250 of the newly discovered sources have been firmly identified, and their
dominant emission mechanisms recognised. The classes of objects discovered
include Seyfert galaxies, blazars, cataclysmic variables, supergiant fast
X-ray transients. INTEGRAL also provides unique measurements of the decay
lines of radioactive isotopes, e.g., from extragalactic supernovae and for the
study of stellar feedback in our Galaxy. The extent and morphology of the
enigmatic positron annihilation in the central regions of the Galaxy, possibly
linked to dark-matter decay, as well as of 26Al and 60Fe continues to be
mapped by INTEGRAL 2016A&A…586A..84S (13), 2020ApJ…889..169W (14),
2022MNRAS.509L..11S (15). INTEGRAL is also performing pioneering and unique
gamma-ray polarization studies of Gamma-ray Burst (GRB)s and bright Galactic
sources, which are fundamental in understanding the emission mechanisms. The
rich science which has resulted from INTEGRAL’s observations over its lifetime
are presented in the various review articles in the special issue of New
Astronomy Reviews "Fifteen-plus years of INTEGRAL science" available at
https://www.sciencedirect.com/journal/new-astronomy-reviews/special-
issue/10K9PV78WN4. Here we provide some examples to illustrate the scientific
impact of INTEGRAL.

Gravitational wave sources: The INTEGRAL and Fermi 2021Ajello (16) detection
of the short GRB (GRB 170817A) linked to a GW event (GW170817) on 17 August
2017 caused by the merger of a binary neutron star 2017ApJ…848L..13A (17),
2017ApJ…848L..15S (18), was a fundamental milestone for the wider astrophysics
and physics communities. Using INTEGRAL, the Fermi localisation area of the
GRB could be improved by more than a factor two; this has been important to
prove, not only the temporal, but also the spatial coincidence of the gamma-
ray emission and the GW event. INTEGRAL has provided instantaneous and
complete coverage of the large localisation regions of most of the other GW
events detected by the LIGO/Virgo collaboration. The majority of them have
been associated with binary black holes, for which most models do not predict
detectable electromagnetic signals: whenever INTEGRAL was above the radiation
belts, it provided strict upper limits.

Bright transients: Several bright black-hole binary transients have led to
unique, extensive ToO campaigns with INTEGRAL. In 2015, V404 Cygni, one of the
closest (2.4 Kilo parsec (kpc)) and best-established accreting black-hole
(about 9 M⊙) binaries, became the brightest object in the X-ray sky (up to 50
times the flux of the Crab). Its outburst showed highly unusual behaviour with
repeated bright optical and X-rays flashes on time scales shorter than one
hour. It was observed almost continuously by INTEGRAL simultaneously in the
optical and hard X-ray bands, which was not possible with any other mission.
More than 40 refereed publications have been written using these data.
Transients of this type can provide unique diagnostics of the physical
processes operating near black holes through the study of optical and hard
X-ray to gamma-ray emission 2015A&A…581L…9R (19), 2015ApJ…813L..22R (20),
2015ApJ…813L..21N (21), 2017ApJ…834..130J (22), 2017ApJ…848….3R (23),
2017A&A…602A..40S (24), 2018A&A…616A.129K (25), 2019ApJ…870…92R (26),
2020A&A…634A..94K (27), 2021A&A…645A..60C (28), 2021ApJ…910…21R (29),
including positron annihilation signals 1991SvAL…17..437G (30),
1992ApJ…389L..79G (31), 2016Natur.531..341S (32), 2019ApJ…870…92R (26), and
gamma-ray polarisation 2017PoS.INTEGRAL2016.022 (33). The sensitive study of
Comptonisation, pair emission, and polarisation above $\sim$100 keV are unique
strengths of INTEGRAL.

Supernovae: INTEGRAL discovered gamma-ray emission from the decay of 56Ni and
56Co in a Type Ia supernova, SN2014J. While confirming theoretical
expectations these measurements significantly added new constraints on how
56Ni ejecta are spatially and kinematically distributed throughout an
expanding envelope 2014Natur.512..406C (34), 2014Sci…345.1162D (35),
2015A&A…574A..72D (36), 2015AN….336..464D (37), 2015ApJ…812…62C (38),
2016A&A…588A..67I (39). For core-collapse supernovae, INTEGRAL spectroscopy of
lines from the decay of 44Ti likewise provided essential kinematic information
from the deep interiors of such supernovae (Cas A; 2006ApJ…647L..41R (40),
2015A&A…579A.124S (41)), which are otherwise inaccessible. Both families of
explosions have now been recognised to be very diverse. Nuclear, as well as
non-thermal, emission occurs within INTEGRAL’s energy range allowing for
essential follow-up observations.

Fast radio bursts: One of the currently intriguing mysteries is the origin of
millisecond duration, 1040 erg, powerful FRBs. The majority of FRB do not
repeat, but a growing number of newly discovered events (now $>$10) are
repeaters 2016Natur.531..202S (42), 2020Natur.582..351C (43), 2020ApJ…891L…6F
(44). In FRB180916.J0158+65 a 16-day periodicity suggests an origin from a
compact object either in a binary system, or in an isolated, precessing,
magnetar 2020Natur.582..351C (43). INTEGRAL is well suited to study such
systems with typical emission at high energies 2020ApJ…896L..40P (45),
2019ATel13073….1G (46). The near all-sky monitoring capability of INTEGRAL
allowed a magnetar flare associated with a Galactic FRB in the soft gamma-ray
repeater SGR 1935+2154 to be found, showing that some FRBs are associated with
magnetars 2020ApJ…898L..29M (9). Since FRBs are extragalactic, this event
provides a "missing link" to a population of extragalactic magnetars.
GRB200415A, a giant flare from a magnetar in NGC 253 2021Natur.589..211S (47)
may be such a source.

Other transients: ToO observations based on multi-wavelength studies are
important for understanding the diversity of transient events. A recent
example is AT2018cow (a supernova variant), also referred to as a fast-rising
blue optical transient, which revealed its internal engine as a temporary hard
X-ray spectral component detected by IBIS up to about 100 keV, in addition to
luminous and highly variable soft X-rays 2019ApJ…872…18M (48). Another case of
a rapidly variable and rising nuclear transient with soft gamma-ray emission
was AT2019pev, which may be evidence of a new class of flares from accreting
supermassive black holes 2019NatAs…3..242T (49), 2019ATel13170….1F (50). This
opens a still unexplored window bridging optical transients to explosive
events, never observed before in this phase.

Serendipitous science: The INTEGRAL Radiation Environment Monitor (IREM) and a
few of the Radiation-sensitive Field-Effect Transistors (RadFETs) on IBIS are
providing important information about the Earth’s local environment and
contributing to interplanetary space weather 2015ITNS…62.2784C (51),
2017SpWea..15..917M (52). IREM is also used as a calibrator of similar
radiation monitors onboard other spacecrafts 2018APh….98…28A (53). Another
area where INTEGRAL provides serendipitous science includes Solar System
research, with the study of Earth’s aurorae and Solar flares
2015JPhCS.632a2081S (54), 2018JSWSC…8A..40G (55).

