Thermal Emission in Gamma-Ray Burst Afterglows

TL;DR

This paper investigates how thermal emission from heated circumstellar shells influences GRB afterglows, explaining observed X-ray plateaus and optical deviations without requiring supernova explosions.

Contribution

It introduces a model using the small STELLA code to show thermal emission's role in GRB afterglow features, proposing a quasi-supernova mechanism independent of progenitor star explosions.

Findings

Thermal emission from heated shells can produce X-ray plateaus.

Thermal radiation pressure causes rapid shell expansion.

The model explains optical bumps without progenitor star supernovae.

Abstract

We study thermal emission from circumstellar structures heated by gamma-ray burst (GRB) radiation and ejecta and calculate its contribution to GRB optical and X-ray afterglows using the modified radiation hydro-code small STELLA. It is shown that thermal emission originating in heated dense shells around the GRB progenitor star can reproduce X-ray plateaus (like observed in GRB 050904, 070110) as well as deviations from a power law fading observed in optical afterglows of some GRBs (e.g. 020124, 030328, 030429X, 050904). Thermal radiation pressure in the heated circumburst shell dominates the gas pressure, producing rapid expansion of matter similar to supenova-like explosions close to opacity or radiation flux density jumps in the circumburst medium. This phenomenon can be responsible for so-called supernova bumps in optical afterglows of several GRBs. Such a `quasi-supernova' suggests interpretation of the GRB-SN connection which does not directly involve the explosion of the GRB progenitor star.