# Gamma-ray Bursts: 50 Years and Counting!
## 7 The High-Energy (AGILE and Fermi) Era (2007-Ongoing)
### Fermi (2008-Ongoing)  
In those same years, another NASA gamma-ray mission, the Gamma-ray Large Area Space Telescope (GLAST), later renamed the _Fermi_ Gamma-Ray Space Telescope (FGST), was launched on 11 June 2008. _Fermi_ carries two scientific instruments on board, the Large Area Telescope (LAT) [124] and the Gamma-ray Burst Monitor (GBM) [125]. The LAT is a gamma-ray imaging detector that detects photons with energies from about 20 MeV to about 300 GeV, with a field of view of about 20% of the sky, capable of exposing all parts of the sky for about 30 min every 3 h in sky survey mode. The GBM consists of 14 scintillation detectors, namely, 12 NaI crystals for the range from 8 keV to 1 MeV, primarily used for onboard triggering, onboard and ground localization, and spectroscopy, and two bismuth germanate (BGO) crystals with sensitivity from 150 keV to 30 MeV. The GBM is used for spectroscopy and is capable of detecting GRBs in this energy range throughout the entire sky not occluded by the Earth. Thanks to these instruments, _Fermi_ offers numerous advantages compared to all previous instruments and, in particular, an enormous range of energies, spanning more than 7 orders of magnitude, including the largely unexplored 10 GeV-100 GeV band and a huge field of view. These allow _Fermi_ to observe \(\sim\)200 GRB/year with observations from 8 keV to 40 MeV and \(\sim\)15 GRB/year with observations from 8 keV to 300 GeV with excellent spectral and timing capabilities, setting _Fermi_ as one of the leading instruments in the current study of the gamma-ray sky. These two instruments have, in many ways, revolutionized our understanding of GRBs. In its years of operation, _Fermi_ observed a lot of GRBs, both with the GBM and with the LAT, and produced various catalogs. The latest ones are dated 2018 (though regularly updated) for both instruments, boasting as many as 2356 GRBs detected by the GBM [126] and 186 (approximately 7-8% of those seen by the GBM) above 100 MeV seen by the LAT [127]. For those detected by both instruments, the LAT band emission usually lasts much longer than the GBM band emission, and it decays as a power law [128; 129]. This has been interpreted as an external-shock origin of the observed \(>\) 100 MeV emission of the GRB, after the prompt-emission phase has ended [128; 129; 130; 131]. Another, yet unexplained, feature that was found confronting the observations of the GBM and LAT is that, in some GRBs, the LAT GeV emission has a delayed onset with respect to the MeV emission. This was not predicted by theoretical models, and there is still no agreement on the origin of such emission, even though many mechanisms have been proposed. Still, the presence (or lack) of delays between the emissions at different energies has been used to put strong constraints on Lorentz Invariance Violations (LIVs), a feature allowed by many models of Quantum Gravity (e.g., GRB090510 [132]). The LAT was able to detect the presence of photons with rest-frame energies exceeding 100 GeV in several bright GRBs. Some examples are GRB080916C [133], i.e., the very first bright LAT burst; GRB090510 [132]; and GRB130427A [134], which, at the time, was a record-setting bright GRB detected both by _[PERSON]_ and by _[PERSON]_. The existence of these photons was not consistent with the predictions of the standard fireball synchrotron internal-shock model of the time and placed strong constraints on GRB physics, including the minimum bulk Lorentz factor (from opacity arguments) [135], particle acceleration mechanisms in relativistic shocks, and relativistic particle radiation mechanisms. These photons can also be used to study the extragalactic background light (EBL) via the expected attenuation of high-energy photons due to two-photon pair production (e.g., [136]).