Thermal and nonthermal emission from a peculiar long-duration GRB 211211A

Abstract

Long-duration GRB 211211A that lacks a supernova emission even down to very stringent limits at such a low redshift $z=0.076$ and is associated with kilonova emission, suggests that its physical origin is from a binary compact star merger. By reanalyzing its data observed with the Gamma-Ray Burst Monitor on board the Fermi mission, we find that both time-integrated and time-resolved spectra can be fitted well by using a 2SBPL plus blackbody (2SBPL+BB) model in the prompt emission. The bulk Lorentz factors ($Γ_{\rm ph}$) of the outflow can be inferred by invoking the observed thermal emission at the photosphere radius within a pure fireball model, and we find out that the temporal evolution of $Γ_{\rm ph}$ seems to be tracking with the light curve. The derived values of $Γ_{\rm ph}$ are also consistent with the $Γ_{\rm ph}$-$L_{γ, \rm iso}$/$E_{γ, \rm iso}$ correlations that had been found in other bursts. Moreover, we also calculate the magnetization factor $σ_{0}$ in the central engine and $σ_{\rm ph}$ at the photosphere radius within the framework of a hybrid jet model, and find that the values of both $1+σ_{\rm 0}$ and $1+σ_{\rm ph}$ are larger than 1 for different time slices. It suggests that at least the Poynting-flux component is indeed existent in the outflow. If this is the case, one possible physical interpretation of thermal and nonthermal emissions in GRB 211211A is from the contributions of both $ν\barν$ annihilation and the Blandford-Znajek mechanisms in the relativistic jet when a stellar mass black hole resides in the central engine.