##  3 Scientific Productivity

Up to the end of 2021, there have been close to 2000 refereed publications
from INTEGRAL that make direct use of data from the mission including from its
primary catalogues, or make quantitative predictions of results from the
mission or describe INTEGRAL, its instruments, operations, software or
calibrations. The scientific productivity of INTEGRAL is screened monthly by
the INTEGRAL SOC, following an approach consistent with those used by the XMM-
Newton 2014Ness (56), Chandra 2012Rots (57) and Hubble Space Telescope (HST)
2010Apai (58) projects. The resulting data are stored in a dedicated database
from which information can be extracted when needed. The INTEGRAL SOC
determines the Observation Identifier (ObsID) and instruments used to provide
the scientific results reported in each publication. The ObsIDs are then used
to access the INTEGRAL Science Legacy Archive (ISLA) to provide detailed
information on the observations and proposal leading to relevant data set. The
information obtained is then combined to allow the scientific productivity of
the mission to be investigated. The major conclusions of this work are the
following:

  * •

An average of 51 scientists per year become lead authors for the first time on
a refereed paper which directly uses INTEGRAL data.

  * •

Each refereed INTEGRAL publication receives an average of around five
citations per year with a long-term citation rate of two citations per year,
more than five years after publication.

  * •

About eighty percent of the articles citing INTEGRAL articles are not
primarily INTEGRAL observational papers.

  * •

The distribution of elapsed time between the execution of the latest
observation used in a publication and the date of that publication peaks at
about 2.5 years. The publication of observations taken under the ToO program
(see below) shows a similar distribution. In other words, no significant
differences are seen by the publication of INTEGRAL Open Time and ToO
observations.

  * •

At least 95% of the science observing time is used in at least one publication
after 7 elapsed years.

  * •

The scientific productivity of INTEGRAL measured by the publication rate,
number of new authors and citation rate, has fluctuated over the time, and
experienced a significant boost in 2017, following the INTEGRAL contribution
to the detection of the electro-magnetic counterpart to GW170817.

##  4 Observing Time

INTEGRAL is an observatory executing observations mainly selected through peer
review. Usually there are annual Announcement of Opportunity (AO)s, see Table
1. The first announcement (AO-1) was issued in November 2000, prior to launch,
and received 277 proposals with a very high over-subscription in time of a
factor 19.7. AO-2 was issued in July 2003 and covered observations for a one
year interval. Due to the impending move of the INTEGRAL Science Operations
Centre (ISOC) from ESTEC to ESAC in 2005, AO-3 covered an interval of 18
months and was issued in September 2004. Subsequent AOs, except for AO-6 and
AO-7, covered intervals of one year and were issued in the Spring of each
year. AO-6 was initially foreseen to cover 1 year, but in November 2008 it was
decided to extend the cycle by two months. AO-7 also covered an interval of 14
months. The following types of observing time are available:

  * •

Normal observations: observations which do not have special scheduling
requirements allowing for the most efficient scheduling. Observation durations
are $<$1 Millions of Seconds (Msec).

  * •

Key Programme observations: Observations intended to carry out scientific
investigations requiring a significant amount of the total non-Target-of-
Opportunity (non-ToO) observing time during an AO cycle ($>$1 Msec), but also
accommodating various scientific aims. These cannot be ToO observations.

  * •

Fixed Time observations: observations with special scheduling requirements
such as phase-dependent observations of a binary system, coordinated multi-
wavelength observations, or a sequence of observations separated by a time
interval. Such observations usually reduce scheduling efficiency, because the
spacecraft must be pointing towards a particular source at a particular time.

  * •

ToO observations that have critical scheduling requirements and are meant as a
fast response to new phenomena, such as X-ray novae outbursts, flares from
active galactic nuclei, supernovae, and bright states of galactic micro-
quasars. ToOs can be for known or unknown sources identified by their class.

  * •

Multi-year proposals spanning two annual AOs for "Key Programmes" requiring
substantial amounts of observing time that cannot be accommodated in a single
AO.

  * •

Joint observations with the XMM-Newton, Swift, NuSTAR, and Fermi missions.

  * •

Guaranteed Time: from 2002 to 2008 the observing programme was divided into
Open Time for the general observer ( General Programme (GP)) and the
Guaranteed Time (GT) for the INTEGRAL Science Working Team (ISWT) ( Core
Programme (CP)). The GT during the CP was the return to the ISWT for their
contributions to the development and execution of the INTEGRAL project. The CP
decreased from 35% in the first year to 20% in the sixth year after launch;
from 2009 there was no further CP (see Table 2). From AO-1 to AO-6 27% of the
Open Time was reserved for scientists from the Russian Federation in return
for the provision of the launcher. From AO-7 this reduced to 25%. The Time
Allocation Committee (TAC) awards this guaranteed return on the same basis of
scientific merit as all other proposals.

  * •

Discretionary Time: The INTEGRAL Project Scientist can grant "Discretionary
Time" which is $\sim$5% of the available observing time. This can be used for
unanticipated ToOs – observations where a timely observation outside the
normal AO cycle is likely to result in a significant scientific impact.
Discretionary Time observations may have a proprietary period of six months,
or may be made publicly available as soon as the relevant data files have been
created.

  * •

Around 5% of the available observing time is used to maintain the calibration
and monitor instrument health.

The detailed contents of the AOs varied. In AO-4 there was a pilot Key
Programme for proposals asking for $>$1 Msec of observing time. In AO-5 and
AO-6 there were dedicated calls for Key Programmes which could continue for
more than one AO. In AO-7 to AO-19 Key Programme proposals which lasted for up
to 2 years could be submitted. If accepted, these had to be resubmitted the
following year. In AO-5 Key Programme proposals accepted for more than one
year were automatically taken over to the next year. From AO-4 to AO-11 there
were separate calls for data rights proposals. These were for targets in the
large IBIS and SPI FOVs that were observed "serendipitously" in other
proposals. Accepted proposals were awarded zero observing time but had the
right to analyse data from the approved target, or targets. A few of the
proposers in AO-1 also asked for serendipitous time in other programmes, and
these were also treated as data rights proposals. From AO-12 to AO-19 the two
types of proposals were merged, and data right proposers had to request time,
but once accepted, they were awarded zero time. Data from fast transient
events (such as GRBs) may be contained in the science data of the operating
instruments. Data on such an event itself can be asked for, and are handled,
in principle, as a ToO event: if there are TAC-approved proposals for such
events in the FOV and it is detected during an INTEGRAL observation, then the
successful proposer will be granted the data rights on the event. Again, these
are serendipitous events, and thus awarded zero time. If the positions of
accepted targets are close-by, proposals could be merged into one observation
with an optimised exposure time (called "amalgamation"), in order to save
slewing time and to maximise the observation efficiency. Those proposals which
were amalgamated to the main proposal were treated as obtaining zero time. In
this chapter we only take into account Open Time proposals.

