Subjects:High Energy Astrophysical Phenomena (astro-ph.HE)
GRB 210121A was observed by Insight-HXMT, Gravitational wave high-energy
Electromagnetic Counterpart All-sky Monitor (GECAM), Fermi Gamma-ray Burst
Monitor (Fermi/GBM) on Jan 21st 2021. 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 those from the other part are
dominant by the saturated regime, and offers an alternative explanation
compared with the previous work. Moreover, the emissions with considering the
intermediate photosphere can well interpret the changes on low-energy photon
index α during the pulses. Besides, the evolution of the outflow is
extracted from time-resolved analysis, and a correlation of Γ0∝L0.24±0.040 is obtained, which implies that the jet may be mainly
launched by neutrino annihilation in a hyper-accretion disk.
