**4. Discussion: A Look into the Future**

With the advent of observations at very high (GeV/TeV) energies by *Imaging Atmospheric Cherenkov Telescopes* such as MAGIC [6] and H.E.S.S. [7], and the perspective being opened by the future multi-messenger environment for gamma-ray bursts, the premise of radiation models will inevitably be revisited. At very high energies, the future Cherenkov Telescope Array (CTA) will provide improved sensitivity to up to an order of magnitude with respect to current IACTs [215,216]. Although the recently provided detection rate is modest (during the prompt phase, it is expected to be . 1 per year [217,218]), if CTA provides GRB observations with high photon statistics, it will help constrain emission models (e.g., the properties in the emission site of high-energy photons). The observed variability could help differentiate between emission mechanisms [217]. As the *Fermi* LAT spectra often displays a hard power-law spectrum extending to GeV energies, the observations of the high energy part could provide the information on the total radiated energy, and the bulk Lorentz factor can be constrained if the high-energy spectral cutoff due to pair production is identified [217]. At the low energy end, the *SVOM* (Space-based multi-band astronomical Variable Objects Monitor) mission aims to survey the high-energy sky and follow-up transients at optical and X-ray wavelengths [219]. Its main goals are observations of the high-redshift GRBs (*z* > 5), and faint/soft nearby events. It will also likely be the alert facility for CTA, opening e.g., the possibility of detecting low luminosity events which are not triggered by the current missions [220]. Other future multi-messenger facilities for GRB-related science include, e.g., the third-generation gravitational-wave observatory Einstein Telescope [221], the development of the extension of the IceCube Neutrino Observatory IceCube-Gen2 [222], and ATHENA [223] satellite for the X-ray domain. Upper limit of neutrino flux from GRBs [224] as well as the observations at longer wavelengths (e.g., using the upcoming Vera Rubin Observatory or the Square Kilometer Array-SKA) could provide information on jet composition—baryonic or magnetic jet. The observational advances need however to be followed by theoretical effort, i.e., numerical simulations of the processes involved in the production of prompt emission, such as energy dissipation and particle acceleration, in order to fully understand the extreme conditions in which gamma-ray bursts are produced.

**Author Contributions:** All authors Ž.B., R.B.D. and A.P. have contributed to writing—review and editing. All authors have read and agreed to the published version of the manuscript.

**Funding:** R.B.D. acknowledges support from the National Science Foundation under grants 1816694 and 2107932. A.P. was partially supported by the EU via the ERC consolidating grant 773062 (O.M.J.). Ž.B. acknowledges support from the Croatian Science Foundation (HrZZ) Project IP-2016-06-9782.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable

**Acknowledgments:** We thank P. Beniamini, D. Giannios and P. Kumar for useful discussions related to this manuscript.
