The anatomy of a long gamma-ray burst: a simple classification scheme for the emission mechanism(s)

TL;DR

This paper proposes a unified classification scheme for gamma-ray burst emission mechanisms based on the outflow's dimensionless entropy, explaining how different mechanisms dominate under different conditions and providing testable predictions.

Contribution

It introduces a simple, unified framework linking GRB emission mechanisms to a single parameter, clarifying their relative contributions during the prompt phase.

Findings

Internal shocks dominate at low entropy values.

External shocks are prevalent at intermediate entropy.

Photospheric emission is dominant at high entropy.

Abstract

Ultra-relativistic motion and efficient conversion of kinetic energy to radiation are required by gamma-ray burst (GRB) observations, yet they are difficult to simultaneously achieve. Three leading mechanisms have been proposed to explain the observed emission emanating from GRB outflows: radiation from either relativistic internal or external shocks, or thermal emission from a photosphere. Previous works were dedicated to independently treating these three mechanisms and arguing for a sole, unique origin of the prompt emission of gamma-ray bursts. In contrast, herein, we first explain why all three models are valid mechanisms and that a contribution from each of them is expected in the prompt phase. Additionally, we show that a single parameter, the dimensionless entropy of the GRB outflow, determines which mechanism contributes the most to the emission. More specifically, internal shocks dominate for low values of the dimensionless entropy, external shocks for intermediate values and finally, photospheric emission for large values. We present a unified framework for the emission mechanisms of GRBs with easily testable predictions for each process.