Evidence for the Transition from Thermal to Nonthermal Emission in the Prompt Emission of GRB 161117A

Abstract GRB 161117A is a long-duration gamma-ray burst with three main overlapping peaks. By analyzing the time-resolved spectra of its data observed with the Gamma-Ray Burst Monitor on board the Fermi mission, we find that the spectral evolution shows a transition from thermal (single blackbody, hereafter BB) to hybrid (power-law, hereafter PL, +BB), and finally to nonthermal (Band and cutoff PL) 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 ph ), radii ( R 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 ph seems to be tracking with the light curve, and R 0 remains a constant at ∼ 10 8 cm. The low magnetization (1 + σ 0 ∼ 1) and high dimensionless entropy ( η ≫ 1) during the first seven time-intervals suggest it to be a pure fireball outflow. Moreover, we also estimate the lower limit of magnetization parameters at the photosphere radius ( σ ph ∼ 1.4 and 0.75) for late phase via the nonthermal spectra, and it indicates that the particle acceleration mechanism is dominated by internal shocks rather than magnetic dissipation processes. Finally, the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>ν</mml:mi> <mml:mover accent="true"> <mml:mrow> <mml:mi>ν</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>¯</mml:mo> </mml:mrow> </mml:mover> </mml:math> annihilation mechanism of NDAF model to explain the thermal emission of GRB 161117A is also discussed.