Spectral and Jet Properties of the Quasi-thermal-dominated GRB 210121A, GRB 210610B, and GRB 221022B

Abstract

Some quasi-thermal (QT)-dominated gamma-ray bursts (GRBs) could be well described by a multicolor blackbody (mBB) function or a combined model of a BB plus a nonthermal (NT) component. In this analysis, two QT radiation-dominated bursts with known emission properties (GRB 210610B, likely from a hybrid jet, and GRB 210121A, with a spectrum consistent with nondissipative photospheric emission from a pure hot fireball) are used to make a comparison between these two models. To diagnose the magnetization properties of the central engine, the “top-down” approach proposed by Gao and Zhang is adopted. It is found that diagnoses based on these two models provide similar conclusions qualitatively; however, the model with mBB (or mBB+NT) may give more reasonable physical explanations. This implies that impacts from the GRB jet structure and geometrical broadening on the observed spectrum should be considered. However, conservatively, these methods may be not sensitive enough to distinguish between a pure hot fireball and a mildly magnetized hybrid jet. Some other information is necessary to provide more evidence when determining the jet properties for similar GRBs. Based on these considerations, we suggest that the photospheric emission of GRB 221022B is from a hot jet, where dissipation is caused by an internal shock mechanism due to the increasing Lorentz factor with time, which makes its prompt emission display typical evolution from thermal to NT.

Keywords

• Gamma-ray bursts