###### Abstract

The development of the latest generation of Imaging Atmospheric Cherenkov
Telescopes (IACTs) over recent decades has led to the discovery of new extreme
astrophysical phenomena in the very-high-energy (VHE, E > 100 GeV) gamma-ray
regime. Time-domain and multi-messenger astronomy are inevitably connected to
the physics of transient VHE emitters, which show unexpected (and mostly
unpredictable) flaring or exploding episodes at different timescales. These
transients often share the physical processes responsible for the production
of the gamma-ray emission, through cosmic-ray acceleration, magnetic
reconnection, jet production and/or outflows, and shocks interactions. In this
review, we present an up-to-date overview of the VHE transients field,
spanning from novae to supernovae, neutrino counterparts or fast radio bursts,
among others, and we outline the expectations for future facilities.

###### keywords:

gamma-ray astrophysics; gamma-ray instrumentation; transients; novae;
supernovae; fast radio bursts; magnetars; neutrinos; tidal disruption events;
gravitational waves

1 1 0 31 January 2024 13 March 2024 15 March 2024
https://doi.org/10.3390/universe10040163 A Very-High-Energy Gamma-Ray View of
the Transient Sky A Very-High-Energy Gamma-Ray View of the Transient Sky
Alessandro Carosi 1,*,† and Alicia López-Oramas 2,*,† Alessandro Carosi and
Alicia López-Oramas Carosi, A.; López-Oramas, A. Correspondence:
<EMAIL_ADDRESS>(A.C<EMAIL_ADDRESS>(A.L.-O.) These authors
contributed equally to this work.

##  1 Introduction

The very-high-energy (VHE, E $>$ 100 GeV) gamma-ray regime is of the utmost
importance in studying extreme astrophysical processes. Transient phenomena,
located at the crossroads of time-domain and multi-messenger astronomy, have
revealed a plethora of new emitters at VHE. During the last twenty years, IACT
experiments have proven to be suitable instruments to perform fast follow-up
of transient events, with 3–4 times improved sensitivities at short time
scales compared to space-based instruments (Fioretti et al., 2019). At the
same time, some IACTs are optimized for a swift reaction and repositioning,
see e.g., (Aleksić et al., 2016), allowing for the study of short-lived
signals during their initial phase. Understanding the recent advancements and
open issues in transient and multi-messenger astrophysics at VHE is the key
for the science to be developed with current IACTs and with future
instrumentation, such as the Cherenkov Telescope Array (CTA) Observatory
(Cherenkov Telescope Array Consortium et al., 2019).

In this contribution, we review the phenomenology of transient events of both
Galactic and extragalactic origin, which are (mostly) related to stellar-size
compact objects and that are shock-powered and/or accretion-powered. Shocks
power several transient phenomena, such as novae, supernovae, binary neutron
star mergers, or tidal disruption events. The shocks and ejecta in (at least
some of) these systems can show morphological resemblances and have similar
characteristics, although at different scales and displaying, e.g., a broad
range of various velocities and densities. Other sources such as magnetars,
even if rotationally powered, can also generate blast waves and shocks.

We review the state of the art in the detection and search for emission of
transient events in the GeV-TeV regime for different types of astrophysical
sources in Sections 2–8, namely novae (Section 2), microquasars, and flaring
gamma-ray binaries (Section 3), supernovae (Section 4), pulsar-wind nebulae
(Section 5), fast radio bursts and magnetars (Section 6), and gravitational
waves (Section 7). We then briefly mention other transient sources in Section
8, such as gamma-ray bursts (GRBs) and tidal disruption events (TDEs), which
are more extensively covered in a dedicated review of this Special Issue
(Vigliano and Longo, 2024). We finally discuss the latest advances and future
perspectives in Section 9.

##  2 Novae

Novae outbursts are thermonuclear explosions that take place on the surface of
a white dwarf (WD) accreting material from a companion star. The transferred
material is mostly hydrogen; however, helium accretion can also occur in some
systems (see (Chomiuk et al., 2021)). The material accumulates on a layer on
the surface of the WD, where hydrogen is burning in degenerate conditions,
increasing the temperature and density. Once a critical mass is reached, the
system undergoes an unstable burning, provoking a thermonuclear runaway. The
ejecta expand at velocities reaching thousands of km s-1 and can interact with
the surrounding material, if any. Depending on the type of companion star,
novae can be classified into classical and symbiotic systems. Classical novae
are cataclysmic variables in which the companion is a main sequence (or
slightly evolved) star. The mass-transfer onto the WD happens via Roche-lobe
overflow. These systems are characterized for having short orbital periods
lasting from hours to days (Hernanz and Tatischeff, 2012). Symbiotic systems
are composed of a red giant (RG) companion and the have longer orbital periods
and show larger component separations (Mikolajewska, 2008). The binary is
embedded in the RG wind and the WD accretes directly from this wind.

Novae explosions do not disrupt the binary system and hence the cycle of
accretion can start again. After enough material is accumulated, another
thermonuclear runaway can happen again. The recurrence timescale of these
outburst is defined as $\tau_{rec}=M_{acc}/\dot{M}$, $M_{acc}$ being the
critical mass to initiate the nuclear burning and $\dot{M}$ the accretion
rate. High recurrence times are then associated to more massive WDs (with mass
close to the Chandrasekhar limit, $M_{Ch}$ $\approx 1.4M_{\odot}$) accreting
at high rates. The typical recurrence times for classical novae are
$10^{4}$–$10^{5}$ years. However, some systems known as recurrent novae have
displayed more than one eruption in a human lifetime ($\tau_{rec}\leq 100$
years). For this to happen, the WD should be close to the Chandrasekhar limit
(with at least $M>1.2M_{\odot}$) and the system shall have high mass-accretion
rates ($\approx$10${}^{-7}M_{\odot}$ y-1) (see (Schaefer, 2010)). There are 10
recurrent systems known in the Galaxy up to date which can be further
classified into two groups (Schaefer, 2010) depending on the mechanism which
leads to the short recurrence: long-period systems (eight in total, period $>$
one-third of a day) hosting a giant companion (also known as symbiotic
recurrent novae) in which the accretion is driven by the RG wind and the
evolution of the companion. These symbiotic recurrent systems can indeed be
the progenitors of type Ia supernovae; and short-period ones (two systems) in
which the accretion is driven by the heating of the WD.

The discovery rate of novae is about 5–15 events per year (Chomiuk et al.,
2021), although the estimated number of eruptions in the Galaxy is much
larger: 20–70 per year. The lower detection rate could be due to dust
obscuration, since many novae happen in the Galactic plane, or simply due to a
scarce monitoring; see, e.g., (Schaefer, 2010), and references therein for a
more detailed discussion on novae rates.

The first evidence of non-thermal emission due to particle acceleration up to
TeV energies in the blast wave of (recurrent) novae was suggested by
Tatischeff and Hernanz (2007). The discovery of symbiotic novae as high energy
(HE; E $>$ 100 MeV) emitters was performed by Fermi-LAT in 2010 (Abdo et al.,
2010) with the detection of V407 Cyg. The HE emission lasted for about two
weeks. Only four years later, Fermi-LAT also established classical novae as HE
sources Ackermann et al. (2014) with the discovery of three systems (V959 Mon,
V1324 Sco, and V339 Del). The spectral energy distribution (SED) of these four
LAT-detected novae is rather soft, mostly described with power laws with
exponential cutoff and with energies up to a maximum of $\sim$10 GeV. Both
hadronic and leptonic scenarios can fit the observed emission and could not be
ruled out.

Since then, the satellite has been detecting an average of $\sim$1 nova per
year.See https://asd.gsfc.nasa.gov/Koji.Mukai/novae/latnovae.html (accessed on
27 March 2024) for the list with LAT-detected novae and sub-significance
hints. Most of the Fermi-LAT novae are located in the Galactic disk, although
some have been discovered in the Galactic bulge, implying detection up to
distances of $\sim$8 kpc. By studying different classical novae, ref.Cheung et
al. (2016) suggests an inverse relationship between the HE emission duration
and the total emitted energies. This could possibly indicate that the presence
of more compact and high-density ejecta produces a higher particle
acceleration, which leads to stronger emissions and shorter duration.

The detection of HE emission from novae clearly demonstrated that non-thermal
mechanisms operate in these cataclysmic binaries. The evident question to pose
was whether novae could accelerate particles to sufficiently high energies to
produce VHE gamma rays. These particles (leptons and protons) are accelerated
at the nova shock and could eventually produce emission at higher energies. In
the case of protons, they could reach high energies and emit TeV gamma rays
(Sitarek and Bednarek, 2012). Since the discovery of HE gamma rays in novae,
searches for a VHE component were performed by IACTs for over a decade,
without achieving any significant detection. VERITAS observed the 2010
outburst of the symbiotic nova V407 Cyg on days 9 to 16 after the eruption,
leading to no signal (Aliu et al., 2012). MAGIC observed the classical nova
V339 Del on the night of the optical peak (although under poor-quality weather
conditions) and a few days after the Fermi-LAT emission, setting upper limits
(ULs) to the VHE emission (Ahnen et al., 2015). In the same work, MAGIC
reported no signal from the symbiotic nova YY Her (taken a week after the
optical maximum) and the dwarf nova ASASSN-13ax, a system in which the
outburst are due to accretion disk instabilities (instead of a thermonuclear
runaway).

The first nova for which VHE gamma-ray emission was discovered is RS Oph, a
recurrent symbiotic system composed of a massive $M_{WD}\approx 1.2\div$1.4
M⊙) carbon–oxygen WD Mikołajewska and Shara (2017) accreting from a M0-2 III
RG star Anupama and Mikołajewska (1999). It shows an orbital period of
$(453.6\pm 0.4)$ days Brandi et al. (2009) and displays major outbursts with a
recurrence time of 14.7 years Schaefer (2010). The fact that the mass of the
WD is so close to the Chandrasekhar limit suggest that RS Oph is a possible
type Ia SN (see Section 4) progenitor candidate (Hachisu and Kato, 2000;
Hernanz and José, 2008; Patat et al., 2011).

The VHE observations were triggered on 9 August, after optical Geary (2021)
and HE Cheung et al. (2021) alerts. A clear gamma-ray signal at VHE was then
detected by H.E.S.S (H. E. S. S. Collaboration et al., 2022), MAGIC (Acciari
et al., 2022), and confirmed by the LST-1 telescope (Abe et al., 2023) during
the 2021 outburst that started on 8 August 2021 (MJD 59435). The multi-
wavelength lightcurve of the RS Oph emission is shown in Figure 1). The VHE
gamma-ray signal is significantly detected up to five days after the nova
eruption. Observations after the full Moon break revealed no significant
signal, with a maximum of 3.3 $\sigma$ hint integrated over 14.6 h between 25
August and 7 September (H. E. S. S. Collaboration et al., 2022).

