The origin of the photospheric emission of GRB 220426A

Abstract

ABSTRACT GRB 220426A is a bright gamma-ray burst (GRB) dominated by the photospheric emission. We perform several tests to speculate the origin of this photospheric emission. The dimensionless entropy η is large, which is not usual if we assume that it is a pure hot fireball launched by neutrino–antineutrino annihilation mechanism only. Moreover, the outflow has larger η with lower luminosity L in the first few seconds, so that the trend of time-resolved η − L can not be described as a monotonically positive correlation between η and L. A hybrid outflow with almost completely thermalized Poynting flux could account for the quasi-thermal spectrum as well as large η. More importantly, the existence of magnetic field could affect the proton density and neutron–proton coupling effect, so that it could account for the observed trend of time-resolved η − L. The other origins for the photospheric emission, such as non-dissipative hybrid outflow or magnetic reconnection, are not supported because their radiation efficiencies are low, which is not consistent with non-detection of the afterglow for GRB 220426A. Therefore, we think the hybrid outflow may be the most likely origin.