Figure 1: The over-subscription of requested INTEGRAL observing time compared to that available for AO-1 to AO-19, covering an interval of 21 years from 2000. Table 1: INTEGRAL AO summary. The numbers of TAC members include the panel chairs. AO | Year | Observation | Total No. of | Over-subs. | TAC
---|---|---|---|---|---
| Issued | Interval | Proposals | (Time) | Members | Panel Chairs
|  |  |  |  | Male | Female | Male | Female
1 | 2000 | 30 Dec 2002 – 16 Dec 2003 | 277 | 19.7 | 22 | 5 | 4 | 0
2 | 2003 | 17 Dec 2003 – 17 Feb 2005 | 151 | 7.9 | 21 | 6 | 4 | 0
3 | 2004 | 18 Feb 2005 – 15 Aug 2006 | 112 | 5.7 | 20 | 6 | 3 | 0
4 | 2006 | 16 Aug 2006 – 16 Aug 2007 | 145 | 7.7 | 15 | 4 | 3 | 0
5 | 2007 | 17 Aug 2007 – 16 Aug 2008 | 183 | 6.7 | 17 | 2 | 3 | 0
6 | 2008 | 17 Aug 2008 – 15 Oct 2009 | 184 | 5.3 | 19 | 0 | 3 | 0
7 | 2009 | 16 Oct 2009 – 31 Dec 2010 | 157 | 3.8 | 13 | 3 | 3 | 0
8 | 2010 | 01 Jan 2011 – 31 Dec 2011 | 132 | 4.1 | 12 | 4 | 3 | 0
9 | 2011 | 01 Jan 2012 – 31 Dec 2012 | 101 | 2.6 | 13 | 3 | 2 | 1
10 | 2012 | 01 Jan 2013 – 31 Dec 2013 | 97 | 2.9 | 13 | 3 | 2 | 1
11 | 2013 | 01 Jan 2014 – 31 Dec 2014 | 87 | 2.1 | 14 | 2 | 1 | 2
12 | 2014 | 01 Jan 2015 – 31 Dec 2015 | 77 | 4.5 | 13 | 3 | 1 | 2
13 | 2015 | 01 Jan 2016 – 31 Dec 2016 | 63 | 4.4 | 12 | 4 | 3 | 0
14 | 2016 | 01 Jan 2017 – 31 Dec 2017 | 58 | 5.2 | 12 | 4 | 3 | 0
15 | 2017 | 01 Jan 2018 – 31 Dec 2018 | 65 | 3.9 | 11 | 5 | 2 | 1
16 | 2018 | 01 Jan 2019 – 31 Dec 2019 | 62 | 5.2 | 11 | 5 | 2 | 1
17 | 2019 | 01 Jan 2020 – 31 Dec 2020 | 63 | 3.8 | 12 | 4 | 1 | 2
18 | 2020 | 01 Jan 2021 – 31 Dec 2021 | 52 | 3.5 | 11 | 5 | 1 | 2
19 | 2021 | 01 Jan 2022 – 31 Dec 2022 | 49 | 3.1 | 11 | 5 | 2 | 1
Total |  |  | 2115 |  | 272 | 73 | 46 | 13
Table 2: INTEGRAL Guaranteed Time (GT) compared to the available observing time. Period | GT (%) | Mission Phase
---|---|---
17 Dec 2002 – 16 Dec 2003 | 35 | Nominal mission
17 Dec 2003 – 16 Dec 2004 | 30 | Nominal mission
17 Dec 2004 – 16 Dec 2005 | 25 | Extended mission
17 Dec 2005 – 16 Dec 2006 | 25 | Extended mission
17 Dec 2006 – 16 Dec 2007 | 25 | Extended mission
17 Dec 2007 – 16 Dec 2008 | 20 | Extended mission

##  5 Time Allocation Committee (TAC)

The TAC reviews all submitted proposals and makes recommendations to the ESA
Director of Science (D-SCI) on the observing programme to be performed by
INTEGRAL. It takes into account the scientific case, justification, merit and
relevance of the proposed observation(s), and the potential contribution of
the overall scientific return of the mission. The TAC recommendations on the
observing programme are given priority assignments of A, B or C, with A being
the highest priority (in AO-1 and AO-2 accepted ToO proposals did not receive
a grade and we treat them as being grade A). Priority A and B targets are of
major scientific importance and are scheduled with the highest priority.
Priority C targets are used as "fillers" and have a significant lower
likelihood to be finally scheduled. Priority A and B targets are automatically
transferred to the next observation period, if their successful observation
commenced (i.e., at least 25% executed of the approved time), but is
uncompleted during the current one. The total recommended observing time is
about quarter more than the total available Open Time, for schedule planning
efficiency. Approximately 45% of the recommended observing time is designated
as Priority A, 30% as Priority B and 25% as Priority C. For ToO proposals, the
actual requested observing time (other than the total time requested based on
the number of targets) has been taken into account.

An average of 110 valid proposals were received in response to each AO (see
Table 1). With the over-subscription111The over-subscription for INTEGRAL is
defined as the ratio of the requested time over the available time. The
available time is the total time expected to be available for Guest Observer
observations in an upcoming AO observing period; for the later AOs this was
about 21 Msec (note that this number does not include perigee passages,
engineering observations, calibration observations, etc.). The requested time
is the total time requested in non-ToO proposals plus 10% of the total time
asked in ToO proposals. The 10% is a kind of rough average probability that a
ToO proposal is triggered. factors (see Fig. 1) scientific assessment of the
proposals is a major undertaking of the astronomical community with over 340
scientists having participated so far. For the first two AOs, the TAC was
divided into four panels to reflect the range of topics proposed. This was
reduced to three panels for subsequent AOs. Much of the assessment is
performed by panels, consisting typically of five scientists selected from the
worldwide community. Each panel is led by a panel chair and the overall TAC is
led by a chairperson. The names of the TAC members (with exception of the
chairperson who sits on the INTEGRAL Users Group (IUG), see Sect. 6) are not
made public.

The genders of the TAC members and panel chairs were assigned through the
personal knowledge of the authors and SOC staff. We appreciate that gender
identity is more complex than a binary issue. However, no attempt was made to
assign genders other than male or female in this study. We then examined the
gender composition of the TAC members and panel chairs (Table 1) from AO-1 to
AO-19 covering 2001 to 2021. TAC Panel members are chosen for their high-level
of relevant scientific knowledge, while selection of the TAC chairs was for
scientists considered to have leadership roles in high-energy astronomy. Fig.
2 shows the fraction of female INTEGRAL TAC members and Fig. 3 the same for
the panel chairs. In total there were 226 male and 60 female panel members of
which 46 males and 13 females were panel chairs. Since members and chairs may
serve for multiple AOs, the numbers of individuals involved is smaller. Most
of the TAC members (and chairs) are based at institutes located in the
European Space Agency (ESA) Member States, with a significant number from
institutes in the Russian Federation222By agreement, 25% of total TAC members
(per AO) must be from Russian Federation scientific community. and the US. An
increasing trend in the fraction of female TAC members from $\sim$0.2 in AO-1
to $\sim$0.4 by AO-19 is evident. Given the small number of panel chairs (59
in a total of 19 AOs), it is more difficult to quantify trends. However, it is
worth noting that there were no female TAC panel chairs until AO-8 in 2010.

Figure 2: The fraction of female INTEGRAL TAC members compared to the total
for AO-1 to AO-19 covering an interval of around 21 years. An increasing trend
in the fraction of females TAC members from $\sim$0.2 in AO-1 to $\sim$0.3 by
AO-19 is evident. Figure 3: The fraction of female INTEGRAL TAC chairs
compared to the total for AO-1 to AO-19 covering an interval of around 21
years. We note that there were no female panel chairs until AO-9 in 2011.

##  6 INTEGRAL Users Group

As well as the TAC, there is another body involved in optimising the
scientific output of the mission. This is the INTEGRAL Users Group (IUG) which
advises ESA, through the Project Scientist, on all matters relating to the
optimisation of the scientific output of the mission. It also acts as a forum
to discuss input from the community of users, and when appropriate advise or
recommend action to ESA regarding INTEGRAL operations. The IUG is smaller than
the XMM-Newton Users’ Group (XUG) with only 6 members (including the chair).
The names of the members are made public. The IUG was established in 2005, in
parallel to the already existing ISWT. As a consequence of the termination of
the CP beyond 2008, the IUG and ISWT teams were merged in November 2007 into
one IUG. The chair of the INTEGRAL TAC is also a member of the IUG to ensure
good coordination between the two bodies. In addition, the INTEGRAL Project
Scientist, Mission Manager and instrument principal investigators attend the
meetings. Note that there was a change in Project Scientist in 2013. Fig. 4
shows the number of female IUG members divided by the total between 2005 and
2021. On average 28% of the total number of members were female with no
obvious trend with time. We note that 53% of the IUG chairs were female.