The lightcurve reported by IACTs varies in shape depending on the energy range
(see Figure 1). MAGIC observed RS Oph between 9 August and 1 September, for a
total of 21.4 h. The signal detected in the VHE regime measured during the
first 4 days corresponds to the optical and HE maxima. However, the 4-day
binned emission $>$100 GeV is best fit to a constant flux (Acciari et al.,
2022), which suggest a migration of the gamma-ray emission toward higher
energies, implying an increase in the energies of the parent particle
population. A constant flux compatible with that measured by MAGIC has been
reported by the LST-1 during the first nights (Kobayashi et al., 2023). On the
other hand, H.E.S.S. observed this between 9 August and 7 September. The
signal was detected by H.E.S.S. at $>$250 GeV peaks a day after the HE
maximum, with a temporal decay of t-(1.43±0.48) compatible with what observed
at HE, explaining the similarities in the lightcurve due to a common origin of
the emission, in which the particles are accelerated at the external shock (H.
E. S. S. Collaboration et al., 2022).

Figure 1: Multi-wavelength lightcurve of the 2021 eruption of RS Oph as seen
by H.E.S.S (top panel), MAGIC (second panel), Fermi-LAT, (third panel), and
optical V magnitude (bottom) observations by ANS (golden points) and TJO
(green points). ULs for the late MAGIC emission and Fermi-LAT are indicated as
inverted triangles. Optical, Fermi-LAT, and MAGIC data points taken from the
supplementary material in Acciari et al. (2022), H.E.S.S. fluxes from
auxiliary material from H. E. S. S. Collaboration et al. (2022).

The VHE component of the SED measured by MAGIC and H.E.S.S. expands from 60
GeV up to 1 TeV Acciari et al. (2022); H. E. S. S. Collaboration et al.
(2022). Both collaborations performed a joint analysis of the HE and VHE data
and suggest that the combined Fermi-LAT + MAGIC and Fermi-LAT + H.E.S.S.
spectra can be described as a single component spanning from 50 MeV to VHE.
The emission would be due to a shock created by the ejecta which expand into
the surrounding medium and the wind of the RG companion, creating a single
shock where particles are accelerated. Both experiments suggest that the
gamma-ray emission is best fit by a hadronic scenario, in which protons are
accelerated in the shock wave formed by the interaction of the novae ejecta
with with the interstellar medium with some contribution of the RG wind. The
daily SEDs (Figure 2 for the MAGIC sample) are also best adjusted to a
hadronic case, with evidence of increase in the energy cutoff, implying and
acceleration of protons and the absence of strong cooling processes. The
leptonic scenario does not properly fit the obtained spectra. A lepto-hadronic
scenario is also tested by (Acciari et al., 2022), providing a poor fit. In
the case of protons, the injected particle spectrum also follows a canonical
distribution ($\Gamma=-2$), while the leptonic and lepto-hadronic cases assume
more complicated injection models with some strong ad hoc spectral breaks
which cannot be fully explained by cooling, and still leading to a poorer fit
of the SED. All this together favored the hadronic scenario as mechanism for
the VHE gamma-ray production and the settlement of novae as proton
accelerators.

Figure 2: Daily SEDs of RS Oph as seen by Fermi-LAT and MAGIC adjusted to an
hadronic scenario from the first night on 9 August 2021 to the fourth night,
12 August. A hint of spectral hardening is observed with increasing time, with
increased cut-off energies. Reprinted with permission from Acciari et al.
(2022).

Ref. (De Sarkar et al., 2023) elaborated a model to explain both the gamma-ray
and radio emissions assuming a single shock, multi-population (lepto-hadronic)
scenario for the first four nights of the outburst. The authors suggest a
possible different origin for the HE and VHE components, the HE one being of
leptonic origin and the VHE hadronic-dominated. This scenario would also
explain the temporal dependence of the measured emissions.

Novae ejecta are not spherical and show some asymmetries whose shape depends
mainly on the densities of the surroundings. It has been proposed that, at
least in classical novae, two components may be at work: a slow
($\sim$hundreds of km/s) dense flow (from binary motion) that moves in the
equatorial plane and a faster ($\sim$few thousand of km/s) less dense
isotropic outflow (wind from the WD) that propagates in the polar direction
(Chomiuk et al., 2014, 2021). This creates a forward shock that is driven into
the slow outflow, while a reverse shock will interact with the faster
component. It is still unknown whether this scenario is universal to all types
of novae or not.

The 2021 eruption of RS Oph could be a first step to answer this enigma. The
2006 outburst already showed some asymmetric structures in the ejecta with
reported extended emission in the east–west direction (Chesneau et al., 2007;
Bode et al., 2007; Ribeiro et al., 2009). A similar asymmetry has been
reported during the latest outburst, displaying a bipolar structure with a
predominant orientation in the same direction (Munari et al., 2022; Nikolov et
al., 2023; Montez et al., 2022). Both in the 2006 and 2021 eruptions, a slow
moving equatorial ring and a faster bipolar ejecta expanding in the polar
(east–west) direction have been reported (Chesneau et al., 2007; Nikolov et
al., 2023). Ref. (Nikolov et al., 2023) claims the formation of a ring-like
structure in the orbital plane due to the interaction between the ejecta and
an enhanced ambient medium in the equatorial plane and argue that similar
torus-like structures are observed in classical novae, although with a
different origin (due to the orbital motion on the ejecta, see (Chomiuk et
al., 2014)).

The recent detection of the RS Oph during the 2021 outburst in the GeV-TeV
domain together with the multi-wavelength data have led some authors to
suggest the presence of multiple shocks (polar and equatorial) in this system.
Recently, ref. (Diesing et al., 2023) argued that the presence of multiple
ejecta components can explain both the gamma-ray SEDs and the shapes of the
lightcurves as seen by Fermi-LAT and H.E.S.S., which are consistent with the
combination of two shocks: a low-velocity shock which expands in a dense
medium and another faster one which expands in a less-dense environment. This
scenario suggests that RS Oph (and probably RS Oph-like symbiotic systems)
show similar properties to classical novae.

The protons that are accelerated in (symbiotic recurrent) novae ejecta can
eventually escape onto the interstellar medium and contribute to the Galactic
cosmic ray sea. Novae are less energetic that other events such as supernovae
(SNe) ($\sim$$10^{43}$ erg vs. $\sim$$10^{50}$ erg) but they occur at a higher
rate (5–15 detected novae in the Galaxy per year vs. 1–2 core-collapse SNe);
hence, their contribution to the cosmic ray budget could be noticeable.
However, considering the energetics of RS Oph as detected at VHE and its
recurrence time of $\sim$15 years, the overall contribution to the cosmic ray
spectrum is negligible, only 0.1% of that of SNe (Acciari et al., 2022).
Nevertheless, novae do create bubbles of enhanced cosmic ray density in their
close environment. In the case of RS Oph-like systems, these bubbles can
extend up to $\sim$10 pc.

##  3 Flaring Binary Systems and Microquasars

Different types of gamma-loud binaries have been found to emit both in the HE
and VHE regime, such as gamma-ray binaries and microquasars (Paredes and
Bordas, 2019). They represent a good opportunity to study particle
acceleration in shocks and jets at relatively short timescales. The so-called
gamma-ray binaries are systems that display the peak of their non-thermal
emission above 1 MeV and are composed by massive stars (O or Be type) being
orbited by a compact object (either NS or BH). Out of eight known gamma-ray
binaries, three of them host a pulsar: PSR B1259–63, PSR 2032+4127, and LS I
+61 303. Even if the powering engine remains unknown for the others, it could
be that all gamma-ray binaries host an NS, due to similarities in the SEDs and
flux patterns, although there are differences in a case-to-case basis. Even
though the emission in gamma-ray binaries is modulated with the orbital period
and some even display super-orbital modulation (Ahnen et al., 2016), the
systems LS I +61 303 and HESS J0632+057 have shown enhanced transient episodes
(Archambault et al., 2016; Adams et al., 2021). Since both systems are
composed by massive Be stars with a circumstellar disk, the origin of this
transient emission is suggested to be associated to clumps or inhomogeneities
in the stellar wind or in the interaction region between the stellar and
pulsar winds (assuming that HESS J0632$+$057 also host a NS). In the case of
long period binaries such as PSR 2032$+$4127 (50 years) and PSR B1259$-$63
(1237 days), their gamma-ray emission is detected during the periastron
passage, where VHE signal is detected (Abeysekara et al., 2018; H. E. S. S.
Collaboration et al., 2020). In the case of PSR B1259$-$63, some additional HE
flares with no flux increase in the TeV counterpart are detected during the
periastron passage.

Microquasars are binary systems composed of a compact object accreting
material from a companion star, generating accretion disks and jets. In the HE
regime, two microquasars, both of them hosting a massive donor star, have been
identified to emit transient emission: Cygnus X-1 (Bulgarelli et al., 2010;
Sabatini et al., 2010, 2013) and Cygnus X-3 (Tavani et al., 2009; Fermi LAT
Collaboration et al., 2009). No HE emission from microquasars with a low-mass
companion (so-called low-mass X-ray binaries, LMXBs) has yet been detected in
the MeV range, with the strongest hint being that at $\sim$4$\sigma$ level of
V404 Cyg during the major outburst of 2015 (Loh et al., 2016; Piano et al.,
2017). No transient emission from microquasars has been detected in the TeV
domain; see, e.g., (Aleksić et al., 2011, 2015; Ahnen et al., 2017; H. E. S.
S. Collaboration et al., 2018; Abe et al., 2022). The strongest TeV hint of
emission up to now is that of Cygnus X-1 reported by (Albert et al., 2007)
during a contemporaneous X-ray flare, reaching a 4.1$\sigma$ (post-trial)
signal in an 80 min observation. Only two microquasars have been discovered to
emit persistent TeV emission: SS433, a microquasar with two persistent jets
that interact with a surrounding nebula, being the interaction regions between
the jet and the nebula the TeV-bright region (Abeysekara et al., 2018), and
the microblazar V4641 Sgr (Tibolla, 2023). Both were discovered by particle
detectors after accumulating few years of data. Only SS433 has been detected
by an IACT (H. E. S. S. Collaboration et al., 2024) after accumulating more
than 200 h of data.

##  4 Supernovae

SNe are explosive energetic events that result from a stellar death. They can
generally be classified into two large groups depending on whether their
spectra is hydrogen-poor (type I SNe) or if they do show Balmer lines (type II
SNe). More refined classifications have been appearing in terms of additional
spectral features, see e.g., (Filippenko, 1997) or even depending on whether
the optical light curve decays (in magnitudes) linearly (II-L) or forming a
plateau (II-P).

