The peak energy of dissipative GRB photospheres

TL;DR

This paper explores how the peak energy of GRB spectra depends on jet properties, especially the Lorentz factor, and suggests continuous sub-photospheric dissipation explains observed spectral features.

Contribution

It introduces a model where continuous energy dissipation in dissipative photospheres determines the GRB peak energy and links it to jet Lorentz factors, explaining observed correlations.

Findings

Peak energy forms at Thomson optical depth of several tens.

Peak energy primarily depends on the bulk Lorentz factor Gamma.

Brightest bursts are predicted to be least baryon loaded.

Abstract

The radiation released at the transparency radius of an ultrarelativistic flow can account for the observed properties of gamma-ray bursts (GRBs) provided that sufficient energy is dissipated in the sub-photospheric region. Here, I investigate how the peak energy of the E*f(E) spectrum and its overall shape depend on the properties of the jet for various "dissipative photospheres". I find that continuous energy release which results in electron heating over a wide range of distances may be the key to explain the GRB emission. In this picture, the peak of the spectrum forms at a Thomson optical depth of several tens. The peak depends mainly on the bulk Lorentz factor Gamma of the flow and can, therefore, be used to determine it. The Gamma is predicted to range from ~10 to 1000 from X-ray flashes to the brightest observed GRBs in agreement with recent observational inferences. The Amati relation can be understood if the brightest bursts are the least baryon loaded ones. Implications from this interpretation of the GRB emission for the central engine are discussed.