GRB 220426A: A Thermal Radiation-Dominated Gamma-Ray Burst

TL;DR

This paper analyzes GRB 220426A, a gamma-ray burst with dominant thermal radiation, revealing its spectral properties, physical outflow parameters, and consistency with thermal emission models, advancing understanding of GRB ejecta composition.

Contribution

It provides the first detailed analysis of a thermal radiation-dominated GRB, using Bayesian inference to support a multicolor blackbody model and deriving physical outflow properties.

Findings

Spectra are narrowly distributed with very hard low-energy indices.

Bayesian inference favors the multicolor blackbody model.

Outflow Lorentz factors range from 274 to 827, with photosphere radii between 1.83e11 and 2.97e12 cm.

Abstract

The physical composition of the ejecta of gamma-ray bursts (GRBs) remains an open question. The radiation mechanism of the prompt gamma rays is also in debate. This problem can be solved for the bursts hosting distinct thermal radiation. However, the events with dominant thermal spectral components are still rare. In this work, we focus on GRB 220426A, a recent event detected by Fermi-GBM. The time-resolved and time-integrated data analyses yield very hard low-energy spectral indices and rather soft high-energy spectral indices. This means that the spectra of GRB 220426A are narrowly distributed. And the Bayesian inference results are in favor of the multicolor blackbody (mBB) model. The physical properties of the relativistic outflow are calculated. Assuming a redshift $z= 1.4$, the bulk Lorentz factors $\Gamma$ of the shells are found to be between $274_{-18}^{+24}$ and $827_{-71}^{+100}$, and the corresponding photosphere radii $R_{\rm ph}$ are in the range of $1.83_{-0.50}^{+0.52} \times 10^{11}$ and $2.97_{-0.15}^{+0.14} \times 10^{12}$ cm. Similar to GRB 090902B, the time-resolved properties of GRB 220426A satisfy the observed $\Gamma-L$ and $E_p-L$ correlations, where $L$ is the luminosity of the prompt emission and $E_{p}$ is the spectral peak energy.