*3.5. Magnetically Dominated Jet*

Some authors have considered models in which the GRB jet is magnetically dominated, that is, where magnetic fields dominate the jet luminosity at the base of the jet [149–152]. In these models, the jet is accelerated as it converts its magnetic energy to bulk kinetic energy. At larger distances from the central engine, the kinetic energy of the jet is transformed to thermal energy, commonly by magnetic reconnection instead of shocks (shocks in this

scenario seems to be too inefficient, e.g., [117,153]), and gamma-rays are produced. For general radiation properties in magnetically-dominated jets, see [154].

Jet acceleration in a magnetic model can occur due to the dissipation of the magnetic field in a striped configuration (such as that of a pulsar wind). This "striped jet" model invokes a magnetized jet with small-scale field reversals or "stripes" [120,155–160], where magnetic reconnection is able to start from small distances and continue as the jet accelerates and collimates. In the case of a black hole central engine, the alternating magnetic fields can be produced by the magneto-rotational instability in the innermost regions of the accretion flow [160]. For the case of a magnetar central engine, the alternating fields can be produced by an oblique dipole rotator [155,156]. Recently, the possibility of a distribution of stripe sizes in a magnetized jet has been considered [160].

In the striped jet, the jet accelerates (as the magnetization drops) up to a saturation radius *R*sat, where the magnetization reaches ∼1. Jet acceleration proceeds, not linearly with the radius as in the hot fireball model, but as Γ(*r*) ∝ *r* 1/3, e.g., [155,156]. Magnetic reconnection energizes particles and their emission spectrum will depend on the location of the Thomson photosphere compared to *R*sat. In this model, the observed gamma-ray prompt spectrum can be dominated by a Comptonized thermal spectrum [158,161,162]. Depending on the particle energy injection, the photospheric emission can be subdominant and a non-thermal spectrum can develop. The details of the non-thermal component in this model depend then on the particle energy injection, and several possibilities can be considered [158,163,164].

Jet acceleration can also occur by adiabatic expansion of the outflow [165,166]. In this case, the jet accelerates also as Γ(*r*) ∝ *r* 1/3 [166]. While there is no magnetic reconnection in this picture, energy dissipation can be driven by internal shocks within the outflow [167,168].

In all models that attempt to explain the prompt gamma-ray emission, reproducing the variability of the observed gamma-ray light curves is crucial. In the case of magnetic reconnection models, including the striped jet model mentioned above, a promising way to explain the light curve variability is to consider small reconnection regions that move relativistically in the co-moving frame of the jet with Lorentz factor ∼ few—10 as considered in the "minijets" or "jet in jet" model, relativistic turbulence model, and ICMART (Internal Collision-induced MAgnetic Reconnection and Turbulence) model [47,142,152,169–172]. It is likely that the directions of motion of these small reconnection regions, instead of being isotropically distributed in the comoving frame of the jet, are primarily perpendicular to the direction of the flow [171,172], and this would explain several of the observed prompt GRB temporal and spectral properties [171]. In this particular scenario, the prompt emission would be delayed with respect to the isotropic case, which would allow for the peak of the GRB afterglow to occur during the prompt emission phase in contrast to the simple isotropic model [172], see Figure 3. It would also explain the observed very steep X-ray emission, which is even steeper than the decay expected in the isotropic case [172].

**Figure 3.** Left: typical prompt emission light curves in the "minijets" model. Degree of anisotropy of minijets' directions increases from top to bottom. Different light curves (normalized) have been shifted vertically for displaying purposes. Anisotropy shifts the overall light curves to later times. The vertical dotted (dashed) lines for each light curve correspond to the *T*<sup>90</sup> duration. We include the observed peaks of the GeV light curves (black crosses) for the sample in [173], scaled for each of the simulated light curves. As the level of anisotropy increases, the peaks of the GeV light curves also shift to later times, making most of them consistent with *t* ≥ ∆/*c*, where ∆ is the shell thickness. In the simplest GRB afterglow model, the deceleration time will occur at times *t* ≥ ∆/*c* (see small black arrow) and if the peak of the GeV light curves correspond to the deceleration time, then anisotropic minijets' directions alleviate the problem of having them at times much less then ∆/*c*. From [172]. Right: light curves of a single pulse at different frequencies. The pulse clearly becomes narrower with lower frequencies in some magnetic reconnection models as observed in the prompt emission phase. From [171]. Reprinted (and modified) permission of Oxford University Press on behalf of the Royal Astronomical Society.

#### *3.6. Radiative Processes*
