**3. Gamma-Ray Bursts Observations at High Energies**

The first systematic and comprehensive study of GRBs population was carried out by the space-based telescope Compton Gamma-Ray Observatory (CGRO), which operated for about 9 years between April 1991 and June 2000. Thanks to its four onboard instruments and, in particular, the Burst And Transient Source Experiment (BATSE: 25 keV–2 MeV) and the Energetic Gamma-Ray Experiment Telescope (EGRET: 20 MeV–30 GeV), it was possible to have an energy coverage ranging from the soft X-rays to the HE gamma rays. This provided the first meaningful interpretation of the GRB phenomenon. Specifically, thanks to EGRET, it was possible to start studying the properties of the high-energy emission (&10–20 MeV) of GRBs for the very first time. A notable event was detected by EGRET on 17 February 1994, GRB 940217 [37]. The burst had a duration of 180 s as measured by BATSE. Ten HE photons were detected by EGRET with energy up to ∼3 GeV during the prompt emission. Eight other HE photons were detected in the following ∼600 s. After the occultation due to the Earth, EGRET registered another 10 photons more than 4700 s after the burst trigger. The highest-energy photon detected in this observation phase had an energy of 18 GeV (see Figure 1), and for many years it represented the highest energy photon ever detected from a GRB. This delay in high-energy emission was observed in other GRBs detected by EGRET, although not as evident as for GRB 940217 (see [38]). Furthermore, the detection of HE photons pointed out to the possible presence of additional spectral components overlapped to the classical sub-MeV band emission. From a different perspective, the presence of a delayed emission was also considered as an opportunity for TeV detectors, such as IACTs, that needed to be repointed for start follow-up but also for extensive air shower (EAS) arrays.

**Figure 1.** EGRET light curve for the GRB 940217 with the detection of an ∼18 GeV photon that occurred about 90 min after the GRB onset. From https://fermi.gsfc.nasa.gov/science/resources/ docs/vwp/. Available online: (accessed on 10 April 2022).

A hint of a distinct emission component in the HE range was found in the case of GRB 941017 [39]. This event showed a &200 MeV signal rising between 14 and 47 s after the *T*<sup>0</sup> and lasted for approximately 200 s in addition to the typical GRB emission which peaked at .few hundred keV. The HE component is well fitted by a power law with index close to −1 up to 200 MeV throughout all the burst duration, while the low-energy spectrum was well described by the classical band function. Data were inconsistent with a simple synchrotron model interpretation, and other theoretical emitting scenarios were considered, such as synchrotron self-Compton (SSC) from the reverse shock, created when the GRB ejecta are decelerated by the ambient medium. Additional interpretations were also considered, such as a possible hadronic origin of the HE component, as well as an HE emission taking place in external shocks [33]. Despite these earliest observations that helped significantly in determining some HE properties of GRB, the limited statistics and the large dead time typical of EGRET did not allow to measure precise spectra and study in detail the short timescale variability in the emission, especially during the prompt phase. Many questions were left unanswered after EGRET stopped operations in 2000, mostly related to the jet physics, particle acceleration, and to the nature of the high-energy emission. In this context, there were many expectations for the launch of AGILE (Astro-Rivelatore Gamma a Immagini Leggero) and *Fermi Gamma-ray Space Telescope* (*Fermi* in short). Using silicon trackers, the limitations of the old generation of gamma-ray imagers, such as the small FoV and the large dead time, were partially solved. AGILE, launched in 2007, was the first instrument with this kind of technology, followed by *Fermi* in 2008. These satellites opened a new era in the studies of GRBs in the HE band.

AGILE's onboard instrumentation includes a gamma-ray imaging detector (GRID) sensitive in the 30 MeV–50 GeV band, a hard X-ray monitor (SuperAGILE: 18–60 keV), and a mini-calorimeter (MCAL) non-imaging gamma-ray scintillation detector sensitive in the 350 keV–100 MeV energy range [40]. With the detection of GRB 080514B [41], AGILE confirmed the presence of a delayed and relatively long-lasting high-energy emission, as seen in the EGRET events. The burst was detected by all the instruments onboard AGILE: GRID detected photons from 25 MeV up to 300 MeV, while in the hard X-ray band (SuperAGILE), the 17–50 keV light curve showed a multi-peaked structure with a total

duration of 7 s. The high-energy emission did not show any correlation with these peaks, and only three photons above ∼30 MeV were detected within 2 s from *T*0. All the other high-energy photons were recorded when the X-ray emission had already faded, up to ∼30 s after the burst onset.