These SNe also differ in the nature and structure of the stellar progenitor.
Type Ia SNe result from the thermonuclear runaway of a WD in a binary system.
The WD has been accreting material from its companion star and it has probably
been producing nova eruptions throughout its life. Due to accretion, the WD
increases its mass until the Chandrasekhar limit ($M_{Ch}$ $\approx
1.4M_{\odot}$) is reached and it will explode as a SNe. As mentioned in
Section 2, symbiotic systems such as RS Oph are type Ia SNe progenitors. Core-
collapse SNe (CCSNe) are the consequence of the death of a massive star
($M>8~{}M_{\odot}$) that has exhausted its fuel, producing a violent explosive
release of the external shells and leading to the collapse of the nucleus.
Type Ib and Ic are stripped envelope SNe, in which the progenitors were Wolf
Rayet stars stripped of their H (Ib) and He (Ic) layers (Georgy et al., 2009).
The progenitors of II-P SNe are generally red-supergiant (RSG) stars and those
of II-n are luminous blue variables (LBVs). These LBVs are very massive
($M>25~{}M_{\odot}$) and show high eruptive mass-loss processes during their
lifetime, leading to the appearance of strong narrow H emission lines due to
the interaction of the SNe with the surrounding circumstellar medium (CSM) in
the spectra of II-n. RSGs can also show some smaller eruptive events and hence
also show early CSM interaction flash features in their early spectra. The
progenitors of IIb SNe are supergiants that were partially stripped from their
H envelope via binary interaction during the pre-SN phase.

CCSNe are of interest due to many aspects. They are the precursors of compact
objects (BHs and NS, depending of the initial mass of the progenitor star),
they help disperse the heavy elements that have been created by the progenitor
star onto the interstellar medium (ISM), and they are sources of cosmic rays,
neutrinos, and (likely) GWs. Finally, some supernova remnants (SNRs) have been
suggested as counterparts of sources detected up to 100 TeV (Cao et al., 2021,
2023) which can contribute to the Galactic cosmic-ray spectrum. The most
common SNe are type II-P, representing 57$\%$ of the population (Smith et al.,
2011).

###  4.1 Expected Gamma-Ray Emission from SNe

SNe are expected gamma-ray production sites due to the acceleration of protons
in the SNe blastwave (Murase et al., 2011; Cristofari et al., 2022; Brose et
al., 2022). They would then contribute to the Galactic cosmic-ray budget. To
account for the measured CR spectrum, about 10$\%$ of the ejecta energy
($\sim$1051 erg) shall be converted into kinetic energy.

###  4.2 Type Ia SNe

Type Ia SNe are thermonuclear explosions generated by carbon–oxygen WDs in
binary systems once they exceed their Chandrasekhar’s limit. These SNe are
used as standard candles since they display similar lightcurves and
homogeneous absolute magnitudes, which are used to estimate cosmological
parameters. We now know that novae, which are the progenitors of this type of
event, are HE (classical and symbiotic) and VHE (recurrent symbiotic novae)
gamma-ray emitters. However, no type Ia SNe has been identified as a gamma-ray
source. The only observation of this kind was performed by the MAGIC
telescopes of SN 2014J (Ahnen et al., 2017), setting the firsts and only ULs
to the VHE gamma-ray emission of these explosive events. SN 2014J was
discovered in 21 January 2014 in M 82 at 3.6 Mpc and was observed by MAGIC for
about 5.4 h starting 6 days after explosion and over a total of four nights.
The integral flux UL set at 300 GeV is 1.3 $\times$ 10-12 photons cm-2 s-1.
The expected gamma-ray emission in type Ia SNe should be of hadronic nature
(which is in line with the hadronic origin of the VHE signal detected in the
nova RS Oph) as described in (Dwarkadas, 2013). This model suggests that the
hadronic emission shall increase with time (for a constant density medium).
The gamma-ray emission should have come from the interaction of the protons
accelerated in the SN shock with the surrounding medium. Adopting this model,
ref. (Ahnen et al., 2017) calculated that the putative gamma-ray flux shall be
at the level of $\sim$1.3 $\times$ 10-24 photons cm-2 s-1, well below the
sensitivity of IACTs.

###  4.3 Core-Collapse SNe

CCSNe are considered the best candidates for gamma-ray factories. The
interaction of the SN ejecta with the surrounding CSM will produce the GeV-TeV
gamma-ray emission via proton–proton interaction with the ambient matter.
Hence, CCSNe with strong CSM interaction (type II-n, Ibn, or near II-P with
early CSM interaction) are the best candidates for gamma-ray production. The
CSM density decreases with increasing distance; hence, the expected GeV-TeV
emission shall take place during the first days of the SNe explosion. However,
eruptive phases of the progenitor star before during the pre-eruption can
cause the CSM to be layered in shells. These shells could then enhance the
expected gamma-ray radiation at later times once the ejecta reaches them.
However, the gamma photons can also interact with the low-energy photons from
the photosphere, producing pair production and hence leading to the strong
absorption of the GeV-TeV signal during the first days after explosion
(Cristofari et al., 2022; Brose et al., 2022).

No transient emission from SNe has been confirmed in the HE regime. Two
candidate sources have been observed in Fermi-LAT data, corresponding to the
position of two CCSNe (Yuan et al., 2018; Xi et al., 2020), but due to the
large uncertainties in the localization regions which overlap with other
gamma-ray sources, their confirmation is challenging. Variable gamma-ray
emission has been detected to correspond with the peculiar luminous type II SN
iPTF14hls located at 150 Mpc (Yuan et al., 2018). It shows similar spectra to
H-rich SNe but with a different lightcurve and it is located at the same
position of another eruption detected in 1954, which is difficult to explain
in an SN context. Also, there is a blazar inside the error box. The HE source
is detected between days 300 and 850 after the explosion. However, if the
association between the HE source and SN iPTF14hls is real, it would be the
first SN to be detected in the gamma-ray domain. Although it is challenging to
explain the gamma-ray emission via particle acceleration in shocks, since the
efficiency should be too high. In the case of Type II-P SN 2004dj located in
the galaxy NGC 2403 (3.5 Mpc), HE gamma-ray emission from the young SNR has
been detected after the accumulation of 11.4 years of data (Xi et al., 2020).
The authors suggest that this source, whose emission is banishing over time,
is the result of the interaction of the SN ejecta with a high-density shell.
Two more candidates have been reported by (Prokhorov et al., 2021) associated
with SN AT2018iwp and SN AT2019bvr, with transient HE signals starting 3 and 6
months after the SN explosion respectively.

No signal from CCSNe has been detected in the VHE regime. The authors in (H.
E. S. S. Collaboration et al., 2019) reported ULs on ten different CCSNe
observed within a year of the explosion. Nine of them where serendipitously
observed, most of them type II-P and located at distances 4–54 Mpc, while ToO
observations were performed on SN 2016adj, a type IIb SN located in Cen A
galaxy at 3.8 Mpc. The exposure time is also different depending on the
target: for four sources including SN 2016adj, observations started around or
shortly after the discovery date, while the rest of the observations happened
months later (up to 272 days after the explosion). The flux ULs above 1 TeV
are of the order of 10-13 TeV cm-2 s-1. The non-detection of this sample may
simply indicate that the putative gamma-ray fluxes are below the sensitivity
of current IACTs or that the CSM is not dense enough for particle
acceleration, but do not rule out the possibility of SNe being VHE emitters.
Most recently (Acharyya et al., 2023) observed the superluminous (SLSN) type I
SN2015bn and SN2017egm. SLSNe are characterized for displaying luminosities 10
to 100 times larger than ordinary CCSNe and for their lightcurves reaching the
peak emission at later times. The VHE observations happened 135 days (49 days
from the peak magnitude) after explosion for SN2015bn (serendipitously
observed) and 670 days from explosion for SN2017egm, targeted due to the
predicted gamma-ray emission derived from the optical lightcurve. No TeV
counterpart was detected and the first ULs on type I SLSNe in this regime are
set (see Figure 3). Although these ULs do not help constrain the scenarios of
a magnetar as central engine or a shock-acceleration they do discard a jet
model powered due to fallback accretion onto a black hole (model LBH as seen
in Figure 3).

Figure 3: Multi-wavelength lightcurves of SN2015bn from days 30–1500 after
explosion (left panel) and SN2017egm spanning 10–1300 days after explosion
(right panel). The gamma-ray ULs from VERITAS (orange arrows) and Fermi-LAT
are shown. Figure adapted and reprinted with permission from (Acharyya et al.,
2023).

The most famous CCSNe is probably SN 1987A, the closest explosion in over 300
yr, located in the Large Magellanic Cloud (LMC), and that reached a peak
magnitude of 2.9, visible to the naked eye. It is classified as type-II
peculiar, since the progenitor was not a RSG but a blue supergiant. Neutrinos
were detected a few hours prior to the arrival of electromagnetic emission,
likely happening during the collapse of the progenitor’s nucleus (Hirata et
al., 1987; Bionta et al., 1987). The central compact object is a neutron star
powering a pulsar-wind nebula (PWN) (Cigan et al., 2019; Greco et al., 2021).
The evolution of the remnant has been studied over decades in which rings have
been formed which are the result of the interaction of the ejecta with pre-
eruption material ejected by the progenitor star; see, e.g., (McCray and
Fransson, 2016), and references therein for a review. This interaction shall
be capable of producing gamma-ray signal via CR acceleration and magnetic
field amplification. However, no VHE signal has detected on the remnant after
a intensive campaign of 210 h (H. E. S. S. Collaboration et al., 2015).

An important parameter to take into account for CCSNe is that the gamma
photons can also interact with the low-energy photons from the photosphere
producing pair production and hence leading to strong absorption of the GeV-
TeV signal during the first days after explosion. Ref. (Cristofari et al.,
2022) created a time-dependent model to estimate the gamma-ray emission from
type II-P CCSNe (the most abundant type of SNe) during the first month after
the explosion, taking into account the expected attenuation. By studying the
evolution and dependence of different parameters such as photosphere
temperature, the authors probe that the expected signal for type II-P CCSNe at
distances $>$1 Mpc is below the sensitivity of current generation of IACTs,
but close-by systems located in the Galaxy or Magellanic Clouds-located SNe
could be detectable now and will undoubtedly be by future-generation of IACTs.
Regarding $\gamma\gamma$ effects, the expected signal shall be strongly
absorbed during the first 10 days approximately (see Figure 4). Ref. (Brose et
al., 2022) developed a model for Type II-n associated to an LBV progenitor and
Type II-P associated to a RSG, accounting also for the strong $\gamma\gamma$
absorption expected during the first days after the explosion. Assuming high
mass-loss rates of the progenitor before the eruption, the maximum energies
reached by protons can reach up to 600 TeV, which could be compatible with the
knee feature of the CR spectrum. However, moderate mass-loss rates show lower
values for these energies, between 70 TeV (type II-P) and 200 TeV (type II-n).
Considering the absorption effects, the expected gamma-ray peak should happen
12 to 30 days after the explosion. The models suggest that current-generation
instrumentation should be able to detect nearby events, up to $\sim$60 kpc for
type II-P and $\sim$1 Mpc for type II-n. Future instruments (such as the
southern array of the CTA observatory) shall detect type II-P up to 200 kpc
and type II-n up to 3 Mpc. The values obtained by (Cristofari et al., 2022;
Brose et al., 2022) are in agreement with the lack of detection of a VHE
counterpart.

