Sub-photospheric shocks in relativistic explosions

Abstract

This paper examines the mechanism of internal shocks in opaque relativistic outflows, in particular in cosmological gamma-ray bursts. The shocks produce neutrino emission and affect the observed photospheric radiation from the explosion. They develop from internal compressive waves and can be of different types depending on the composition of the outflow: (1) Shocks in "photon gas," with negligible plasma inertia, have a unique structure determined by the force-free condition---zero radiation flux in the plasma rest frame. Radiation dominance over plasma inertia suppresses formation of collisionless shocks mediated by collective electromagnetic fields. (2) If the outflow is sufficiently magnetized, a strong collisionless subshock develops, which is embedded in a thicker radiation-mediated structure. (3) Waves in outflows with a free neutron component lead to dissipation through nuclear collisions. At large optical depths, shocks have a thickness comparable to the neutron free path, with an embedded radiation-mediated and collisionless subshocks. The paper also presents first-principle simulations of magnetized flows filled with photons, demonstrating formation of shocks and their structure. Simple estimates show that magnetized sub-photospheric shocks are efficient producers of photons and have a great impact on the observed photospheric radiation. The shock structure changes as the outflow expands toward its photosphere. The dissipation is accompanied by strong $e^\pm$ pair creation, and the $e^\pm$-dressed shock carries the photosphere with it up to two decades in radius, emitting a strong pulse of nonthermal radiation.