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

TL;DR

This study analyzes time-resolved GBM spectra of GRB 161117A to test jet composition evolution from a thermal photosphere toward non-thermal emission. Using Bayesian, MCMC-based fits across nine intervals, the authors find a progression from a single blackbody (BB) to a PL+BB hybrid, and finally to Band/CPL–dominated spectra, interpreted within fireball and hybrid jet models. Physical parameters reveal $Γ_{ m ph}$ tracking the light curve, a nearly constant $R_{0}$ around $10^{8}$ cm, and $R_{ m ph}$ in the range $[1.7\times10^{12}, 6\times10^{12}]$ cm; early intervals imply a matter-dominated outflow ($1+σ_0\approx1$, $η\gg1$), while late-time non-thermal emission requires at least modest magnetization ($σ_{ m ph}$ lower limits ≈ $1.4$ and $0.75$). The authors also explore an NDAF-based scenario with $ν\bar{ν}$ annihilation potentially powering the thermal component, and discuss internal shocks as the likely mechanism for late-time non-thermal emission, highlighting the coexistence of photospheric and dissipation physics in GRB jets.

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 ($Γ_{\rm ph}$), radii ($R_{\rm ph}$ and $R_{0}$), magnetization factor at the central engine ($σ_0$), and dimensionless entropy ($η$) 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 $Γ_{\rm ph}$ seems to be tracking with the light curve, and $R_{0}$ remains a constant at $\sim$ $10^{8}$ cm. The low magnetization ($1+σ_0 \sim$ 1) and high dimensionless entropy ($η\gg$ 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 ($σ_{\rm ph}\sim 1.4$ 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 $ν\barν$ annihilation mechanism of NDAF model to explain the thermal emission of GRB 161117A is also discussed.