*3.4. The Role of Neutrons in the GRB Jet*

It is possible that the GRB jet is also composed of a population of neutrons [144–147]. These neutrons may change the GRB jet dynamics and have an effect on the resulting prompt emission phase [92]. As mentioned before, in the case of the hot fireball model without neutrons, the fireball accelerates as long as the photons are coupled to the electrons. Due to the smaller proton-neutron cross section, when neutrons are present in GRB jet they decouple the protons at a smaller distance from the central engine than the Thomson photosphere. If the decoupling radius is also smaller than the radius *R<sup>s</sup>* where protons attain their maximum speed, then neutrons attain a Lorentz factor Γ*<sup>n</sup>* < Γ*<sup>s</sup>* . This two-fluid state or "compound" state of the jet, similarly to the internal shock model, extracts the kinetic energy of internal motions of the jet. More specifically, it extracts the energy of the streaming of plasma through the neutron component throughout a volume instead of being solely confined to the shock front as in internal shocks [92].

Since this jet is prone to collisions between neutrons and protons, it creates multiple *e* ± pairs, which can have an effect on the emerging gamma-ray spectrum, by cooling via synchroton and inverse Compton. These cooled pairs form a thermalized pair population which is Coulomb-heated by collisions with protons. This mechanism is able to produce a peak near 1 MeV and a "Band" spectrum with *f<sup>ν</sup>* ∝ *ν* 1.4 and *f<sup>ν</sup>* ∝ *ν* <sup>−</sup>1.5 below and above the peak, respectively [92].

Magnetic fields change the spectrum below the peak by significantly cooling the pairs produced in the neutron-proton collisions via the synchroton process [148]. This does not significantly alter the peak of the spectrum, but does flatten the spectrum below the peak, see Figure 2. It also steepens the spectrum above the peak since inverse Compton emission by pairs becomes less important above the peak in lieu of stronger synchrotron emission below it, which may be in tension with spectral observations [14].

**Figure 2.** Spectrum of the magnetized, collisionally heated jet. The solid, short-dashed, long-dashed, dotted, dot-dashed, and triple dot-dashed curves correspond to magnetizations of 0, 10−<sup>3</sup> , 0.01, 0.1, 0.5, and 2, respectively. From ©AAS. Reproduced with permission [148].
