Off-axis prompt X-ray transients from the cocoon of short gamma-ray bursts

TL;DR

This paper uses numerical simulations to study off-axis X-ray transients from the cocoon of short gamma-ray bursts, revealing their properties, detectability, and dependence on radiation assumptions.

Contribution

It introduces detailed simulations of cocoon emission in short gamma-ray bursts, highlighting off-axis transient characteristics and detection prospects.

Findings

Cocoon material confined between 15° and 45° off-axis angles.

X-ray transients last a few to ~10 seconds, delayed from merger.

Detectability depends on radiation spectrum assumptions, with thermal transients limited to a few Mpc and Comptonized ones detectable up to tens of Mpc.

Abstract

We present the results of numerical simulations of the prompt emission of short-duration gamma-ray bursts. We consider emission from the relativistic jet, the mildly relativistic cocoon, and the non-relativistic shocked ambient material. We find that the cocoon material is confined between off-axis angles 15<theta<45 degrees and gives origin to X-ray transients with a duration of a few to ~10 seconds, delayed by a few seconds from the time of the merger. We also discuss the distance at which such transients can be detected, finding that it depends sensitively on the assumptions that are made about the radiation spectrum. Purely thermal cocoon transients are detectable only out to a few Mpc, Comptonized transients can instead be detected by the FERMI GBM out to several tens of Mpc.