Figure 4: The fraction of female INTEGRAL IUG members compared to the total
membership between 2005 and 2021.

##  7 Proposers

Between AO-1 and AO-19 there were 2115 proposals submitted in response to
INTEGRAL calls for observing opportunities. This provides a rich data set to
quantify many aspects of the proposal process. When submitting a proposal,
INTEGRAL proposers are required to indicate the country where their institute
or university is located. These are selected from a drop-down menu. This
information allows an examination of the nationality distribution of the
proposers’ institutes. Proposers are not required to submit information on
gender, age or type of position held and these have to be derived by other
means if required.

###  7.1 Proposers’ Institute Countries

Table 3 shows the number of proposals submitted and accepted for a range of
countries including all the ESA Member States that submitted at least one
proposal and other countries from which significant numbers of proposals
originated. Only eight proposals were received from countries that are not
listed in Table 3.

Table 3: The number of proposals submitted and accepted from scientists located in different countries. Country | No. Proposals | Percentage | No. | Acceptance
---|---|---|---|---
| Submitted | of Total | Accepted | Percentage
BELGIUM | 8 | 0.38 | 2 | 25.0
CZECH REPUBLIC | 3 | 0.14 | 0 | 0.0
DENMARK | 35 | 1.65 | 4 | 11.4
FINLAND | 5 | 0.24 | 4 | 80.0
FRANCE | 245 | 11.58 | 94 | 38.4
GERMANY | 328 | 15.51 | 155 | 47.3
GREECE | 1 | 0.05 | 0 | 0.0
IRELAND | 28 | 1.32 | 1 | 3.6
ITALY | 472 | 22.32 | 169 | 35.8
NETHERLANDS | 70 | 3.31 | 33 | 47.1
POLAND | 18 | 0.85 | 6 | 33.3
SPAIN | 142 | 6.71 | 88 | 62.0
SWEDEN | 4 | 0.19 | 0 | 0.0
SWITZERLAND | 149 | 7.04 | 45 | 30.2
UNITED KINGDOM | 74 | 3.50 | 33 | 44.6
AUSTRALIA | 9 | 0.43 | 6 | 66.7
CHINA | 21 | 0.99 | 10 | 47.6
INDIA | 2 | 0.09 | 0 | 0.0
ISRAEL | 1 | 0.05 | 0 | 0.0
JAPAN | 18 | 0.85 | 4 | 22.2
MEXICO | 1 | 0.05 | 0 | 0.0
RUSSIAN FEDERATION | 189 | 8.94 | 101 | 53.4
TURKEY | 14 | 0.66 | 9 | 64.3
UNITED STATES | 270 | 12.77 | 107 | 39.6

Scientists located at institutes and universities within Italy submitted the
most proposals – nearly a quarter of the total (22.3%). This is followed by
Germany (15.5%), the USA (12.8%), France (11.6%), the Russian Federation
(8.9%), and Switzerland (7.0%). Figure 5 shows the percentages of proposal
numbers versus AO for the six countries whose scientists submitted the most
proposals. These show relatively stable proposal submission fractions with AO
number. There is evidence for a decline in the percentage of proposals from
Principal Investigator (PI)s located in the USA as the mission progressed
(grey line in Fig. 5). This may indicate a lessening of interest in the
mission from US based PIs as the mission progressed. We note that NASA funding
for US INTEGRAL proposals was only available from AO-1 to AO-7.

In order to be able to draw reliable conclusions, we examined the number of
accepted proposals for the seven countries whose scientists have submitted at
least 100 proposals (Table 3). the countries with the highest accepted
fractions are Spain where 62.0% of 142 proposals were accepted, followed by
the Russian Federation with 53.4% accepted of 189 proposals, and Germany with
47.3% accepted of 328 proposals.

###  7.2 Proposers’ Gender

Figure 7 shows that the fraction of proposals with female PIs compared to the
total increased from $\sim$10% to $\sim$30% between AO-1 and AO-19.

We next examined the number of proposals with male and female PIs from the
same seven countries (Table 4). Since proposers are not asked to specify their
gender (nor age or type of position held etc.) gender information for each
proposer was obtained through examining publicly-accessible web-based data in
a similar way as for the HST study by I.N. Reid 2014Reid (59) and from
knowledge of the community they serve by the Project Scientist and SOC staff.
We appreciate that gender identity is more complex than a binary issue,
however, no attempt can be made to assign genders other than male or female as
this information is not readily available.

The country with the highest fraction of female PIs is Italy (54.4%) followed
by Spain (45.1%). The countries with the lowest fraction of female PIs are the
Russian Federation (0.0%) and the USA (9.3%). Figure 6 shows the acceptance
fraction differences for male and female PIs compared to the average for that
country for each of the seven countries with $>$100 proposals. It can be seen
that Switzerland is the only country to have a better female PI acceptance
rates of +5.1% compared to the average for all Swiss proposals. Female PIs
located in Germany and Spain have acceptance rates of $-$13.2% and $-$10.4%,
respectively compared to the averages for these countries. The accepted
fraction difference for the Russian Federation comes out to 0, because the
number of female PIs has been 0; see Table 4.

Table 4: Proposal statistics for PIs located in countries with $>$100 proposals. The % differences are with respect to the acceptance percentages given in Table 3 which are for all proposals from each country Country | Submitted | Female | Accepted | % Difference
---|---|---|---|---
| Male PI | Fem. PI | /Total (%) | Male PI | Fem. PI | Male PI | Fem. PI
FRANCE | 209 | 36 | 14.7 | 80 | 13 | 0.3 |  $-$1.8
GERMANY | 281 | 47 | 14.3 | 139 | 16 | 2.2 |  $-$13.2
ITALY | 215 | 257 | 54.4 | 92 | 77 | 7.4 |  $-$5.8
SPAIN | 78 | 64 | 45.1 | 55 | 33 | 8.5 |  $-$10.4
SWITZERLAND | 132 | 17 | 11.4 | 39 | 6 |  $-$0.7 | 5.1
RUSSIAN FEDERATION | 189 | 0 | 0.0 | 101 | 0 | 0.0 | …
USA | 245 | 25 | 9.3 | 98 | 8 | 0.4 |  $-$7.6
Figure 5: Proposals submitted from the six countries with the largest numbers
of proposals against AO number. There are no large changes in the percentages
during the mission, suggesting that the level of interest from the six
countries was unchanged. The only exception is a decline in the percentage of
proposals from PIs located in the USA (grey line) as the mission progressed.
Note that the Guaranteed Time ended in 2008 around the time of AO-6. Figure 6:
The acceptance fraction differences of INTEGRAL proposals with male and female
PIs compared to the average for all proposers from that country. The countries
are those with at least 100 submitted INTEGRAL proposals.