Figure 4: Temporal evolution of the integrated fluxes for type II-P CCSNe at E
$>$ 100 GeV (left) and E $>$ 1 TeV (right) at a distance of 1 Mpc and a mass-
loss of 10${}^{-6}~{}M_{\odot}$ yr-1 for different mass ejecta (blue dotted,
red dashed, and green solid) and progenitor radius (thin and thick lines).
Figure adapted and reprinted with permission from (Cristofari et al., 2022).

The second brightest type II CCSNe since the discovery of SN 1987A is SN
2023ixf which went off on May 2023 in M101 galaxy at $\sim$6.8 Mpc and reached
a peak magnitude of about B = 10.6. It is a type II-P SNe with a RSG
progenitor that showed strong interaction with the CSM, revealed via flash
spectroscopy. No gamma-ray emission has been detected at HE (Marti-Devesa,
2023) and none has been reported at VHE. However, considering the type of
explosion and its distance, a non-detection is compatible with respect to
current models.

##  5 Flaring Pulsar-Wind Nebulae

Pulsars are highly magnetized neutron stars which are the aftermath of a
massive star death. They accelerate leptons (electrons and positrons) in a
relativistic wind that halts at the termination shock, creating diffuse
structures known as pulsar-wind nebulae (PWNe). In early times, as pulsars are
the result of a SN explosion, both the pulsar and its associated nebula are
initially surrounded by an SNR. PWNe represent one of the largest VHE source
population in the Galaxy. The recent detection (and highly probable
association with PWNe counterparts in other cases) of several systems at
$>$100 TeV (ultra high energies, UHEs) has revealed that (at least) a fraction
of PWNe are leptonic PeV accelerators or leptonic PeVatrons (Cao et al., 2021,
2023).

PWNe show different evolutionary stages; see, e.g., (Gaensler and Slane, 2006;
Giacinti et al., 2020; Olmi, 2023): (i) an early phase (typical time t $\leq$
10 kyr) of free–free expansion in which the PWNe are contained inside the SNR
and there is no interaction yet with the inward SNR reverse shock. At this
early stage, the pulsar is located near the SNR center close to its
birthplace. The TeV emission should come, in this case, from within the nebula
itself; (ii) the reverberation or second phase begins once the reverse shock
collides with the PWN forward shock (t$\sim$few tens of years kyr), creating a
compression on the PWNe that then leads to an expansion, creating oscillations
or reverberations. At this point, the PWN becomes disrupted, provoking that
the electrons that produce the TeV emission start to escape from the PWNe onto
the SNR and possibly into the ISM. The PWNe start suffering morphological
changes and the pulsar can start moving from its birthplace, but it is still
contained within the nebula; (iii) the final post-reverberation or bow-shock
phase is reached once the pulsar abandons the SNR onto the ISM (at least t
$\geq$ 40 kyr), creating a bow-shock structure. At this stage, the escaped
leptons can propagate further into the ISM in a region larger than the PWN,
creating extended TeV halos. Two TeV halos were initially discovered by HAWC
around the evolved pulsars Geminga and Monogem (Abeysekara et al., 2017) and
several more have been identified by the Large High Altitude Air Shower
Observatory (LHAASO) as counterparts for some of the sources on their first
catalog (Cao et al., 2023).

The Crab Nebula is probably the most studied PWNe in the VHE regime. It was
the first TeV source ever detected (Weekes et al., 1989) and since then it has
served as standard candle for VHE astronomy. It is the result of an SN
explosion that happened in 1054 CE; hence, it is powered by a young 33-ms
pulsar (PSR J0534$+$220). This central pulsar has largely been studied by
IACTs and its pulsations have been detected from as low as 25 GeV (Aliu et
al., 2008) up to 1.5 TeV energies (Ansoldi et al., 2016). It is the most
powerful pulsar in the Galaxy with a spin-down luminosity of 4.6 $\times$ 1038
erg s-1. The multi-wavelength emission of the Crab Nebula is described by
synchrotron radiation detected from radio to HE gamma rays, while the TeV
component is produced by inverse Compton up-scattering of those synchrotron
photons by the relativistic electrons accelerated in the pulsar wind. The TeV
PWNe has been resolved to an extension of $\sim$52 arcsec (H. E. S. S.
Collaboration, 2020). Its spectrum is measured over 22 decades in energy,
described as leptonic emission. The existence of photons at energies $>$100
TeV was first evidenced by the UHE detectors Tibet AS$\gamma$ and HAWC
(Amenomori et al., 2019; Abeysekara et al., 2019) and by the MAGIC telescopes
via very large zenith angle (VLZA) observation techniques (MAGIC Collaboration
et al., 2020). LHAASO has further established the Crab Nebula as a leptonic
PeVatron with the detection of photons up to 1.1 PeV (Cao et al., 2021, 2023),
implying that the parent electrons shall have energies of $\sim$2 PeV.

For a long time, the Crab Nebula was considered a steady source with a stable
flux both in the HE and VHE gamma-ray regimes. However, strong flaring
emission was discovered at energies $>$100 MeV by the space-borne instruments
AGILE and Fermi-LAT (Tavani et al., 2011; Abdo et al., 2011). These flux
enhancements show a short few-hour timescales duration. The flux increase has
been found to vary for a factor 3–30 with respect to the nebula average value,
as seen in (Bühler and Blandford, 2014), and references therein. These flaring
episodes can last for several days/weeks and they show shorter-scale
structures. Also, the spectral index shows differences among flares. During
these flaring events, no variability in the pulsar flux or significant glitch
is detected. Also, no variability in the synchrotron component is detected in
the radio, infrared of X-ray bands (Tavani et al., 2011; Bühler and Blandford,
2014). These flares have been repeatedly appearing at rates of about $\sim$1–2
per year.

This enhanced emission could be extended up to TeV emission and be potentially
observable by IACTs. Two scenarios are possible: the detection of the
synchrotron tail at the low-end of the VHE regime (up to few tens of GeV) or
that the enhanced emission is transferred to the IC component and detected
deep in the TeV range (in Klein–Nishina regime) due to synchrotron self
Compton process, since the electrons that produce the enhanced MeV emission
should upscatter the photons in the nebula to produce TeV emission.However,
the IC component seems to remain stable during the HE flares, since IACTs have
searched for variability in the TeV component, with no significant flux
enhancement detected. Both MAGIC and VERITAS observed during the 2010 HE flare
(58 min in one night and 120 min over four nights, respectively), with no VHE
enhancement reported (Mariotti, 2010; Ong, 2010). The HE flare of 2013 (Ojha
et al., 2013), which lasted for about 2 weeks at HE, was observed by H.E.S.S.
for five consecutive nights and by VERITAS during a period of about three
weeks (see Figure 5), with similar results (H. E. S. S. Collaboration et al.,
2014; Aliu et al., 2014). Whether a flux enhancement deep in the TeV component
exists remains yet unknown. A search in the TeV end with the VLZA technique
with MAGIC revealed that the stereoscopic system should be able to detect
fluctuations 2.25 times brighter that the constant PWNe value (van
Scherpenberg et al., 2019); hence, given the right conditions, these flares
could potentially be detected by the current generation of IACTs.

Figure 5: SED of the Crab Nebula during quiescence (black squares for Fermi-
LAT data and blue dots for VERITAS data) and during the 2013 flare (magenta
triangles for Fermi-LAT and red dots for VERITAS). While the synchrotron
component detected by Fermi-LAT shows hardening and enhanced flux, the IC
spectrum shows no deviation. The y-axis error bars represent the statistical
uncertainties in the flux. The downside arrow in the Fermi-LAT flaring
component is a flux UL. Reprinted with permission from (Aliu et al., 2014).

.

It is not trivial to understand the undergoing mechanism of these rapid
flares. In the PWNe scenario, the MeV-GeV component is described as
synchrotron produced by electrons and positrons in a shocked pulsar wind and
hence with energy limited by the synchrotron burn-off (assuming an MHD
outflow). The flares surpass this value, hence excluding this ideal scenario.
On the other hand, rapid flares cannot be explained in an IC context.
Different scenarios have been proposed to account for the origin of these HE
gamma-ray flares: (explosive) magnetic reconnection events in a highly
magnetized plasma (Lyutikov et al., 2018) or inductive spikes (Kirk and
Giacinti, 2017), among others. The absence of flux enhancement at other
wavelengths could indicate that the HE flares are produced by a single
population of electrons. However, it is the lack of multi-wavelength
detections and possible correlations that make the study of the origin of this
flaring emission challenging. The fast variability and rapid enhancement at HE
implies that the emission should come from a compact region in the PWNe of
$\sim$10-4 pc (Bühler and Blandford, 2014).

PSR J0534$+$220, powering the Crab Nebula, is the most energetic pulsar in the
Galaxy (4.8 $\times$ 1038 erg s-1) and it is among the youngest ones. Up to
now, the Crab Nebula is the only PWNe showing variable HE gamma-ray emission
in the Milky Way. However, two young pulsars in the LMC show similar spin-down
powers to Crab: PSR J0537$-$6910 (1.5 $\times$ 1038 erg s-1) and PSR
J0540$-$6919 (4.9 $\times$ 1038 erg s-1). Flaring-like activity has been
detected with Fermi-LAT in different bands: 100–300 MeV, 100–300 MeV, and 1–10
GeV (Nizamov and Pshirkov, 2023). Due to the spatial closeness of the two
pulsars, it is not possible to identify which of them is responsible for the
flares in the first two energy regimes. However, since the angular resolution
improves at higher energies, it has been found that both pulsars flare at GeV.
Gamma-ray flares from these pulsars were already predicted by (Kirk and
Giacinti, 2017). The detection of flaring gamma-ray emission from other PWNe
apart from the Crab Nebula could then indicate that this type of variability
(e.g., inductive spikes) is common to young powerful pulsars.

