The $E_{\rm p}$ - $E_{\rm iso}$ relation and the internal shock model

TL;DR

This study investigates whether the observed $E_{p}$ - $E_{iso}$ correlation in gamma-ray bursts can be explained by internal shock models, using synthetic populations and comparing them with observed data to identify necessary conditions.

Contribution

The paper introduces a toy model of internal shocks limited to two-shell collisions and compares synthetic burst populations with observed correlations to test the model's validity.

Findings

Model can reproduce the correlation under specific constraints.

Agreement requires particular relations between flow dynamics and microphysics.

Synthetic and observed data comparison highlights key conditions for internal shock viability.

Abstract

The validity of the $E_{\rm p}$ - $E_{\rm iso}$ correlation in gamma-ray bursts and the possibility of explaining the prompt emission with internal shocks are highly debated questions. We study whether the $E_{\rm p}$ - $E_{\rm iso}$ correlation can be reproduced if internal shocks are indeed responsible for the prompt emission, or conversely, if the correlation can be used to constrain the internal shock scenario. We developed a toy model where internal shocks are limited to the collision of only two shells. Synthetic burst populations were constructed for various distributions of the model parameters, such as the injected power in the relativistic outflow, the average Lorentz factor, and its typical contrast between the shells. These parameters can be independent or linked by various relations. Synthetic $E_{\rm p}$ - $E_{\rm iso}$ diagrams are obtained in the different cases and compared with the observed correlation. The reference observed correlation is the one defined by the BAT6 sample, a sample of Swift bursts almost complete in redshift and affected by well-known and reproducible instrumental selection effects. The comparison is then performed with a subsample of synthetic bursts that satisfy the same selection criteria as were imposed on the BAT6 sample. A satisfactory agreement between model and data can often be achieved, but only if several strong constraints are satisfied on both the dynamics of the flow and the microphysics that governs the redistribution of the shock-dissipated energy.