##  8 Proposal Selection

We decided to only investigate the outcomes of proposals that requested
observing time – so data rights or similar proposals were not included in the
analysis of success rates. Data rights proposals have different factors
affecting their selection and so their success rates cannot be directly
compared with other proposals. In addition, data right proposals are a
peculiarity of the INTEGRAL mission and cannot easily be compared with other
missions. We note that, examination of the 607 data rights proposals shows
that 511 were awarded data rights. This is 84% of the proposals, a much higher
percentage than for proposals requesting dedicated observing time. For the
proposals requesting observing time, we investigated the number of accepted
proposals and the amount of observing time allocated. We examined the
statistics for all proposals – Priority A, B and C and only for those ranked
in Priority A and B by the TAC since these are of major scientific importance
and had a much higher chance of being scheduled than the "filler" Priority C
targets.

###  8.1 Proposers’ Age

In order to determine the "academic age" of proposers we used the difference
between the year that their Doctor of Philosophy Degree (PhD) was awarded and
the year that an AO to which they submitted a proposal was issued. Thus a
proposer who submits multiple proposals to the same AO will be counted
multiple times in the same "academic year", whilst one who submits multiple
proposals to different AOs will be counted in different "academic years".

The year the proposer obtained their PhD (or equivalent) were determined for
99.7% of the proposals by searching the internet, particularly sites such as
the Astrophysics Data Service (ADS), LinkedIn, the Astronomy Genealogy Project
(astrogen.aas.org), ORCID.org, IEEE Xplore
(https://ieeexplore.ieee.org/Xplore/home.jsp), and, for French theses
https://www.theses.fr. Some proposers were contacted and provided their PhD
dates by email. The proposers for which the PhD could not be found are often
retired, deceased or have left astronomy for some other reason. For PhD
students who had not yet completed their degrees, the expected year of
submission was used. For the small number of proposers who did not have a PhD
and were not enrolled in a PhD programme, their dates were assumed to be
arbitrarily far in the future. For some senior scientists in Italian
institutes who do not have a PhD, three years after they obtained their Laurea
was used. It should be noted that using the year of PhD to indicate the number
of years experience neglects time spent outside of astronomy such as as a
carer. We note that the "youngest" proposer was 13 years before obtaining a
PhD and the oldest 53 years after.

Figure 7: The fraction of submitted INTEGRAL proposals with female PIs compared to the total. This increased from $\sim$10% to $\sim$30% between AO-1 and AO-19. Table 5: INTEGRAL AO proposal numbers showing for proposals with male and female PIs and the number of proposals accepted. Only proposals that requested observing time are included. AO | Submitted | Accepted
---|---|---
| No. of | Male PI | Female PI | Male PI | Female PI
| Proposals | Proposals | Proposals | Proposals | Proposals
1 | 265 | 236 | 29 | 75 | 15
2 | 143 | 124 | 19 | 71 | 12
3 | 103 | 84 | 19 | 48 | 11
4 | 93 | 82 | 11 | 40 | 5
5 | 93 | 79 | 14 | 49 | 5
6 | 70 | 59 | 11 | 35 | 8
7 | 70 | 59 | 11 | 38 | 5
8 | 61 | 51 | 10 | 32 | 6
9 | 46 | 39 | 7 | 33 | 5
10 | 57 | 46 | 11 | 31 | 7
11 | 53 | 40 | 13 | 32 | 10
12 | 72 | 54 | 18 | 29 | 8
13 | 57 | 39 | 18 | 23 | 9
14 | 54 | 34 | 20 | 27 | 11
15 | 61 | 43 | 18 | 31 | 9
16 | 58 | 39 | 19 | 27 | 7
17 | 59 | 37 | 22 | 29 | 14
18 | 48 | 32 | 16 | 27 | 10
19 | 45 | 31 | 14 | 27 | 9
Total | 1508 | 1208 | 300 | 704 | 166

###  8.2 Priority A, B and C Proposal Acceptance Rates

Of the 1508 proposals which requested observing time that were submitted to
AO-1 to AO-19, 870 were awarded an amount of observing time. This corresponds
to an overall success rate of 57.7%. This is higher than for XMM-Newton (see
2024XMM (60)), consistent with the more moderate over-subscription factors
given in Table 1 compared to those in 2024XMM (60). We next examined the
number of proposals for each gender that were awarded any observing time. A
total of 1210 such proposals with male PIs and 298 proposals with female PIs
were submitted, of which 704 and 166 were awarded observing time. This gives
success rates of 58.2% and 55.7% for male and female PIs, respectively. This
is a difference in favour of males of 4.4%.

Proposal submissions and proposal acceptance are unlikely to be independent or
random processes and for Poisson statistics to apply events need to be
independent of each other. If this is assumed to be the case then the success
rates are $58.2\pm 2.8$% and $55.7\pm 5.4$% for male and female PI proposals,
respectively and a difference in favour of males of $4.4\pm 6.7$%
corresponding to a male to female success ratio of $1.053\pm 0.113$. These
uncertainties are given only to illustrate the outcomes if Poisson statistics
were to apply to the selection process. We note that actual uncertainties on
the proposal numbers for each AO are zero. We further note that there are
systematic uncertainties associated with this process due to misappropriated
genders and incorrect PhD "academic ages", but these are likely to be too
small to significantly affect our calculations.

The success rates are shown for male, female and all proposers for each AO in
Fig. 9. In contrast to the results from HST Cycles 11 to 20 reported in
2014Reid (59) where proposals with male PIs had a consistently higher success
rate than those with female PIs, no such trend is visible in INTEGRAL
proposals. There is no obvious evolution in the male and female PI proposal
acceptance rates with AO number, which cover an interval of more than 20
years.

Figure 10 shows the variation in acceptance rates more clearly. For each AO,
it shows the difference between the expected number of accepted proposals,
calculated using the overall acceptance rate, and the number actually
accepted. It shows that the largest discrepancies occurred during AO-1 and
AO-2 where proposals with female PIs had higher success rates of 17.9% and
6.6%, respectively, compared to proposals with a male PI. The most successful
AO for male PI proposals was AO-19 where proposals led by male PIs were 4.9%
more likely to be accepted than those with a female PI.

Figure 8: The acceptance fraction for female PIs for each AO (labelled)
against the fraction compared to the total of female TAC members. There is no
obvious correlation between the two, or with the number of female panel
chairs.

We examined the TAC membership genders to see if these are correlated with
proposal acceptance rates. Figure 8 shows the acceptance fraction of proposals
with female PIs against the fraction of female compared to the total number of
TAC members. It can be seen that there is no obvious correlation between the
success rates of female-led proposals and the fraction of female TAC members.
We note that there were no female chairs for AO-1 to AO-8, or AO-13 and AO-14
(Fig. 3) and the results from these AOs are indistinguishable from the other
AOs suggesting that having females in leadership positions in the TAC does not
make a measurable difference to female PI proposal success rates.

Figure 11 shows the same excesses depicted as a quasi-Poisson quantity,
normalised by dividing by the square root of the expected number of accepted
proposals. While Poisson statistics do not necessarily apply to the peer-
review process, scientists often adopt such a statistic in assessing the
difference between observation and prediction. It is interesting to note that
from the 19 AOs examined, nine had higher female PI success rates than for
male PIs, while the opposite occurred during 10 AOs.