##  6 Fast Radio Bursts and Magnetars

Fast Radio Bursts (FRBs) are a relatively newly-discovered Jy-level-class of
$\sim$millisecond radio transient events of (mainly) extragalactic origin. At
present, since the first discovery of the so-called Lorimer burst (Lorimer et
al., 2007), about 1000 FRBs have been detected. However, once taken into
account, various factors such as the sky coverage of the different
instruments, survey threshold, and selection effects, it is possible to
conclude that FRBs occur at an extraordinary rate, up to $\sim$$10^{4}$ per
day distributed over the entire sky. This correspond to a rate of $10^{-3}$
yr-1 per galaxy, much larger than the GRB rate. Nonetheless, only few tens of
FRBs have been associated to their host galaxy (with kpc precision) and only a
handful have been localized with enough accuracy to be associated with
specific regions within those galaxies. Although from the observational point
of view FRBs are similar to the pulses detected from Galactic radio pulsars,
the observed flux density coupled with their extragalactic origin indicates a
total emitted isotropical equivalent energy ranging from $\approx$$10^{35}$ to
$\approx$$10^{43}$ erg, extremely high compared to the radio pulsar standard,
but several orders of magnitude smaller than GRBs; for a general review, see,
e.g., (Cordes and Chatterjee, 2019; Petroff et al., 2019).

Most FRBs are one-off events. However, within the population of FRBs detected
so far, around 50 events have been observed to produce multiple bursts, the
so-called repeating FRBs (Chime/Frb Collaboration et al., 2023). Although
sporadic, the repeating behavior of some FRBs allowed for the first time to
perform targeted observations to localize the source using interferometry
techniques. The first known repeater, FRB 121102, was associated with a low-
metallicity star-forming dwarf galaxy at redshift z = 0.19 (Tendulkar et al.,
2017), while a persistent and compact ($<$$0.7$ pc) radio source of unclear
nature was discovered in association with the FRB direction (Chatterjee et
al., 2017). High-resolution optical and infrared observations by the Hubble
space telescope and the Spitzer telescope showed that the galaxy optical
emission is dominated by an inner star-forming region whose position is
consistent (within uncertainties) with the persistent radio source (Bassa et
al., 2017). Such type of galaxy is also the typical host galaxy for extreme
transient events such as GRBs or super-luminous supernovae. While the
association of FRBs with cataclysmic events may sound natural and was
originally proposed as counterpart of FRBs, the bursts of FRB 121102 have not
revealed any signature of an afterglow emission and have been found to repeat
at a rate short enough to rule any possible explosive mechanism to power them
out. A second localized repeater, FRB 20180916B, shows an apparent
$\sim$16-day ($\sim$4 days active followed by 12 days of inactivity)
periodicity (Chime/Frb Collaboration et al., 2020). It was found to be located
at the edge of a star-forming region within a spiral galaxy, without any
persistent counterpart associated. A possible periodicity of $\sim$150 days
has been found also for FRB 121102 (Rajwade et al., 2020). Some significant
differences between the repeaters and the apparent one–off FRBs have been also
reported in the literature (Pleunis et al., 2021). In particular, repeater
bursts seem to be intrinsically broader in width and narrower in bandwidth.
The position of active repeating FRBs seems to be consistent with the one of
young extreme objects such as magnetars. Magnetars are isolated NS with an
extremely powerful magnetic field of the order of $10^{14}$–$10^{15}$ G, about
1000 times stronger than a normal NS. In these objects, the observed
persistent electromagnetic radiation is likely powered by the decay in the
intense magnetic field. On the other hand, magnetars can also undergo flaring
episodes with outbursts on different timescales, detectable in X-rays and
radio. These are probably caused by large-scale rearrangements of the surface
and/or magnetospheric field. Interestingly, magnetars can additionally produce
giant flares (GFs), which are among the most energetic ($10^{44}$–$10^{47}$
erg s-1) Galactic events.

On April 2020, the event FRB 200428 was detected by the Canadian Hydrogen
Intensity Mapping Experiment (CHIME) telescope from a direction consistent
with the Galactic magnetar (and soft gamma repeater) SGR 1935$+$2154
(CHIME/FRB Collaboration et al., 2020), located at a distance of 6.6–12.5 kpc
and embedded in the supernova remnant SNR G57.2$+$0.8. This discovery
represented the first detection of an FRB event from a known object, as well
as the first FRB of Galactic origin. Contemporaneously to the FRB event, the
detection of several X-ray flaring episodes was achieved by a wide range of
instruments (Mereghetti et al., 2020; Tavani et al., 2021; Ridnaia et al.,
2021). Figure 6 shows the X-ray light curve as measured by the INTEGRAL
satellite where the radio emission is found to be in time coincidence with the
X-ray flaring activity (Mereghetti et al., 2020). Furthermore, a long-lasting
high-energy flaring activity in the form of a forest of intense X-ray bursts
was detected by Swift (Palmer, 2020) and Fermi-GBM (Fletcher and Fermi GBM
Team, 2020) up to several hours after the initial episode. The discovery of
the connection between hard X-ray bursts (HXRBs) of SGR 1935$+$2154 and FRBs
significantly boosted the long-lasting idea of the theoretical interpretation
of magnetars as progenitors of FRBs. However, deeper observations performed by
the FAST radio telescopehttps://fast.bao.ac.cn/ (accessed on 27 March 2024).
showed that the majority of the X-ray bursts emitted by SGR 1935$+$2154 are
actually not correlated with the FRBs (Lin et al., 2020). Additionally, the
further surprisiny detection of the repeater FRB 200120E in a position
consistent with a globular cluster within the nearby galaxy M81 (Kirsten et
al., 2022) challenges the young magnetar scenario as the only engine of FRB.
Globular clusters are old enough to not have massive stars able to originate
magnetars within. However, they do show high star densities and host short-
period binaries which can lead to the production of magnetars via more exotic
channels such as accretion-induced collapse (of a WD) or merger-induced
collapse (of WDs and NSs). Thus, as a matter of fact, the progenitor of FRB
remains a unanswered question. Nonetheless, in light of the so-far only robust
hint of association with SGR 1935$+$2154, the magnetar paradigm is still
considered the leading interpretative scenario and it has been discussed
extensively in the literature (a non-exhaustive list in Popov and Postnov,
2010; Kulkarni et al., 2014; Katz, 2016; Zhang, 2020).

Figure 6: INTEGRAL (20–200 keV) light curve of one of the flare of SGR
1935$+$2154 referred to T0 = 14:34:24 UTC of 28 April 2020. The vertical
orange lines represent the time of the detected radio pulses. Reprinted with
permission from (Mereghetti et al., 2020).

Within this framework, the proposed emission processes involve coherent
radiation mechanisms such as synchrotron maser radiation in magnetar internal
(Beloborodov, 2020) and external shock models (Metzger et al., 2019) as well
as magnetospheric pulsar-like models. The latter, however, do not foresee keV-
MeV emission as observed in SGR 1935$+$2154\. In (Metzger et al., 2020), it is
predicted that if FRBs are produced by magnetar flares, an afterglow emission
peaking at the MeV-GeV band is expected with a total energy release in the
X-ray/gamma-ray band at least a factor $\approx$$10^{4}$ larger than the
emitted radio energy. At the time of writing, the only magnetar flaring event
detected in the gamma-ray regime was the GF from a magnetar in NGC 253,
detected by Fermi-GBM (Bissaldi et al., 2020) and Fermi-LAT with a photon of
up to 1.7 GeV (Omodei et al., 2020). However, recent results published by
Fermi-LAT on individual FRBs analysis reported no significant emission in the
LAT energy band Principe et al. (2023). Nonetheless, the detection of hard
X-ray bursts with a non-thermal spectrum in SGR 1935$+$2154 shows that at
least some FRBs are able to accelerate particles and produce MeV non-thermal
emission. Despite the puzzling scenario, FRB (and magnetars) are also an
interesting target for IACTs. In fact, some theoretical models also predict
VHE (Lyubarsky, 2014; Murase et al., 2016; Margalit et al., 2020; Metzger et
al., 2020) emission correlated in time with FRBs. Not surprisingly, the
flaring episodes of SGR 1935$+$2154 in April 2020 were also followed-up by
current Cherenkov telescopes and monitoring campaigns on this magnetar have
been active since then. The campaigns organized by MAGIC (López-Oramas et al.,
2021) and H.E.S.S. (Abdalla et al., 2021), coordinated within a larger multi-
wavelength framework, did not reveal VHE emission to be neither persistent nor
on shorter (minutes to milliseconds) time scales.

Current-generation IACTs have active follow-up programs on FRBs although no
particularly stringent flux ULs in the VHE band were reported so far (Holder
et al., 2019). Some specific events have been the subjects of dedicated
follow-up campaigns such as in the case of H.E.S.S. that obtained the first
ULs on the potential VHE afterglow emission from FRB 20150418A (H. E. S. S.
Collaboration et al., 2017) (Figure 7 right panel). This FRB was of particular
interest as it showed hint of a $\approx$$6$ days-long-lasting radio afterglow
detected by the Australia Telescope Compact Array (ATCA) (Keane et al., 2016).
The achieved limit on the VHE luminosity was of the order of $5\times 10^{47}$
erg s-1 at the energy of 1 TeV. The MAGIC and VERITAS collaborations reported
VHE ULs on the repeater FRB 20121102A conducting coordinated observations with
Arecibo (MAGIC Collaboration et al., 2018; Bird and VERITAS Collaboration,
2017). In the case of MAGIC follow-up, five contemporaneous radio bursts were
detected (at a central frequency of 1.38 GHz) although no millisecond
timescale burst emission was detected in VHE or the optical band (Figure 7
left panel). Follow-up results on a sample of other repeaters (FRB 180814, FRB
180916, FRB 181030, and FRB 190116) were more recently reported by VERITAS
(Veritas et al., 2022), again with no detection achieved.

Figure 7: (Left plot): Optical light curves obtained by MAGIC and spanning 200
ms around the trigger times of 5 bursts from FRB 121102 detected
simultaneously with MAGIC data. The vertical axis is proportional to the
U-band flux. No significant excess is observed simultaneously with any of the
5 bursts. Reprinted with permission from (MAGIC Collaboration et al., 2018).
(Right plot): Significance sky map from the H.E.S.S. follow-up observations of
FRB 150418. Reprinted with permission from (H. E. S. S. Collaboration et al.,
2017).

It is important to remark that IACTs are versatile instruments that, although
designed to detect nanosecond pulses of Cherenkov light, are generally
sensitive to millisecond timescale optical signals. Despite the modest quality
of their mirrors when compared to standard optical telescopes, the typical
large diameter of their primary mirror and their fast-response readout
electronic make them effective high-time-resolution photometers. Current IACTs
are indeed able to perform parallel VHE and optical observation on very short
timescales up to a limiting magnitude significantly lower than standard
optical telescopes (Hoang et al., 2020; Griffin et al., 2011; Franzen et al.,
2003). This is a key feature that make IACTs excellent instruments for fast
transient astronomy and with a relevant application in the case of FRBs. In
fact, FRBs may be potentially accompanied by fast optical bursts (FOBs) via
different mechanisms (Yang et al., 2019). Optical counterparts have been
detected in association with magnetars flaring episodes (Stefanescu et al.,
2008) and can therefore provide important insights into the physics of FRBs.
The possibility to have parallel VHE and fast optical observations made IACTs
key instruments for future follow-up. The improvement in VHE sensitivity as
promised by next-generation instruments such as the CTA will finally allow for
observations up to a gamma emission values comparable to the ones expected by
magnetars.

