Collisional mechanism for GRB emission

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Abstract
GRB jets are expected to carry a neutron component. Strong collisional heating operates in such jets. It is shown to produce bright emission with a spectrum consistent with observations. The radiative mechanism involves inelastic collisions between neutrons and protons, which create multiple e+- pairs with energies ~ m_\pi c^2 = 140 MeV. The created e+- pairs quickly radiate their energy and thermalize. The thermalized e+- population is heated by Coulomb collisions with protons. Numerical simulations of collisionally heated jets are presented. The simulations track the evolution of e+- plasma and radiative transfer in the expanding jet. Jets with Lorentz factors \Gamma > 500 produce the photon spectrum that peaks near 1 MeV and extends to GeV energies with a slope of 2.3-2.5, which is the typical spectrum reported by GRB observations. Most of this radiation is emitted near the photosphere of the jet. The sub-photospheric collisional heating converts > 30 per cent of the explosion energy to escaping radiation. Additional emission is expected at later stages when neutrons decay and produce collisionless heating.

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