Figure 9: The fraction of accepted proposals compared to the number submitted. The histograms show the acceptance fractions for male PIs, female PIs and all PIs. Figure 10: The relative success rates of INTEGRAL proposals.The histograms show the difference between the actual number of successful proposals and the expected number based on the overall acceptance rate. Figure 11: The relative success rates of INTEGRAL proposals. The histograms show the difference between the actual number of successful proposals and the expected number based on the overall acceptance rate normalised by dividing by the square root of the number of expected proposals. Table 6: INTEGRAL requested and accepted observing time for male and female proposers between AO-1 and AO-19. The last two columns give the ratio of accepted to proposed observing time for each gender. A proposal with a male PI which requested 1000 Msec of observing time in AO-1 has been excluded. AO | Time (Msec) | Success Rate
---|---|---
| Requested | Accepted | Time
| Male | Female | Male | Female | Male | Female
1 | 283.0 | 39.8 | 32.7 | 5.4 | 0.116 | 0.135
2 | 132.4 | 14.1 | 49.4 | 5.7 | 0.373 | 0.402
3 | 102.3 | 12.7 | 48.3 | 6.7 | 0.472 | 0.526
4 | 121.0 | 15.5 | 27.6 | 6.3 | 0.228 | 0.406
5 | 132.9 | 23.6 | 46.0 | 6.4 | 0.346 | 0.272
6 | 75.0 | 11.6 | 26.8 | 5.1 | 0.357 | 0.439
7 | 92.5 | 11.3 | 41.0 | 5.7 | 0.443 | 0.499
8 | 71.9 | 14.1 | 33.1 | 7.4 | 0.461 | 0.527
9 | 52.3 | 10.1 | 32.9 | 7.1 | 0.628 | 0.701
10 | 48.7 | 9.8 | 30.2 | 6.8 | 0.619 | 0.698
11 | 36.1 | 14.5 | 29.0 | 12.2 | 0.805 | 0.838
12 | 77.6 | 21.6 | 33.3 | 7.4 | 0.430 | 0.342
13 | 81.0 | 21.7 | 27.7 | 9.0 | 0.342 | 0.415
14 | 74.3 | 37.2 | 25.6 | 9.6 | 0.345 | 0.258
15 | 47.9 | 33.5 | 34.8 | 8.6 | 0.726 | 0.258
16 | 76.3 | 33.2 | 32.1 | 7.9 | 0.421 | 0.237
17 | 44.5 | 36.0 | 32.7 | 12.0 | 0.735 | 0.332
18 | 38.2 | 36.2 | 35.3 | 10.5 | 0.923 | 0.290
19 | 38.3 | 26.8 | 37.2 | 13.2 | 0.971 | 0.492
Total | 1626.2 | 423.2 | 655.6 | 152.7 |  |

###  8.3 Priority A and B Proposal Acceptance Rates

Of the 1508 proposals which requested observing time that were submitted to
AO-1 to AO-19, 690 were awarded Priority A or B observing time. This
corresponds to an overall success rate of 45.8%. We next examined the number
of proposals for each gender that were awarded high-priority observing time. A
total of 1210 such proposals with male PIs and 298 proposals with female PIs
were submitted, of which 556 and 134 proposals were awarded observing time.
This gives success rates of 46.0% and 45.0% for male and female PIs,
respectively. This is a difference in favour of males of 2.2%. Again, assuming
square root errors on the numbers of submitted and accepted proposals gives
success rates of $46.0\pm 2.4$% and $45.0\pm 4.7$% for male and female PI
proposals, respectively. This corresponds to a male to female success ratio of
$1.022\pm 0.118$.

###  8.4 Priority A, B and C Observing Time Acceptance Rates

We examined the acceptance rates for the genders using awarded times. Many
INTEGRAL proposals are allocated less observing time than requested, often
because not all the requested targets are approved. If the allocation of
observing time is handled differently for males and female PIs, then this will
show as a difference in the relative amounts of time approved, compared to the
number of proposals accepted. The results for each AO are summarised in Table
6. This shows the total time requested by proposals with male and female PIs
to be 2626.2 and 423.2 Msec, respectively. 655.6 Msec and 152.7 Msec of
observing time was awarded to proposals with male and female PIs,
respectively. This gives average success rate of 40.3% for male PIs and 36.1%
for female PIs. This is a difference of 11.5% in favour of male PIs. We note
that a single proposal with a male PI which requested 1000 Msec of observing
time (or about 32 years!) and was awarded 1 Msec (in Priority C in AO-1) has
been excluded from this analysis as the request was deemed unrealistic.

Figure 12 shows the distributions of male and female INTEGRAL PIs who obtained
their PhDs or equivalent in one year bins between 10 years before the PhD date
to 50 years after. The male PIs who requested observing time (ie. data time
proposals are excluded) have an mean "academic age" of 14.7 years post-PhD
compared to that of the female PIs of 10.8 years. The mean for all PIs is 13.7
years. A two-sample Anderson-Darling Test (e.g., 2006Babu (61)) shows that the
hypothesis that both samples come from the same underlying population can be
rejected at $>$99% confidence. This difference can be more clearly seen in
Fig. 13 which shows the ratio of the two curves. There appears to be an
increase in the number of early career (less than around 5 years after
obtaining a PhD) female PIs compared to males. Beyond this the ratio of
females to the total is $\sim$20%.

We next investigated the acceptance rates for the proposals against PhD year.
This is shown for years $-$4 to 35 years after PhD in Fig. 14. A least squares
fit to the data gives an intercept of ($45.7\pm 0.4$)% and a gradient of
($0.73\pm 0.26$)% year-1 assuming that the uncertainties are the square root
of the number of people in each bin. The value of R2 is 0.45. Indicative error
bars show 1$\sigma$ standard deviations assuming that the number of proposers
in each age bin follows a Poisson distribution. There as an increase in the
acceptance rate from $\sim$45% for PhD students to $\sim$70% for late career
researchers. We note that this gradient is almost a factor two stronger than
measured with XMM-Newton. This increase could result from a combination of
factors including:

  1. 1.

An increase in the success rate of researchers as they gain experience and
have expanded networks of collaborators.

  2. 2.

Less successful proposers who leave astronomy.

  3. 3.

Less successful proposers who remain in astronomy, but do not propose in
subsequent INTEGRAL AOs.

  4. 4.

A bias in the selection process towards late career scientists.

The positive gradient of this relation indicates that the female early career
proposer population is less likely to have proposals accepted simply due to
having less experience. To calculate the size of this effect, we took the male
and female PI distributions shown in Fig. 12 and multiplied the number of PIs
each year by the linear value derived from the fit to the overall success
rates. This gave a predicted difference in the acceptance ratio of 2.4% for
male and female PIs. This indicates that the underlying "academic age"
differences between the two populations may be responsible for about half of
the difference in gender acceptance rates of about 4.4% in proposal numbers or
a quarter of the difference in accepted time (11.5%).

The acceptance fractions of female and male INTEGRAL PIs with the binned year
of their PhD are shown in Fig. 15. The proposal success rate of females PIs
tends to increase with "academic age" whilst that of male PIs shows a smaller
increase, or may be constant. This is in contrast to results reported from HST
2014Reid (59) and XMM-Newton 2024XMM (60) where the success rates for late
career ($\sim$20 years post PhD) female PIs appear to be lower than their
early career counterparts. Indicative error bars show 1$\sigma$ standard
deviations assuming that the number of proposers in each age bin follows a
Poisson distribution. Error bars are only shown for female PIs as these are
much larger than for male PIs due to the smaller number of female PIs.