##  7 Gravitational Waves

The possibility of performing astrophysical observations by means of non-
electromagnetic signals such as gravitational waves (GWs) has become reality
with the first scientific runs of the LIGOhttps://www.ligo.org/ (accessed on
27 March 2024). and Virgo Scientific Collaborationshttps://www.virgo-gw.eu/
(accessed on 27 March 2024). (LVC). The first LVC scientific observation run,
named O1, opened the era of gravitational wave astronomy by means of the first
direct detection of a GW signal (Abbott et al., 2016) from a binary stellar-
mass black hole merger (BBH). Not long after, during the O2 scientific run,
the first GW signal (GW 170817) from the coalescence of a binary system
composed of two NSs (BNS) was discovered (Abbott et al., 2017) together with a
new sample of BBH signals (Abbott et al., 2019). Approximately 2 s after the
detection of GW 170817, the Fermi and INTEGRAL satellites detected a sub-
threshold short GRB (sGRB), namely GRB 170817A (Goldstein et al., 2017;
Savchenko et al., 2017). The identification of a sGRB as electromagnetic
counterpart of a GW signal triggered by a binary neutron star merger
represented a groundbreaking observation that provided the first firm evidence
on the nature of sGRB’s progenitors. The potential link between GWs (from BNS
mergers) and sGRBs has been widely explored and discussed in the literature in
the past; see, e.g., (Levan et al., 2016), and references therein for a
review. The discovery of GRB 170817A triggered an unprecedented follow-up
campaign at all wavebands. It is important to remark that these observations
are particularly challenging due to the very large localization uncertainties
provided by GWs interferometers, up to tens of thousands of square degrees.
Nonetheless, approximately 11 h after the GW trigger, an optical/IR
counterpart, named AT 2017gfo (IAU naming) and hosted in the 40 Mpc-distant
galaxy NGC 4993, was detected by the One-Meter Two-Hemisphere (1M2H)
collaboration (Coulter et al., 2017) and interpreted as a kilonova. Unlike BBH
mergers, BNS mergers are expected to be source of optical/near-IR emission
powered by the decay of radioactive nuclei generated by r-process
nucleosynthesis in the outflow formed after the coalescence; see, e.g., (Li
and Paczyński, 1998; Metzger et al., 2010). The detection of AT 2017gfo
represented the first confirmation of this theoretical prediction. In the days
after the burst, an X-ray counterpart was detected and identified as the GRB
afterglow non-thermal emission (Troja et al., 2017). The late-time rising of
an X-ray afterglow fits within the interpretation that the GRB is observed
off-axis, with the jet-beamed ejecta pointing away from Earth. According to
the hydrodynamic of a generic relativistic shock-wave model, the bulk Lorentz
factor ($\Gamma(t)$) of the outflow is reduced by the deceleration of the jet,
causing the relativistic beaming angle ($\propto 1/\Gamma(t)$) to increase
(Blandford and McKee, 1976). The opening angle of the emission widens,
eventually including the line of sight of the observer. From the observational
point of view, a delayed emission, whose intensity and delay depends on the
off-axis angle, may arise. The case of GW/GRB 170817A confirms this paradigm
as successive radio observations did confirm the interpretation of the radio
to X-ray emission as originated by an off-axis, structured jet (i.e., the
energy and velocity of the ejected material scale with the angular distance
from the jet axis) with a viewing angle of $\approx$$30^{\circ}$ (Troja et
al., 2017; Pian et al., 2017; Ghirlanda et al., 2019). The radio and X-ray
emission increased in the weeks following the initial trigger, peaking
approximately 155 days after the merger.

Figure 8: (Left plot): Sky map covering the 90% confidence-level region for
the GW 170817 position. The positions of galaxies used in cross-correlating
the large localization area and defining an optimized pointing strategy for
the Swope telescope on 17–18 August 2017 are shown as gray circles. The size
of the circle indicates the probability of a particular galaxy being the host
galaxy for GW 170817. The square regions are individual Swope pointings
labeled in the order that they were observed. Solid and dashed squares
represent the square chosen to contain multiple and individual galaxies,
respectively. The position of NGC 4993 and SSS17a are in the blue square.
Reprinted with permission from (Coulter et al., 2017). (Right plot): H.E.S.S.
pointing directions during the monitoring campaign of SSS17a. The circles
denote an FoV with radius of $1.5^{\circ}$ and the shown times are the start
times of each observation with respect to GW 170817A. Colored background is
the GW localization map. Reprinted with permission from (Abdalla et al.,
2017).

The extensive multi-wavelength campaign triggered by the detection of GW
170817A also included follow-up at VHE by IACTs (see Figure 8). Less than two
years after GW 170817A, the first detection of VHE gamma-ray emission from GRB
180720B (Abdalla et al., 2019), GRB 190114C (MAGIC Collaboration et al.,
2019), and GRB 190829A (H. E. S. S. Collaboration et al., 2021) was announced
by the H.E.S.S. and the MAGIC collaborations, bringing an end to a quest
lasting for more than twenty years. Although all of the GRBs detected so
farMore GRBs have been detected at VHE since then such as GRB 201216C by MAGIC
and the remarkable GRB 221009A, although not detected by IACTs, observed by
LHAASO up to 13 TeV. by current IACTs were long GRBs, sGRBs are also expected
to emit VHE radiation. In this regard, a hint of VHE emission has been
observed by MAGIC in the case of the short GRB 160821B (Acciari et al., 2021),
providing a compelling clue on the detectability of TeV emission from GW
counterparts from compact object mergers. Few attempts by IACTs in following-
up GW alerts were reported before the breakthrough of GW 170817A such as for
GW 151226 (Antonelli et al., 2015; de Lotto et al., 2017), GW 170104
(Mukherjee and VERITAS Collaboration, 2017), and GW 170814 (Ashka et al.,
2019). However, the VHE campaign organized for GW 170817A represented a step
forward and a fundamental test-bench in exploring IACTs’ capabilities in this
challenging observations. The H.E.S.S. telescopes started a series of pointing
over the uncertainty region of GW 170817A about 5 h after the first trigger
(Abdalla et al., 2017), that made it the first ground telescope to point at
the source location. Although the detection of AT 2017gfo was not yet
announced (Coulter et al., 2017), the pointing strategy proved to be efficient
with the NGC 4993 location within the field of view of the H.E.S.S. first
pointing. Nonetheless, no evidence of VHE emission was detected during this
early monitoring campaign of SSS17a (Abdalla et al., 2017). Starting from mid-
December 2017, the sky position of the optical transient SSS17a became visible
also to the MAGIC telescopes’ site. This observation window roughly overlaps
with the afterglow peaking time. Late time follow-up was then performed by
MAGIC and, again H.E.S.S. covering the peak and the onset of the fading phase
in the X-ray and radio lightcurves. Although no detection was achieved, the
obtained ULs were used by the two collaborations to constrain physical
emission models, although with a rather different prediction on the intensity
of the TeV component (see Figure 9).

Figure 9: (Left plot): Expected SSC emission evaluated by MAGIC 155 days after
the merger, using fit parameters constrained by the radio, optical, and X-ray
data. Reprinted with permission from (Salafia et al., 2022). (Right plot): SSC
spectra foreseen in (Abdalla et al., 2020) 110 days after the merger. The blue
and red curves represent two possible geometry and expansion speed of the
remnant: an isotropic, non-relativistic expansion (blue curves) and a
relativistic jet (red SSC curves). The minimum magnetic field strength imposed
by the H.E.S.S. ULs (green arrows) are also reported. Reprinted with
permission from (Salafia et al., 2022).

Both in (Abdalla et al., 2020; Salafia et al., 2022), the broad-band SED is
modeled by means of a synchrotron + synchrotron self-Compton (SSC) processes.
However, the two proposed models are not directly comparable. In the
structured jet approach used within the MAGIC interpretation and described in
details in (Salafia et al., 2019), the full time-evolution of the jet is taken
into account in the evaluation of the expected emission.As the jet evolves,
the observed radiation is the result of the convolutions of photons emitted at
different times and different locations behind the shock. Such an evolution is
not considered within the H.E.S.S. modeling wherein the emission is evaluated
at specific single times. Within the uncertainties of assumed physical
parameters for GRB 170817A, the structured jet model foresees a rather low TeV
emission component, significantly lower than MAGIC upper limits, challenging
the possibility of detection of such an event with current-generation IACTs.
However, more favorable conditions in the emitting geometry and circumbusrt
properties may mitigate these prospects as we will discuss in Section 9.

##  8 Other Transient Sources: Tidal Disruption Events and Gamma-Ray Bursts

The science topics of GRBs and tidal disruption events (TDEs) are discussed in
more details in dedicated papers within this Special Issue Vigliano and Longo
(2024). Nevertheless, we briefly touch upon them in the following subsections
for the completeness of this review.

###  8.1 Tidal Disruption Events and Neutrino Connection

Tidal disruptions events (TDEs) are powerful events that occur when a star is
disrupted by tidal forces when approaching a massive BH. They are considered
of extreme importance in particular in the framework of multi-messenger
astrophysics. It is thought that the disruption of a stellar object may
trigger the launch of a relativistic jet able to shock-accelerate particles
from the star remnants. This material is naturally rich in light and heavy
nuclei so that TDE may be a plausible acceleration site for ultra-high-energy
cosmic rays (UHECR $>10^{20}$ eV) and neutrino; see, e.g., (Farrar and Piran,
2014; Guépin et al., 2020). While the cosmic neutrino flux has already been
established through the measurements of the IceCube Neutrino Observatory
(Aartsen et al., 2015), the association of this flux with specific
astrophysical sources is still challenging. So far, few sources have been
correlated with neutrinos: the flaring blazar TXS 0506$+$056, identified as
the potential source of the IceCube neutrino alert IC 170922A (IceCube
Collaboration et al., 2018); the nearby star-forming galaxy NGC 1068, in
spatial coincidence with a cluster of IceCube-detected neutrinos (IceCube
Collaboration et al., 2022); and the TDE AT 2019dsg, discovered in the optical
band by the Zwicky Transient Facility (ZTFhttps://www.ztf.caltech.edu/
(accessed on 27 March 2024). (Bellm et al., 2019)) and identified as the
source of the event IC 191001A (Stein et al., 2021). While NGC 1068 and TXS
0506$+$056 have already been the target of observations with IACTs (Acciari et
al., 2022, 2019), TDE is a relatively unexplored class of sources in the VHE
band. In 2011, the remarkable TDE event Sw J1644$+$57, originally triggered as
a GRB by Swift, was the subject of an extensive follow-up campaign by MAGIC
(Aleksić et al., 2013) and VERITAS (Aliu et al., 2011). Although no
significant VHE detection was found, these observations may potentially pave
the road to future follow-ups with both current IACTs and the CTA Observatory.

