3.1.4. Particle Acceleration

To produce the non-thermal GRB spectra, part of the kinetic energy needs to be dissipated and used to increase the random motion of the outflow particles and/or accelerates some fraction of them to a non-thermal distribution. Once accelerated, these high-energy particles emit non-thermal photons.

The most widely proposed particle acceleration mechanism within the internal shock scenario is the Fermi mechanism [88]. In this process, the accelerated particles cross the shock fronts, and, during each crossing, their energy increases at a constant rate ∆*E*/*E* ∼ 1. The accelerated particles have a power-law energy distribution *N*(*E*) ∝ *E* <sup>−</sup>*<sup>δ</sup>* with index *δ* ≈ 2.0–2.4 [89].

Dissipation mechanisms in magnetized outflows have been discussed at length (see Ref. [90] and references therein). Furthermore, particles may also be accelerated via Fermi mechanism in shock waves, but it has been pointed out that, in highly magnetized plasma, this process may be inefficient [91].

## 3.1.5. Radiative Processes

After kinetic energy dissipation and particle acceleration, energy conversion is needed to produce the non-thermal spectra observed in GRBs. The most discussed radiative model in the literature is the optically thin synchrotron emission [74,92–95], accompanied by synchrotron-self Compton (SSC) at high energies [96–98].

Recently, with mounting evidence of thermal components in GRB spectra [63,99], the photospheric model has acquired growing relevance [61,100,101]. This model is not in contrast with and has to be considered complementary to the synchrotron+SSC emission, which originates from a different region of the outflow.