Figure 12: The "academic age" distribution (Years since PhD) for male and
female INTEGRAL PIs. PIs have a mean "academic age" of 13.7 years. Male PIs
have a mean "academic age" of 14.7 years compared to 10.8 years for female
PIs. Figure 13: The ratio of the "academic age" (years from PhD) of female to
all INTEGRAL PIs in two year bins showing an increase in early career (less
than around 5 years after obtaining a PhD) female PIs compared to the total.
The indicative error bars show 1$\sigma$ standard deviations assuming that the
number of proposers in each age bin follows a Poisson distribution.  Figure
14: The acceptance fraction of all INTEGRAL PIs with year of their PhD. This
shows an increase in the acceptance rate from $\sim$45% for PhD students to
$\sim$70% for "senior" researchers. The indicative error bars show 1$\sigma$
standard deviations assuming that the number of proposers in each age bin
follows a Poisson distribution.  Figure 15: The acceptance fractions of female
and male INTEGRAL PIs with year of their PhD. The proposal success rate of
females PIs tends to increase with "academic age" whilst that of male PIs
shows a smaller increase, or may be constant. The indicative error bars show
1$\sigma$ standard deviations assuming that the number of proposers in each
age bin follows a Poisson distribution. Error bars are only shown for female
PIs as these are much larger than for male PIs due to the smaller number of
female PIs.

###  8.5 Priority A and B Observing Time Acceptance Rates

We next examined the acceptance rates for the genders awarded high-priority (A
or B) observing time. The total time requested by proposals with male and
female PIs is 2626.2 and 423.2 Msec, respectively. 551.1 Msec and 139.2 Msec
of Priority A or B observing time was awarded to proposals with male and
female PIs, respectively. This gives average success rate of 33.9% for male
PIs and 32.9% for female PIs. This is a difference of 2.9% in favour of male
PIs. As before, a single proposal with a male PI which requested 1000 Msec of
observing time (or about 32 years!) and was awarded 1 Msec (in Priority C in
AO-1) has been excluded from this analysis.

##  9 Proposal Selection Summary

We have examined the outcomes of 1508 proposals submitted to ESA requesting
observing time in response to the AO-1 to AO-19 calls for INTEGRAL observing
time. The proposal acceptance rates are summarised in Table 7. A total of
58.2% of proposals with male PIs and 55.7% with female PIs were awarded any
observing time on INTEGRAL. This is a difference in favour of males of 4.4%.
When the amount of awarded time is evaluated this changes to being 11.5% in
favour of proposals with male PIs. This difference implies that proposals with
female PIs were awarded a smaller fraction of the requested observing time
than their male counterparts. For the proposals that were awarded Priority A
or B time, the difference in the number of accepted proposals is 2.2% and in
terms of awarded time 2.9% in favour of male PIs. Both of these differences
are smaller than when Priority C time is included. This implies that female
PIs have much lower success rates than their male counterparts for proposals
awarded Priority C time. We interpret these four different numbers to imply an
overall male to female success rate difference of 2–11% for INTEGRAL
proposals.

The acceptance rate for data rights proposals was 84% – much higher than for
proposals requesting observing time. This suggests that submitting data rights
proposals is a good strategy for obtaining INTEGRAL data.

Scientists located at institutes within Italy submitted the most proposals –
almost a quarter of the total followed by those located in institutes in
Germany, the USA and Switzerland. Amongst the seven countries submitting
$>$100 proposals, Spain had 62% of 142 proposals accepted, followed by the
Russian Federation with 53% of 189 proposals, and Germany with 47% of 328
proposals. Italy is the country with the highest fraction of proposals from
female PIs (55%) followed by Spain (45%). The countries with the lowest
fraction of proposals from female PIs are the Russian Federation (0%) and the
USA (9%). Switzerland is the only country to have a better female PI proposal
acceptance rate than for male PIs of +5% compared to the average for all Swiss
proposals. Proposals from female PIs located in Germany and Spain have
acceptance rates of $-$13% and $-$10%, respectively compared to the averages
for their countries.

The fraction of proposals from female PIs increased from $\sim$10% of the
total during the early AOs to $\sim$30% of the total in the later AOs.
However, there is no marked evolution in the gender balance of the proposal
acceptance rates with AO number and hence between 2000 and 2021. There was an
increase in the fraction of female TAC members from $\sim$0.2 of the total for
the first AOs to $\sim$0.3 for the latest AOs and a larger increase in the
number of female TAC panel chairs from none before AO-9 to one or two out of
three chairs in subsequent AOs. These results suggest that increasing
fractions of female TAC members or panel chairs does not make a measurable
difference to the success rate of female PI proposals.

Using the year of obtaining a PhD, or equivalent degree, as a proxy for
"academic age" shows that the mean "academic age" of female PIs is 3.9 years
less than for males. A linear fit to the proposal acceptance rate against PhD
year shows an increase in acceptance rate from $\sim$45% for students to
$\sim$70% for senior astronomers. The proposal success rate of females PIs
tends to increase with "academic age" whilst that of male PIs shows a smaller
increase, or may be constant. This is in contrast to results reported from HST
2014Reid (59) and XMM-Newton 2024XMM (60) where the success rates for late
career ($\sim$20 years post PhD) female PIs appear to be lower than their
early career counterparts.

Table 7: Summary of INTEGRAL Proposal Acceptance Rates for AO-1 to AO-19 for proposals requesting observing time. A proposal with a male PI which requested 1000 Mec of observing time in AO-1 has been excluded from the analysis. The uncertainties given in brackets assume that the number of submitted and accepted proposals have square root uncertainties and for guidance only. Priority | Parameter | Proposal PI
---|---|---
|  | All | Male | Female
| Proposals Submitted | 1508 | 1208 | 300
A, B, C | Number Accepted | 870 | 704 | 166
| Percentage Accepted | 57.7 | 58.2 | 55.7
| Ratio Male/Female |  | 1.044
A, B | Number Accepted | 690 | 556 | 134
| Percentage Accepted | 45.8 | 46.0 | 45.0
| Ratio Male/Female |  | 1.022
| Time Requested (Msec) | 2049.4 | 1626.2 | 422.7
A, B, C | Time Accepted (Msec) | 808.3 | 655.6 | 152.7
| Percentage Time Accepted | 39.4 | 40.3 | 36.1
| Ratio Male/Female |  | 1.115
A, B | Time Accepted (Msec) | 690.2 | 551.1 | 139.2
| Percentage Time Accepted | 33.7 | 33.9 | 32.9
| Ratio Male/Female |  | 1.029
| Overall Male/Female Difference |  | 2–11%

##  10 Discussion

It is interesting to compare our results with those from other missions and
facilities. XMM-Newton and HST are the obvious missions for comparison as they
are similarly long-lived as INTEGRAL and both have much higher average numbers
of proposals in response to each AO. There have also been investigations of
the outcomes of the observing selection processes for European Southern
Observatory (ESO), Canadian facilities and the National Radio Astronomy
Observatory (NRAO) facilities, which are also discussed in Sect.
LABEL:subsec:XMMDiscussion.

For XMM-Newton the difference observed in the proposal selection process of
5–15% in favour of male PIs is comparable to that observed for INTEGRAL of
2–11%. Both of these ranges are small in comparison to those reported for some
ESO, and NRAO facilities 2016Patat (62), 2016Lonsdale (63), 2020Carpenter (64)
and for HST prior to the implementation of dual anonymous reviewing. 2014Reid
(59) reports on a study of gender-based systematic trends in the HST proposal
review process for Cycles 11 through 21 (2001 to 2013). From this study of the
outcomes of 9400 proposals there appears to be a similar trend to that seen
here in that male PIs are more likely to succeed in achieving a successful HST
proposal (23.5% success rate) than female PIs (18.1% success rate). This is a
difference of 30% in favour of proposals led by male PIs. With dual anonymous
reviewing the names of the reviewers and the investigators are made known to
each other only after the review has been completed. Once dual-anonymous
reviewing was implemented for HST the male and female PI proposal acceptance
difference falls to 9.3% 2023Reid (65), comparable to that seen with INTEGRAL
and XMM-Newton.