In the near future, the Vera Rubin Observatory (Ivezić et al., 2019), will
start operation and overlap with the CTA Observatory era. The Legacy Survey of
Space and Time (LSST) will also dramatically enlarge the sample of detected
TDEs, thereby providing an unprecedented number of possible triggers to CTA
that may be able to detect VHE gamma-signature at least for nearby events
($\lesssim$$20$ Mpc) (Chen et al., 2016).

###  8.2 Gamma-Ray Bursts

GRBs are transient events last from milliseconds up to hundreds of seconds.
They are the brightest electromagnetic events known and they are able to
release an enormous amount of energy ($10^{52}\div 10^{54}$ erg). They show
their phenomenology mainly in the 10 keV–1 MeV energy band. According to a
relativistic shock model, described for example in (Paczynski, 1986; Piran,
1999), GRB emission is powered by the conversion of the kinetic energy of a
relativistic outflow into electromagnetic emission. The details of this
conversion remain poorly understood. A largely discussed possibility is that
the observed photons are radiation from particles accelerated to ultra-
relativistic energies by successive collisions within the magnetized medium.
During the so-called prompt phase, GRB dynamic is thought to be driven by
relativistic collisions between shells of plasma emitted by a central engine
(internal shocks). Similarly, the emission during the afterglow phase seems to
be connected to the shocks between these ejecta with the external medium
(external shocks). The results of such internal/external shocks is the
acceleration of particles through Fermi mechanisms. The accelerated particles
can emit the observed high-energy photons through many possible non-thermal
mechanisms. Within this framework, synchrotron emission has largely been
considered as the most natural to explain the GRB sub-MeV emission (Sari and
Esin, 2001; Zhang and Mészáros, 2001; Gupta and Zhang, 2007). Although alone
it cannot fully explain the observed prompt spectrum for the majority of the
events, synchrotron is believed to play an essential role in GRB dynamic. In
particular, it has been suggested that the HE emission observed by Fermi-LAT
extending after the end of the prompt emission is synchrotron radiation
produced in the external shock that is driven by the jet into the circum-burst
medium (Ghisellini et al., 2010). However, the recent detection of a VHE
counterpart challenged the synchrotron-alone scenario, confirming the
existence of a second emission component above the synchrotron burnoff limit.
In the near future, the CTA will open the possibility of detecting
$\sim$hundreds (or more) of photons from moderate-to-bright GRB, allowing for
a significant improvement in the photon statistics and for the possibility to
have good-quality time-resolved spectra. The first prototype of the 23 m class
diameter LST-1, particularly suited for the follow-up of transient events due
to the fast repositioning ($\sim$30 s for 180∘) and the relatively low energy
threshold, is currently ending its commissioning phase at the CTA northern
site. LST-1 already reported the follow-up of different GRBs and neutrino
events, although with no reported significance yet (Carosi et al., 2021).

Furthermore, many events have shown (somehow surprisingly) that long-lasting
TeV signatures can also be detectable under favorable conditions. The close-by
and very low luminosity burst GRB 190829A H. E. S. S. Collaboration et al.
(2021) was detected by the H.E.S.S. telescope up to a few TeV for three
consecutive nights while the recent detection of GRB 221009A by the LHAASO
experiment up to 13 TeV LHAASO Collaboration et al. (2023) has definitively
proven that instruments operating in the energy range above few TeV band such
as the ASTRI Mini-Array (Scuderi et al., 2022), although not specifically
designed for transients and time-domain astrophysics, may also play a key role
in the future follow-up programs of these events (Stamerra et al., 2021;
Carosi et al., 2023). For more detailed review on GRBs, see (Berti and Carosi,
2022; Vigliano and Longo, 2024)

##  9 Discussion and Prospects

The last two decades have proven to be the starting point of a golden era for
multi-messenger time domain astrophysics. New facilities for non-
electromagnetic astronomy such as neutrino and GW detectors have reached their
nominal operational phase, joining a large network of telescopes and
satellites covering an unprecedentedly wide energy band. New synergies and
improving communication channels between these facilities have led to
breakthrough discoveries such as the connection between sGRBs and GWs. The
physics of extreme transient events both inside and outside our Galaxy has an
intuitive connection with the highest energetic X- and gamma-ray radiation.
Non-thermal emission processes, typical of the HE and VHE band, represent the
signature of shock-powered radiation mechanisms, often invoked in explaining
the dynamics of a wide range of extreme cosmic accelerators. Shock
interactions may be at work as a particle acceleration mechanism in both a
relativistic (like in GWs/GRBs) and non-relativistic (like in SNe) flavor. The
corresponding radiation mechanisms at work may be shared among these sources,
although showing a diverse phenomenology given the differences in shocks
expanding velocity, external density, and surrounding environment. Hence, VHE
observations provide a privileged channel to shed light into the physics of
transient events in an energy range particularly important for the
discrimination among different emitting scenarios. Although in operation since
the beginning of the 2000s, current-generation IACTs can still lead to the
discovery of new transient phenomena and to deepen our understanding of the
TeV physics of these newly identified VHE sources. However, the IACT community
is working toward the construction of the CTA Observatory, which is the next-
generation ground-based observatory for Cherenkov astronomy. It will be
composed by two arrays composed of telescopes of up to three different sizes,
located in the northern (Roque de Los Muchachos Observatory, Spain) and
southern (Paranal Observatory, Chile) hemispheres. It will cover the energy
range between 20 GeV and 300 TeV and it will count with improved sensitivity
with respect to current IACT experiments. It will have unprecedented
sensitivity at short timescales (see Figure 10), making it a unique laboratory
for VHE transient astrophysics. The Transients Key Science Project of the CTA
Observatory (Cherenkov Telescope Array Consortium et al., 2019) defines the
core program for the follow-up of transient sources (Bošnjak et al., 2021;
Carosi et al., 2022; López-Oramas, 2023), including GRBs, GWs, neutrino
counterparts or the large zoo of Galactic transient sources (novae,
microquasars, magnetars, flaring PWNe, etc.), among other serendipitous
transitory events.

Figure 10: Differential flux sensitivity versus time of the CTA Observatory
Northern Array in its Alpha configuration (blue), which accounts for the
initial construction phase, compared to Fermi-LAT (green) at different
energies. Figure taken from https://www.cta-observatory.org/science/ctao-
performance/ (accessed on 27 March 2024).

The improved sensitivity of CTA, together with its better angular and energy
resolution and large energy coverage, will allow for the discovery of new
transient and multi-messenger phenomena, widening the population of current
known sources.

###  9.1 Novae

We can expect to detect other recurrent symbiotic systems in the VHE regime.
There are 10 confirmed recurrent novae in the Galaxy with recurrency times
between 10 and 80 years approximately (Schaefer, 2010). This number could,
however, be larger since other systems with very massive WDs could also be
recurrent, they have simply not yet been identified as so. The next imminent
eruption is that of T CrB, a symbiotic nova which shows a recurrence of about
80 years Schaefer (2023) and for which the next explosion is calculated to
happen on 2024.4 ± 0.3 (Schaefer et al., 2023). The latest reports indicate
that the source entered the so-called pre-eruption dip and its B and V
magnitudes are slowly decreasing, as seen in Figure 11. The accretion disk
reached a minimum in August–September 2023 and it is showing a fast
rebrightening (Munari et al., 2024). T CrB is closer than RS Oph (0.9 pc vs.
2.4 pc) and it is expected to reach a flux of about 10 times larger than RS
Oph. Its peak optical magnitude is can reach magnitude 2.9 as in previous
eruptions, being one of the brightest novae observed.

Figure 11: (Left) Optical lightcurve of T CrB during the 1946 eruption in B
(blue dots) and V (green circles) magnitudes. A similar lightcurve is expected
in the next eruptive event; (Right) Current fading of the B (blue circles) and
V (green circles) of T CrB, revealing that the source has entered the pre-
eruption dip. Figures from (Schaefer et al., 2023; Schaefer, 2023).

HE emission from classical novae was an unexpected discovery, due to the low
density of the surrounding environment, but they are now the most frequently
detected type of novae. Whether classical novae can emit at VHE is still an
open question, although if shocks operate similarly in symbiotic and classical
systems, then we could potentially expect VHE emission also from these
systems.

###  9.2 Microquasars

The improved sensitivity of CTA will likely lead to the detection of TeV
transient emission from flaring microquasars. When extrapolating the Cygnus
X-1 hint observed by (Albert et al., 2007) in the VHE regime, we see that the
CTA northern array will be able to detect a similar flare with high
significance in only 30 min of observation (see Figure 12) (Bošnjak et al.,
2021). We can expect the future CTA observatory to detect transient emission
from other microquasars, probing particle acceleration in jets.

Figure 12: Simulated SED of Cyg X-1 as seen by CTA-North (black points) during
a flaring episode similar to that reported in Albert et al. (2007) (magenta
points). Reprinted from (Bošnjak et al., 2021) .

###  9.3 Supernovae

As discussed in Section 4, the CTA observatory is expected to detect CCSNe up
to a few Mpc of distance. According to (Sarmah, 2023), an SN like the recent
type II-P SN 2023ixf should be detectable by future experiments such as the
CTA even at 7–10 Mpc. In the case Galactic CCSNe, there is the open
possibility that current IACTs could detect a VHE counterpart. Since neutrino
bursts take place during a core-collapse event, they are expected to precede
the electromagnetic radiation from the SN when reaching the Earth, as it
happened in SN 1987A. Hence, neutrino bursts are good alert trigger systems
for a Galactic CCSNe event, which are rare events in our Galaxy. However, it
is expected that the VHE emission is absorbed due to $\gamma$$\gamma$
annihilation during the first 7–10 days, approximately. It can be worth trying
to catch the VHE counterpart during the first hours after explosion, since
models do not manage to simulate the expected gamma-ray emission so early on
and since an observation like this will definitely help constrain the
theoretical scenarios for such unique events.

###  9.4 Crab Nebula Flares

Next-generation instrumentation such as the CTA observatory will count with an
increased sensitivityhttps://www.cta-observatory.org/science/ctao-performance/
(accessed on 27 March 2024). to short timescale transient events (López-
Oramas, 2023). It has been explored how the northern array of the CTA
observatory will be sensitive to flaring emission from the Crab Nebula (Mestre
et al., 2021; López-Oramas et al., 2022). The high sensitivity of the array
will likely allow for the detection of both the synchrotron end at low
energies in few ($\leq$5) hours in the case of hard synchrotron flares for
magnetic fields with similar or larger intensity than that of the nebula. Even
current facilities such as MAGIC could potentially detect bright flares
(similar to that of 2011) or at least set strong constraints (see Figure 13).
In the case of the IC component, TeV emission could be detectable if the
energy of the electrons is boosted and under certain scenarios, such as soft
spectra and mG magnetic fields (right panel of Figure 13).

