GRB 210121A: A Typical Fireball Burst Detected by Two Small Missions

TL;DR

This paper reports the detection and comprehensive analysis of GRB 210121A by multiple missions, revealing its spectral properties, potential redshift, and origin as a typical fireball with photosphere emission.

Contribution

It presents the first joint observation of GRB 210121A by multiple missions and constrains its redshift and physical origin using spectral and physical modeling.

Findings

High peak energy and thermal-like low energy indices.

Redshift constrained between 0.3 and 3.0.

Burst likely originated from a fireball with photosphere emission.

Abstract

The Chinese CubeSat Mission, Gamma Ray Integrated Detectors (GRID), recently detected its first gamma-ray burst, GRB 210121A, which was jointly observed by the Gravitational wave high-energy Electromagnetic Counterpart All-sky Monitor (GECAM). This burst is confirmed by several other missions, including \fermi and \textit{Insight}-HXMT. We combined multi-mission observational data and performed a comprehensive analysis of the burst's temporal and spectral properties. Our results show that the burst is relatively special in its high peak energy, thermal-like low energy indices, and large fluence. By putting it to the $E_{\rm p}$-$E_{\rm\gamma, iso}$ relation diagram with assumed distance, we found this burst can be constrained at the redshift range of [0.3,3.0]. The thermal spectral component is also confirmed by the direct fit of the physical models to the observed spectra. Interestingly, the physical photosphere model also constrained a redshift of $z\sim$ 0.3 for this burst, which help us to identify a host galaxy candidate at such a distance within the location error box. Assuming the host galaxy is real, we found the burst can be best explained by the photosphere emission of a typical fireball with an initial radius of $r_0\sim$ 3.2 $\times 10^7$ cm.