An investigation of the time allocation process at ESO is reported in
2016Patat (62). This covers an interval of 8 years and involved about 3000
PIs. Female PIs were found to have a significantly lower chance of being
awarded a top rank compared to male PIs with a male/female ratio of 1.39 $\pm$
0.05. The paper suggests that the principal explanation for the difference may
be due to the average higher seniority of the male PIs assuming that more
senior scientists of both genders write better proposals and thus succeed more
often at obtaining ESO telescope time. However, no attempt was made to
quantify if this effect can account for the observed differences.
Nevertheless, 2016Patat (62) concludes that the ESO review process itself
introduces extra gender differences.

An investigation of the gender-related systematics in the proposal review
processes for the four facilities operated by NRAO: the Jansky Very Large
Array (JVLA), the Very Long Baseline Array (VLBA), the Green Bank Telescope
(GBT) and Atacama Large Millimeter/sub-millimeter Array (ALMA) are reported in
2016Lonsdale (63) and 2020Carpenter (64). Similarly to HST, before the
introduction of double-anonymous reviewing, and similarly as well to the ESO
study there are significant gender-related effects in the proposal rankings in
favour of male PIs compared to female PIs for all four facilities with varying
degrees of confidence reflecting the different number of proposals.

In summary, the difference observed in the INTEGRAL proposal selection process
of 2–11% in favour of male PIs is smaller than those reported for some ESO,
and NRAO facilities 2016Patat (62), 2016Lonsdale (63) and for HST prior to the
implementation of dual anonymous reviewing 2014Reid (59). Once dual-anonymous
reviewing was implemented for HST the male and female PI proposal acceptance
difference falls to 9.3% 2023Reid (65), comparable to that seen here with
INTEGRAL of 2–11%. This suggests that the INTEGRAL proposal selection
processes produces outcomes consistent with, or at least close to, the best
available science. It does not however provide gender parity and it would be
useful to investigate reasons for this. The acceptance rates of male and
female proposers for different geographical regions could be examined. This
could reveal that regional differences play a role, perhaps related to
language skills. It would also be useful to investigate the uncertainties in
the proposal selection process. This could be accomplished by comparing the
results of different representative TACs evaluating the same set of proposals.
Finally, it would be interesting to examine the the publication rates of early
and late career scientists who have been awarded INTEGRAL observing time. Are
late career scientists better at exploiting their observations and so produce
relatively more publications and citations than their early career colleagues?
More generally, what is the effect on the science return of a mission if the
observations are dominated by late or early career scientists. This is a
complex issue as e.g., a late career scientist may have made substantial
contributions to a proposal from an early career colleague, so improving its
chances of success.

Acknowledgements. We thank the INTEGRAL SOC and ISDC staff for their support.

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## Acronym List

ADS

    Astrophysics Data Service
AO

    Announcement of Opportunity
ALMA

    Atacama Large Millimeter/sub-millimeter Array
CP

    Core Programme
D-SCI

    ESA Director of Science
ESA

    European Space Agency
ESAC

    European Space Astronomy Centre
ESO

    European Southern Observatory
ESOC

    European Space Operations Centre
ESTEC

    European Science and Technology Centre
FOV

    Field of View
FRB

    Fast Radio Burst
FWHM

    Full-Width at Half Maximum
GBT

    Green Bank Telescope
GP

    General Programme
GRB

    Gamma-ray Burst
GT

    Guaranteed Time
GW

    Gravitional Wave
HST

    Hubble Space Telescope
IREM

    INTEGRAL Radiation Environment Monitor
ISDC

    INTEGRAL Science Data Centre
ISWT

    INTEGRAL Science Working Team
IBIS

    Imager On-Board the INTEGRAL Spacecraft
INTEGRAL

    INTErnational Gamma-Ray Astrophysics Laboratory
ISGRI

    INTEGRAL Soft Gamma-Ray Imager
ISOC

    INTEGRAL Science Operations Centre
IUG

    INTEGRAL Users Group
JEM-X

    Joint European X-ray Monitor
JVLA

    Jansky Very Large Array
keV

    Kilo Electron Volt
kpc

    Kilo parsec
MeV

    Million Electron Volt
MOC

    Mission Operations Centre
Msec

    Millions of Seconds
NASA

    National Aeronautics and Space Administration
NRAO

    National Radio Astronomy Observatory
ObsID

    Observation Identifier
OMC

    Optical Monitor Camera
OSA

    Off-line Scientific Analysis
PhD

    Doctor of Philosophy Degree
PI

    Principal Investigator
PICsIT

    Pixellated Imaging Caesium iodide Telescope
SOC

    Science Operations Centre
SPI

    Spectrometer on INTEGRAL
TAC

    Time Allocation Committee
ToO

    Target of Opportunity
VLBA

    Very Long Baseline Array
XUG

    XMM-Newton Users’ Group
  *[INTEGRAL]: INTErnational Gamma-Ray Astrophysics Laboratory
  *[NASA]: National Aeronautics and Space Administration
  *[keV]: Kilo Electron Volt
  *[MeV]: Million Electron Volt
  *[IBIS]: Imager On-Board the INTEGRAL Spacecraft
  *[SPI]: Spectrometer on INTEGRAL
  *[FWHM]: Full-Width at Half Maximum
  *[FOV]: Field of View
  *[ISGRI]: INTEGRAL Soft Gamma-Ray Imager
  *[PICsIT]: Pixellated Imaging Caesium iodide Telescope
  *[JEM-X]: Joint European X-ray Monitor
  *[OMC]: Optical Monitor Camera
  *[MOC]: Mission Operations Centre
  *[ESOC]: European Space Operations Centre
  *[SOC]: Science Operations Centre
  *[ESTEC]: European Science and Technology Centre
  *[ESAC]: European Space Astronomy Centre
  *[ISDC]: INTEGRAL Science Data Centre
  *[OSA]: Off-line Scientific Analysis
  *[GW]: Gravitional Wave
  *[FRB]: Fast Radio Burst
  *[ToO]: Target of Opportunity
  *[GRB]: Gamma-ray Burst
  *[kpc]: Kilo parsec
  *[IREM]: INTEGRAL Radiation Environment Monitor
  *[HST]: Hubble Space Telescope
  *[ObsID]: Observation Identifier
  *[AO]: Announcement of Opportunity
  *[ISOC]: INTEGRAL Science Operations Centre
  *[Msec]: Millions of Seconds
  *[GP]: General Programme
  *[GT]: Guaranteed Time
  *[ISWT]: INTEGRAL Science Working Team
  *[CP]: Core Programme
  *[TAC]: Time Allocation Committee
  *[D-SCI]: ESA Director of Science
  *[IUG]: INTEGRAL Users Group
  *[ESA]: European Space Agency
  *[XUG]: XMM-Newton Users’ Group
  *[PI]: Principal Investigator
  *[PhD]: Doctor of Philosophy Degree
  *[ADS]: Astrophysics Data Service
  *[ESO]: European Southern Observatory
  *[NRAO]: National Radio Astronomy Observatory
  *[JVLA]: Jansky Very Large Array
  *[VLBA]: Very Long Baseline Array
  *[GBT]: Green Bank Telescope
  *[ALMA]: Atacama Large Millimeter/sub-millimeter Array