GRB 100427B [42] is another notable GRB detected by AGILE. Both the MeV and GeV light curves show two bumps where the second peak is broader than the first, with no significant delay with respect to the lower-energy emission in the X-ray band. The second bump resulted harder then the first, and spectral evolution between the bumps and the inter-bump region in MCAL data were detected at the level of 4.0*σ*. A single power law was shown to be adequate to model the spectrum from 500 keV to 3.5 GeV, given that the spectral index of the MCAL + GRID data and GRID data only were compatible with each other. Even if the redshift was not measured for this GRB, given the highest energy photon of 3.5 GeV, the minimum Lorentz factor during the prompt emission was constrained to be between 50 and 900. For other GRBs observed by AGILE, not detected by GRID, upper limits were derived and found to be consistent with an extrapolation of the band spectrum up to GeV energies; see [43].

*Fermi* was launched in 2008, approximately one year after AGILE. *Fermi* uses the same detector technology as AGILE and it was designed to be a proficient gamma-ray satellite with improved capabilities with respect to previous-generation gamma-ray detectors. The spacecraft hosts two instruments on board. The gamma-ray burst monitor (GBM) is composed of 14 scintillators (twelve sodium iodide and two bismuth germanate) and covers the energy range from a few keV to ∼30 MeV [44]. With a field of view of almost 4*π*, it is devoted to the detection of GRBs or other burst-like sources and to the quick distribution of GRB localizations. The second instrument is the pair-production telescope LAT [17], operating in the energy range 20 MeV–300 GeV. The adoption of the silicon strips detector technology for the *Fermi*-LAT led to substantial improvements in terms of angular resolution and timing capabilities. Thanks to a big calorimeter, the sensitive energy range of the *Fermi*-LAT extends up to few hundreds of GeV, also providing a good energy resolution. Owing to its efficient design, *Fermi* delivered and is still delivering more detailed results and the highest statistics for studying GRBs in the HE regime. At the same time, it is providing an invaluable overlap with ground-based VHE facilities.

GRB 080825C [45] was the first GRB detected by *Fermi*-LAT, a long burst with *T*<sup>90</sup> = 27 s. The highest-energy photon was a (572 ± 58) MeV photon detected at ∼*T*<sup>0</sup> + 28 s, just after the low-energy emission measured in *Fermi*-GBM faded almost completely. The spectrum of GRB 080825C in different time bins is well fitted by a band function with a hard-to-soft evolution of the *νF<sup>ν</sup>* spectrum peak energy (*E*peak). In the last time bin, the spectrum is well described by a power law with a harder index −1.95 ± 0.05. This property and the low flux ratio between the first two peaks in the *Fermi*-LAT light curve may suggest a different region of origin for their emission: within the internal and external shock respectively [45].

GRB 080916C [20] is the second *Fermi*-LAT detected burst and one of the brightest in the *Fermi*-LAT GRB sample, with a measured redshift of *z* = 4.35 ± 0.15 and a total isotropic energy release of 8.8 <sup>×</sup> <sup>10</sup><sup>54</sup> erg. Compared to the signal measured in *Fermi*-GBM, this GRB showed a delayed onset of the LAT pulse and a longer-lived emission in the &100 MeV band. These features will be confirmed in other GRBs detected at HE. The comparison between the *Fermi*-GBM and the *Fermi*-LAT light curves (Figure 2) showed that the first *Fermi*-GBM peak has no corresponding peak in the *Fermi*-LAT light curve. The first *Fermi*-LAT pulse is instead temporally coincident with the second *Fermi*-GBM peak. A common origin for the two peaks but in spatially different regions is the most likely explanation, with different pairs of colliding shells within the internal shock scenario. The long-lasting emission above 100 MeV was detectable up to *T*<sup>0</sup> + 1400 s, well after the low energy emission faded. The time decay of the high energy flux is well fitted by a power law *t* <sup>−</sup>*<sup>α</sup>* with *<sup>α</sup>* <sup>=</sup> <sup>−</sup>1.2 <sup>±</sup> 0.2, a value that is typical for other *Fermi*-LAT-detected GRBs. The *Fermi*-GBM flux decays as *t* <sup>−</sup>0.6 up to *<sup>T</sup>*<sup>0</sup> <sup>+</sup> 55 s with a steepening in the index (*α*∼−3.3) afterward. This might indicate a different nature of the high-energy emission, although no

spectral hardening is seen in the *Fermi*-LAT late spectrum, as in the case of GRB 080825C. As in other HE detected burst, GRB 080916C data were used to set a lower limit to the Lorentz factor of the blast-wave, Γmin = 887 ± 21. Even if most of the *Fermi*-LAT-detected GRBs belong to the long class, it helped to study the high-energy emission of short GRBs as well. Among them, some interesting cases are GRB 081024B [46] and GRB 090510 [21,47].

