Radiation mediated shocks in gamma-ray bursts: Subshock photon production

TL;DR

This paper demonstrates that mildly magnetized radiation mediated shocks in gamma-ray burst jets can efficiently produce copious photons through synchrotron emission in a collisionless subshock, explaining observed photon numbers.

Contribution

It introduces a new mechanism for photon production in GRB shocks involving collisionless subshocks in magnetized plasma, contrasting previous non-magnetized models.

Findings

Photon number per proton Z=10^5-10^6, matching observations.

Synchrotron emission in collisionless subshocks is a key photon source.

Soft photons gain energy via bulk Comptonization, shaping the GRB spectrum.

Abstract

Internal shocks provide a plausible heating mechanism in the jets of gamma-ray bursts (GRBs). Shocks occurring below the jet photosphere are mediated by radiation. It was previously found that radiation mediated shocks (RMSs) inside GRB jets are inefficient photon producers, and the photons that mediate the RMS must originate from an earlier stage of the explosion. We show that this conclusion is valid only for non-magnetized jets. RMSs that propagate in moderately magnetized plasma develop a collisionless subshock which locally heats the plasma to a relativistic temperature, and the hot electrons emit copious synchrotron photons inside the RMS. We find that this mechanism is generally effective for mildly relativistic shocks and may be the main source of photons observed in GRBs. We derive a simple analytical formula for the generated photon number per proton, $Z$, which gives $Z=10^5$-$10^6$, consistent with observations. The photons are initially injected with low energies, well below the observed GRB peak. Their number is controlled by two main factors: (1) the abundance of electron-positron pairs created in the shock, which is self-consistently calculated, and (2) the upper limit on the brightness temperature of soft radiation set by induced downscattering. The injected soft photons that survive induced downscattering gain energy in the RMS via bulk Comptonization and shape its nonthermal spectrum.