Consistency with synchrotron emission in the bright GRB 160625B observed by Fermi

TL;DR

This study analyzes the prompt emission spectrum of GRB 160625B, introducing a new spectral model that better fits the data and supports synchrotron emission as the underlying mechanism.

Contribution

The paper presents a new spectral fitting function, 2SBPL, and demonstrates its superiority over standard models in explaining GRB 160625B's emission, supporting synchrotron emission with a low-energy cutoff.

Findings

The 2SBPL model significantly improves spectral fits.

The spectral slopes are consistent with synchrotron emission expectations.

The magnetic field in the emission region is estimated to be around 10 Gauss.

Abstract

We present time resolved spectral analysis of prompt emission from GRB 160625B, one of the brightest bursts ever detected by Fermi in its nine years of operations. Standard empirical functions fail to provide an acceptable fit to the GBM spectral data, which instead require the addition of a low-energy break to the fitting function. We introduce a new fitting function, called 2SBPL, consisting of three smoothly connected power laws. Fitting this model to the data, the goodness of the fits significantly improves and the spectral parameters are well constrained. We also test a spectral model that combines non-thermal and thermal (black body) components, but find that the 2SBPL model is systematically favoured. The spectral evolution shows that the spectral break is located around $E_{\rm break}\sim$ 100 keV, while the usual $\nu F_{\nu}$ peak energy feature $E_{\rm peak}$ evolves in the 0.5-6 MeV energy range. The slopes below and above $E_{\rm break}$ are consistent with the values -0.67 and -1.5, respectively, expected from synchrotron emission produced by a relativistic electron population with a low energy cut-off. If $E_{\rm break}$ is interpreted as the synchrotron cooling frequency, the implied magnetic field in the emitting region is $\sim$ 10 Gauss, i.e. orders of magnitudes smaller than the value expected for a dissipation region located at $\sim 10^{13-14}$ cm from the central engine. The low ratio between $E_{\rm peak}$ and $E_{\rm break}$ implies that the radiative cooling is incomplete, contrary to what is expected in strongly magnetized and compact emitting regions.