Observational evidence of dissipative photospheres in gamma-ray bursts

TL;DR

This paper presents observational evidence that photospheric emission in gamma-ray bursts can produce both thermal and non-thermal spectra, with spectral evolution explained by subphotospheric dissipation and flow parameter changes.

Contribution

It demonstrates that photospheric emission can account for diverse GRB spectra and their evolution, highlighting the role of subphotospheric dissipation and flow dynamics.

Findings

Spectral shape evolution from Planck to Band function in GRB090902B.

Spectral broadening explained by subphotospheric dissipation.

Correlation between peak energy and spectral slope explained by flow changes.

Abstract

The emission from a gamma-ray burst (GRB) photosphere can give rise to a variety of spectral shapes. The spectrum can retain the shape of a Planck function or it can be broadened and have the shape of a Band function. This fact is best illustrated by studying GRB090902B: The main gamma-ray spectral component is initially close to a Planck function, which can only be explained by emission from the jet photosphere. Later, the same component evolves into a broader Band function. This burst thus provides observational evidence that the photosphere can give rise to a non-thermal spectrum. We show that such a broadening is most naturally explained by subphotospheric dissipation in the jet. The broadening mainly depends on the strength and location of the dissipation, on the magnetic field strength, and on the relation between the energy densities of thermal photons and of the electrons. We suggest that the evolution in spectral shape observed in GRB090902B is due to a decrease of the bulk Lorentz factor of the flow, leading to the main dissipation becoming subphotospheric. Such a change in the flow parameters can also explain the correlation observed between the peak energy of the spectrum and low-energy power law slope, alpha, a correlation commonly observed in GRBs. We conclude that photospheric emission could indeed be a ubiquitous feature during the prompt phase in GRBs and play a decisive role in creating the diverse spectral shapes and spectral evolutions that are observed.