Figure 13: Simulated SEDs of the Crab Nebula during different flares compared
to steady state. (Left) Synchrotron regime (Right) IC component. The 5 h
sensitivity CTA-North array (bold line), the 5 h sensitivity of the 4 LSTs of
CTA-North (dashed line), and the 10 h sensitivity of the MAGIC telescopes
(dotted–dashed line) are represented. Figure rearranged and reprinted with
permission from (Mestre et al., 2021).

###  9.5 FRB and Magnetars

The nature of FRBs represents one of the most enigmatic (and recent) hot
topics in time-domain astrophysics. The discovery of the association within an
FRB-like emission and the Galactic magnetar SGR 1935$+$2154 provided possible
evidence about the origin of these events. Magnetars already triggered the
interest of IACT at VHE in the search for persistent emission; see, e.g.,
(Aleksić et al., 2013) and more recently as transient sources as they may
undergo important GFs: rare and brief ($\sim$$0.1$ s) bursts of hard X-rays
and soft gamma rays, recently detected up to the GeV range (Fermi-LAT
Collaboration et al., 2021). The energy release of a GF may be remarkable,
reaching a total value of $10^{44}\div 10^{46}$ erg. Although many theoretical
models do not envisage magnetars as VHE emitters during their quiescent state,
the possibility of having VHE emission during flaring episodes cannot be ruled
out. The April 2020 flaring activity of SGR 1935$+$2153 gathered an
exceptional extended multi-wavelength coverage, mainly thanks to the above-
mentioned FRB connection. The observed X-ray activity showed a harder spectra
with respect to the typical bursts from SGR 1935$+$2154 (and other magnetars)
although its intensity was relatively moderate and significantly too faint to
be classified as a GF. Observations by H.E.S.S. and MAGIC ruled out possible
extended emission up to the VHE band for this event (Abdalla et al., 2021;
López-Oramas et al., 2021). Very recently, a candidate magnetar GF from the
nearby galaxy M82 has been followed up by MAGIC with a (preliminary) non-
detection at VHE (Paneque, 2023).

The high sensitivity to short timescale signals foreseen for the CTA
observatory will make it a perfect instrument to magnetars flaring activity
follow-up. Furthermore, the new radio facilities that will operate at the time
of the CTA will provide the detection of up to hundreds of FRBs per day. Many
of these will have good localizations and will be inside the CTA field of
view, making it possible to search for prompt and/or delayed VHE emission
corresponding with radio activity, unveiling the still-puzzling connection
between FRB and magnetars.

###  9.6 GRBs and GWs

Whether the sources of GWs are BNS merger or CCSNe, electromagnetic emission
up to the VHE may be envisaged. Expectations for VHE emission from CCSNe
likely pose these sources out of reach for current IACTs (see Section 4). On
the other hand, although challenging, the VHE counterpart of BNS mergers
stands in a better chance of detection for running facilities. The link
between sGRB and GWs has indeed been proven by the detection of GW/GRB
170817A, while a hint for VHE also from sGRB (long GRBs are now known to be
VHE emitters) has been achieved by MAGIC in the case of the short GRB 160821B
(Acciari et al., 2021). Thus, it is justified to assume that each BNS merger
may result in an sGRB launching a relativistic jet. However, GW 170817, the
only event with a firmly detected electromagnetic counterpart, did not show
any hint of GeV-TeV emission and detailed emission models for this event do
not foresee a VHE component strong enough to be detectable with current IACTs
(Salafia et al., 2022). Regardless, such negative prospects have to be
considered as not conclusive. Within the framework of an off-axis GRB as the
source of electromagnetic radiation in a BNS merger, geometry plays a key role
in the expected emission at all wavebands. In the case of GW 170817, the
relatively large viewing angle of $\theta$$\sim$$15^{\circ}\div 25^{\circ}$
played a key role in suppressing the VHE emission component. Viewing angles
closer to an on-axis geometry may certainly increase the flux expected at VHE
although anticipating the peak time of the emission. Furthermore, the
circumburst conditions may also have a significant impact on the expected
spectrum. The low interstellar medium density for GW 170817 ($10^{-4}$ cm-3)
stands as a disadvantage for a detectable VHE signal. In (Salafia et al.,
2022), an example of light curves at 1 TeV for a jet with the same parameters
as that of GW 170817, but with a denser circumburst medium ($5\times 10^{-2}$
cm-3) (Figure 14 left plot).

Figure 14: (Left) 1 TeV light curve expected for a GW 170817-like event under
more favorable conditions. The circumburst medium density is fixed to
($5\times 10^{-2}$ cm-3), while different viewing angles are plotted.
Reprinted with permission from (Salafia et al., 2022). (Right) Detectability
of sGRBs with CTA-South (upper panels) and CTA-North (lower panels) array for
the simulated events given latency and exposure time. The left panels show a
subset of the sources with viewing angle $<$$10^{\circ}$, while the right
panels show all sources with view $<$$45^{\circ}$. Reprinted with permission
from (Gershon Green et al., 2023).

It is important to remark that one of the keys for a successful GW follow-up
lies in the synergies with other facilities and in the optimization of the
observing strategy. As a matter of fact, GRB 180817A was a sub-threshold
event, several orders of magnitudes less luminous than a standard GRB,
although located much closer to us compared to the average GRB population.
Hence, this event may not have been followed up if no gravitational wave was
detected. The extensive multi-waveband follow-up has proved to be the key to
identify the counterpart and its nature, representing a takeaway message for
future observations. Within this framework, a large effort is currently taking
place within the CTA consortium to optimize VHE follow-up strategy for near
future observations (LVK run O5, planned for 2027
https://observing.docs.ligo.org/plan/ (accessed on 27 March 2024).), where the
parallel operation of more GW interferometers will allow for the localization
of new merger events with much better precision.

In (Gershon Green et al., 2023), a preliminary estimation of CTA detection
capabilities on GWs/GRBs is reported. A set of simulated BNS mergers and their
associated GW signals (Petrov et al., 2022) are used taking into account
realistic astrophysical distributions of masses, spins, distances, and sky
locations of the neutron stars. Each merger is associated with a simulated
sGRB. The authors use an empirical approach that does not need to assume any
specific particle population or radiative process for the production of gamma-
rays according to the empirical evidence collected by IACT observations of
GRBs. Then, the luminosity in the TeV range is assumed to be comparable to the
one at lower energies (in the soft X-ray range), and the spectra are assumed
to have a photon index around $-$2.2. The synthetic spectra are then analyzed
by means of CTA analysis tools and Instrument Response Functions (IRFs). The
estimation of the integration time required to achieve a detection with CTA is
reported in Figure 14 (right panel) as a function of the time needed by the
telescopes to point at the region of interest.

It is clear that the CTA will represent a unique instrument to achieve a VHE
detection of a GW counterpart, shedding light into the physics of GRB and BNS
mergers dynamics and setting a key step for the future multi-messenger
astronomy.

##  10 Conclusions

The past decade marked the beginning of the era of multi-messenger
observations accompanied, in parallel, by the remarkable development of time-
domain astronomy. In the GeV-TeV energy range, in particular, new sources of
VHE gamma rays have been identified, opening new perspectives for transient
astrophysics in this energy regime.

One of the newly identified class of Galactic VHE emitters are novae thanks to
the recent discovery of VHE signal of hadronic origin in the recurrent
symbiotic nova RS Oph. These novae create bubbles of enhanced cosmic-ray
density in their close environment at $\sim$pc scales. Other (recurrent)
symbiotic systems such as T CrB are expected to be detected by the current
generation of IACTs in the very near future. The discussion on whether
classical novae are TeV emitters is still open and will hopefully be addressed
over the next years.

At extragalactic distances, the detection of the TeV counterpart of GRBs was
finally achieved by current IACTs after a quest which lasted for more than 20
years. TeV GRBs were first detected in 2018–2019 and since then a total of
four long GRBs located at redshifts between 0.0785 and 1.1 have been reported
by IACTs during the afterglow phase. Furthermore, the brightest GRB of all
times, GRB 221009A, has been recently detected by LHAASO above the 10 TeV,
opening new possibilities for GRB study with instruments not originally
thought for GRB follow-up due to their relatively large energy threshold such
as particle array detectors or the ASTRI Mini-Array. The hint of detection at
VHE from the short-GRB 160821 also proved the possible link between VHE and
the GW emission from BNS mergers. In the near future, thanks to the improved
sensitivity in the GW interferometers and the new-generation IACTs, follow-up
observations of GW + VHE will connect the gamma-ray emission with the
formation and evolution of the GW-central engine, shedding light into the
physics of these extreme cosmic events.

New intriguing transients such as FRBs and their connection with magnetars
represent a very recent development and a still marginally explored field for
VHE transient astrophysics. Although magnetars per se are not found to be
steady gamma-ray emitters, they have been detected in the GeV range by Fermi-
LAT during giant flare episodes. Furthermore, magnetar-based models predict
emission up to the VHE correlated in time with FRBs. The higher sensitivity to
short IACTs compared to space-based instruments represents a unique feature
for exploring the wide and complex range of transient phenomenology embedded
in the magnetar–FRB scenario.

The current generation of IACTs is still on the catch of other transient
events that are known HE emitters such as the enhanced flaring emission from
PWNe, notably the Crab Nebula or flaring (massive) microquasars as, e.g.,
Cygnus X-1 or Cygnus X-3. Other transient phenomena are still elusive both in
the HE and VHE regimes, such as core-collapse SNe (despite some candidate
associations at HE), the VHE counterpart of kilonovae from GWs, or TDEs. The
identification of any of these sources as TeV emitters will undoubtedly push
the boundaries of our knowledge and open new research areas. In this regard,
future instrumentation such as the CTA observatory, with enlarged energy
range, improved (short timescale) sensitivity, and fast response capabilities
will set new frontiers in time-domain TeV astrophysics.

The authors contributed equally to this work. All authors have read and agreed
to the published version of the manuscript.

This research is part of the project RYC2021-032991-I, funded by
MICIN/AEI/10.13039/ 501100011033, and the European Union
“NextGenerationEU”/PRTR.

Not applicable.

The authors declare no conflicts of interest.

Abbreviations The following abbreviations are used in this manuscript:
BH Black hole CCSNe Core-collapse supernovae FRB Fast radio burst GRB Gamma-
ray burst GW Gravitational wave HE High energy IACT Imaging Air Cherenkov
Telescopes IC Inverse Compton NS Neutron star RG Red giant SSC Synchrotron
self Compton SED Spectral energy distribution sGRB Short gamma-ray burst SNe
Supernovae TDE Tidal disruption event UL Upper limit VHE Very-high-energy WD
White dwarf

[custom]

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