GRB 210121A: Observation of photospheric emissions from different regimes and the evolution of the outflow

TL;DR

This paper analyzes the photospheric emissions of GRB 210121A, revealing different emission regimes, the evolution of the outflow, and suggesting a neutrino annihilation origin for the jet, based on multi-instrument observations.

Contribution

It presents the first observation of intermediate photosphere emission in GRB 210121A and offers an alternative explanation for the prompt emission phases.

Findings

First detection of intermediate photosphere emission in a GRB

Correlation between initial Lorentz factor and luminosity

Support for neutrino annihilation as jet launching mechanism

Abstract

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 $\alpha$ during the pulses. Besides, the evolution of the outflow is extracted from time-resolved analysis, and a correlation of $\Gamma_0 \propto L^{0.25\pm0.05}_0$ is obtained, which implies that the jet may be mainly launched by neutrino annihilation in a hyper-accretion disk.