Evidence for the transition from thermal to non-thermal emission in the prompt emission of GRB 161117A

Abstract

GRB 161117A is a long-duration GRB with three main overlapping peaks. By analyzing the time-resolved spectra of its data observed with the Gamma-Ray Burst Monitor (GBM) on board the Fermi mission, we find that the spectral evolution shows a transition from thermal (single BB) to hybrid (PL$+$BB), and finally to non-thermal (Band and CPL) emissions. Such a transition suggests that the jet composition of GRB 161117A should be changed from a fireball to a Poynting-flux-dominated jet. The bulk Lorentz factor (${\Gamma }_{\mathrm{ph}}$), radii ($R_{\mathrm{ph}}$ and $R_0$), magnetization factor at the central engine (${\sigma }_0$), and dimensionless entropy ($\eta$) of the outflow can be inferred by invoking the observed quasi-thermal component within two models (e.g., pure fireball and hybrid). It is found that ${\Gamma }_{\mathrm{ph}}$ seems to be tracking with the light curve, and $R_0$ remains a constant at $\sim$ $10^8$ cm. The low magnetization ($1+{\sigma }_0\sim$ 1) and high dimensionless entropy ($\eta \gg$ 1) during the first seven time-intervals suggest to be a pure fireball outflow. Moreover, we also estimate the lower limit of magnetization parameter at the photosphere radius (${\sigma }_{\mathrm{ph}}\sim 1.4$ and 0.75) for late phase via the non-thermal spectra, and it indicates that the particle acceleration mechanism is dominated by internal shocks rather than magnetic dissipation processes. Finally, the $\nu \bar{\nu }$ annihilation mechanism of NDAF model to explain the thermal emission of GRB 161117A is also discussed.

18 pages, 7 figures, and 2 tables. Accepted for publication in ApJ