GRB 210121A: Observation of Photospheric Emissions from Different Regimes and the Evolution of the Outflow

GRB 210121A was observed by Insight-HXMT, by the Gravitational wave high-energy Electromagnetic Counterpart All-sky Monitor (GECAM), and by the Fermi Gamma-ray Burst Monitor (Fermi/GBM) on 2021 January 21. In this work, photospheric emission from a structured jet is preferred to interpret the prompt emission phase of GRB 210121A, and emissions from different regimes are observed on-axis. Particularly, the emission from the intermediate photosphere is first observed in the first 1.3 s of the prompt emission, while emissions from the other part are dominant by the emissions from the saturated regime. This offers an alternative explanation compared with previous work. Moreover, the emissions that consider the intermediate photosphere can well interpret the changes in the low-energy photon index α during the pulses. In addition, the evolution of the outflow is extracted from a time-resolved analysis, and a correlation of Γ 0 ∝ L 0 0.25 ± 0.5 is obtained, which implies that the jet may be mainly launched by neutrino annihilation in a hyper-accretion disk.