High-efficiency photospheric emission of long-duration gamma-ray burst jets: the effect of the viewing angle

TL;DR

This study uses numerical simulations to analyze the photospheric emission from long gamma-ray burst jets, revealing high efficiency, a dependence on viewing angle, and the need for additional mechanisms to explain observed non-thermal spectra.

Contribution

It provides a detailed numerical analysis of photospheric emission in gamma-ray bursts, highlighting the effects of viewing angle and the limitations of thermal models.

Findings

Photospheric emission efficiency increases slightly with off-axis angle.

The peak spectrum frequency scales with the square root of luminosity.

Thermal models alone cannot fully explain observed non-thermal spectra.

Abstract

We present the results of a numerical investigation of the spectra and light curves of the emission from the photospheres of long-duration gamma-ray burst jets. We confirm that the photospheric emission has high efficiency and we show that the efficiency increases slightly with the off-axis angle. We show that the peak frequency of the observed spectrum is proportional to the square root of the photosphere's luminosity, in agreement with the Amati relation. However, a quantitative comparison reveals that the thermal peak frequency is too small for the corresponding total luminosity. As a consequence, the radiation must be out of thermal equilibrium with the baryons in order to reproduce the observations. Finally, we show that the spectrum integrated over the emitting surface is virtually indistinguishable from a Planck law, and therefore an additional mechanism has to be identified to explain the non-thermal behavior of the observed spectra at both high and low frequencies.