Extremely narrow spectrum of GRB110920A: further evidence for localised, subphotospheric dissipation

TL;DR

This paper analyzes the gamma-ray spectrum of GRB110920A, showing it can be explained by photospheric emission with localized dissipation near the saturation radius, resulting in an extremely narrow spectrum.

Contribution

It provides evidence that the narrow spectrum of GRB110920A is produced by photospheric emission with local dissipation at high optical depth, clarifying the emission mechanism.

Findings

Spectrum becomes extremely narrow towards the end of the burst

Emission explained by Comptonisation of thermal photons by cold electrons

Local dissipation occurs near the saturation radius at optical depth ~20

Abstract

Much evidence points towards that the photosphere in the relativistic outflow in GRBs plays an important role in shaping the observed MeV spectrum. However, it is unclear whether the spectrum is fully produced by the photosphere or whether a substantial part of the spectrum is added by processes far above the photosphere. Here we make a detailed study of the $\gamma-$ray emission from single pulse GRB110920A which has a spectrum that becomes extremely narrow towards the end of the burst. We show that the emission can be interpreted as Comptonisation of thermal photons by cold electrons in an unmagnetised outflow at an optical depth of $\tau \sim 20$. The electrons receive their energy by a local dissipation occurring close to the saturation radius. The main spectral component of GRB110920A and its evolution is thus, in this interpretation, fully explained by the emission from the photosphere including localised dissipation at high optical depths.