Early Thermal X-ray Emission from Long Gamma-ray Bursts and Their Circumstellar Environments

TL;DR

This study uses hydrodynamical simulations to explore how early thermal X-ray emissions in long gamma-ray bursts relate to their circumstellar environments, revealing that such emissions can probe the surrounding matter.

Contribution

It demonstrates that early thermal X-ray emission can be explained by cocoon photospheric emission, linking observable signals to circumstellar environment properties.

Findings

Early thermal X-ray emission can be produced by cocoon photospheric emission.

The circumstellar environment influences the characteristics of early X-ray emission.

Different circumstellar environments lead to distinguishable thermal X-ray signatures.

Abstract

We performed a series of hydrodynamical calculations of an ultra-relativistic jet propagating through a massive star and the circumstellar matter to investigate the interaction between the ejecta and the circumstellar matter. We succeed in distinguishing two qualitatively different cases in which the ejecta are shocked and adiabatically cool. To examine whether the cocoon expanding at subrelativistic speeds emits any observable signal, we calculate expected photospheric emission from the cocoon. It is found that the emission can explain early thermal X-ray emission recently found in some long gamma-ray bursts. The result implies that the difference of the circumstellar environment of long gamma-ray bursts can be probed by observing their early thermal X-ray emission.