**Figure 2.** *Fermi*-GBM and *Fermi*-LAT combined light curve for GRB 080916C. Reprinted with permission from Ref. [20].

GRB 081024B is the first short GRB detected by *Fermi*-LAT, with a duration of 0.8 s. In addition for this GRB, the emission above 100 MeV is delayed and is long-lasting (*T*<sup>90</sup> = 2.6 s above 100 MeV).

GRB 090510 is a short GRB which was detected by both AGILE and *Fermi*. Both instruments confirmed the presence of a ∼0.1 s-delayed HE emission component after the onset measured in *Fermi*-GBM. The detection of a 30.5 GeV photon during the prompt phase allowed an evaluation of the bulk Lorentz factor that resulted in a very high lower limit of Γ*min* & 1200, assuming the estimated redshift *z* = 0.903. However, the most remarkable feature of GRB 090510 is that its time-integrated spectrum for the GBM+LAT prompt emission data cannot be fitted with a simple band function. An additional power law component with index −1.62 ± 0.03, dominant below 20 keV and above 100 MeV, is needed to describe the spectrum (see Figure 3) [21]. In the afterglow phase, a signal has been detected by *Fermi*-LAT up to ∼ *T*<sup>0</sup> + 150 s, which prompted several theoretical interpretations for both the prompt and afterglow phases. Some of them consider synchrotron radiation as theoretical interpretation of the low-energy (band) emission while the hard extra-component is generated by the synchrotron photons Compton upscattered by the same electrons accelerated in the shock (synchrotron self-Compton); see, e.g., [48]. This scenario is commonly used to model emission in other VHE sources, such as blazars, and the SSC component results are stronger for a large ratio of non-thermal electron to magnetic-field energy density and low values of Γ. However, in the case of GRB 090510, such an interpretation has difficulties in explaining the delayed onset of the high-energy emission. For example, the SSC model predicts a too-short delay in the assumptions of weak magnetic field [21]. Hadronic scenarios were also proposed but the proton injection isotropic-equivalent energy required is more than two orders higher than the one actually measured for the burst [49]. These observations of short GRBs show that they can be as relativistic as long GRBs and that they seem to have a better efficiency in emitting gamma rays, given that the energy emitted in the high-energy (100 MeV–10 GeV) band is greater than the one at low energy (20 keV–2 MeV). However, the statistic is still limited to few bursts to draw a definitive conclusion.

**Figure 3.** Time−integrated spectrum (upper panel) and model spectra (with ±1*σ* error contours) used to fit the emission of GRB 090510 in different time bins. Reprinted with permission from Ref. [21].

As a final example, it is worth to report the *Fermi*-LAT detection of GRB 130427A, one of the most powerful GRBs observed at redshift *z* = 0.34 [50]. The event showed the highest fluence (4.2 <sup>×</sup> <sup>10</sup>−<sup>3</sup> erg/cm<sup>2</sup> from 10 keV to 20 MeV), the highest energy photon (95 GeV at *T*<sup>0</sup> + 244 s), and the longest-lasting HE emission extending up to 100 ks after the trigger. It had a total apparent isotropic gamma-ray energy release of <sup>∼</sup>1.4 <sup>×</sup> <sup>10</sup><sup>54</sup> erg. The event showed a delayed emission starting about 10 s after the trigger when the *Fermi*-GBM brightest emission already ended. Therefore, the *Fermi*-LAT emission is temporally distinct from the one in *Fermi*-GBM (see, e.g., Figure 1 in [50]), and this suggests different regions or mechanisms for the two emissions. Having a 95 GeV photon in the early afterglow and a 32 GeV one at *T*<sup>0</sup> + 34.4 ks, it is difficult to accommodate them within the standard synchrotron emission from electrons accelerated in the external shock or in the SSC scenario, at least according to [50]. In [51], the combined X-ray, GeV, and optical data were used to fit the spectrum with a single synchrotron component while authors in [32] proposed an afterglow SSC emission to explain the long-lasting emission. These results show the puzzling interpretative scenarios of GRBs at HE and the lack of a clear physical explanation, both in the prompt and in the afterglow phases.

Summarizing, AGILE and *Fermi* showed that HE emission from GRBs share some common features:


These conclusions are the same as those resulting from the first *Fermi*-LAT GRB catalog, presented in [52], an in-depth systematic study of *Fermi*-LAT-detected GRBs in the first three years of the mission.
