Constraints on Cold Magnetized Shocks in Gamma-Ray Bursts

TL;DR

This paper evaluates the efficiency of magnetized shocks in gamma-ray bursts and finds they are too inefficient to explain observations unless magnetic energy is converted to thermal energy, suggesting alternative models like thermal pressure-driven jets.

Contribution

It provides a quantitative analysis of magnetized shock efficiency in GRBs and compares it with observational data, highlighting the need for additional processes or alternative models.

Findings

Magnetized shocks are radiatively inefficient in GRBs.

Converting magnetic energy to thermal energy can increase radiative efficiency.

Thermal pressure-driven jets can produce sufficient prompt emission.

Abstract

We consider a model in which the ultra-relativistic jet in a gamma-ray burst (GRB) is cold and magnetically accelerated. We assume that the energy flux in the outflowing material is partially thermalized via internal shocks or a reverse shock, and we estimate the maximum amount of radiation that could be produced in such magnetized shocks. We compare this estimate with the available observational data on prompt gamma-ray emission in GRBs. We find that, even with highly optimistic assumptions, the magnetized jet model is radiatively too inefficient to be consistent with observations. One way out is to assume that much of the magnetic energy in the post-shock, or even pre-shock, jet material is converted to particle thermal energy by some unspecified process, and then radiated. This can increase the radiative efficiency sufficiently to fit observations. Alternatively, jet acceleration may be driven by thermal pressure rather than magnetic fields. In this case, which corresponds to the traditional fireball model, sufficient prompt GRB emission could be produced either from shocks at a large radius or from the jet photosphere closer to the center.