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Gamma-Ray Bursts at TeV Energies: Theoretical Considerations

Gamma-ray bursts (GRBs) are the most luminous explosions in the Universe and are powered by ultra-relativistic jets. Their prompt $γ$-ray emission briefly outshines the rest of the $γ$-ray sky making them detectable from cosmological distances. It is followed by, and sometimes partially overlaps with, a similarly energetic but very broadband and longer-lasting afterglow emission. While most GRBs are detected below a few MeV, over a hundred were detected at high ($\gtrsim0.1\;$GeV) energies and several have now been observed up to tens of GeV with the \textit{Fermi} Large Area Telescope (LAT). A new electromagnetic window in the very high energy (VHE) domain ($\gtrsim0.1\;$TeV) was recently opened with the detection of afterglow emission in the $(0.1$\textendash$1)\,$TeV energy band by ground-based imaging atmospheric Cherenkov telescopes. The emission mechanism for the VHE spectral component is not fully understood, and its detection offers important constraints for GRB physics. This review provides a brief overview of the different leptonic and hadronic mechanisms capable of producing VHE emission in GRBs. The same mechanisms possibly give rise to the high-energy spectral component seen during the prompt emission of many \textit{Fermi}-LAT GRBs. Possible origins of its delayed onset and long duration, well into the afterglow phase, with implications for the emission region and relativistic collisionless shock physics are discussed. Key results for using GRBs as ideal probes for constraining models of extra-galactic background light and intergalactic magnetic fields, as well as for testing Lorentz invariance violation, are presented.