**1. Introduction**

In the past few years, new observations have led to several breakthroughs in the field of high energy astrophysics. The first detection of the binary neutron star merger event GW170817 by the Laser Interferometer Gravitational-Wave Observatory (LIGO) and the Virgo Consortium coinciding with a short-duration gamma-ray burst (GRB) [1–3] was a watershed moment in astronomy. For the first time, both gravitational waves and electromagnetic waves were detected from the same astrophysical source. Furthermore, this detection firmly placed the merger of neutron star binaries as progenitors of (at least, some) short GRBs. This event was accompanied by a "kilonova", also robustly establishing neutron star mergers as critical contributors of the production of heavy elements in the Universe [4,5]. These exciting observations have reinvigorated the interest of the astronomical community in understanding the underlying physics of gamma-ray bursts, their associated jets, and progenitors.

A second major breakthrough was the detection of the very high energy (>100 GeV) emission from GRBs by the Major Atmospheric Gamma Imaging Cherenkov (MAGIC) telescopes and High Energy Stereoscopic System (H.E.S.S.) [6,7]. These discoveries provided crucial data for relativistic jets models in which gamma-ray bursts are produced, as well as the nature of high energy radiation processes. On the other hand, neutrinos from GRBs are expected following the interactions of energetic protons that may be accelerated in the GRB environment, however no neutrinos from GRBs have been firmly detected yet [8]. As a result of this lack of detection, one critical piece of information regarding the possible GRB radiation mechanism is still missing. With the advent of new multi-messenger observations, it is becoming increasingly important to revise theoretical models to understand the physics in the vicinity of black holes and neutron stars, the nature of relativistic jets, and the origin of GRBs as the most energetic events in the Universe.

These recent observations add and extend the knowledge gained in the past several decades about the nature of GRBs. Observationally, we know that the vast majority of GRBs have the following common features: (i) Most GRBs consist of highly variable pulses of gamma-ray photons typically lasting dozen of seconds, having a non-thermal spectrum peaking at ∼a few 100 keV. (ii) The occurrence rate is approximately once per

**Citation:** Bošnjak, Ž.; Barniol Duran, R.; Pe'er, A. The GRB Prompt Emission: An Unsolved Puzzle. *Galaxies* **2022**, *10*, 38. https:// doi.org/10.3390/galaxies10020038

Academic Editors: Elena Moretti and Francesco Longo

Received: 13 December 2021 Accepted: 4 February 2022 Published: 22 February 2022

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day from random directions in the sky [9–11]. (iii) The prompt emission is followed by the afterglow emission detected at lower energies (X-ray, optical, and radio) lasting for days, weeks, months, and (in radio band) years after the main event. (iv) For a number of GRBs, long lasting gamma-ray photons with energy >100 MeV have been observed during the afterglow phase.

The extreme nature of these events—short variability time scales ∼10 ms, extreme energy of up to (isotropic equivalent) 10<sup>55</sup> ergs [12], emission over a broad energy scale, from optical to TeV, and the connection of the origin of these explosions with black-hole formation, have posed a challenge for the theoretical modeling of these events. In this review we will focus on the prompt emission of gamma-ray bursts, and provide a short summary of some of the most recent results, and of the proposed models for this emission episode. For more extensive reviews see, e.g., [13–17].
