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

Some quasi-thermal (QT) dominated gamma-ray bursts (GRBs) could be well described by a multi-color blackbody (BB) function or a combined model of BB plus non-thermal (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 a non-dissipative photospheric emission from a pure hot fireball) are used to make a comparison between these two modelings. To diagnose the magnetization properties of the central engine, the `top-down' approach proposed by Gao \& Zhang is adopted. It is found that diagnoses based on these two modelings could provide similar conclusions qualitatively; however, the modeling with mBB (or mBB+NT) may give more reasonable physical explanations. This implies that impacts from the GRB jet structure and the geometrical broadening on the observed spectrum should be considered. However, conservatively, these methods may be not sensitive enough to distinguish between the pure hot fireball and a mildly magnetized hybrid jet. Some other information is necessary to provide more evidence for the determination of jet properties for similar GRBs. Based on these considerations, we suggest that the photospheric emission of GRB 221022B is from a hot jet; a dissipation is caused by a internal shock (IS) mechanism due to the increasing Lorentz Factor with time, which makes its prompt emission behaves a typical evolution from thermal to NT.
Comment: Published in ApJ on Jan 19th 2024. Accepted for publication in The ApJ on 19 Nov. 16 pages, 6 figures, 3 tables, submitted to ApJ on 06-May-2023