Non-Thermal emission from the photospheres of Gamma-Ray Burst outflows. I: High frequency tails

TL;DR

This paper models high-frequency gamma-ray emission from gamma-ray burst photospheres, showing inverse Compton scattering can produce power-law tails up to GeV energies, explaining observed spectral diversity.

Contribution

It introduces a model where impulsive electron heating and inverse Compton scattering generate high-energy tails in GRB spectra, applicable to both baryonic and magnetic outflows.

Findings

Power-law tails extend from hundreds of keV to GeV energies.

Spectral slope varies significantly during a burst.

Model accounts for observed spectral diversity in BATSE data.

Abstract

We study the spectrum of high frequency radiation emerging from mildly dissipative photospheres of long-duration gamma-ray burst outflows. Building on the results of recent numerical investigations, we assume that electrons are heated impulsively to mildly relativistic energies by either shocks or magnetic dissipation at Thomson optical depths of several and subsequently cool by inverse Compton, scattering off the thermal photons of the photosphere. We show that even in the absence of magnetic field and non-thermal leptons, inverse Compton scattering produces power-law tails that extend from the peak of the thermal radiation, at several hundred keV, to several tens of MeV, and possibly up to GeV energies. The slope of the high-frequency power-law is predicted to vary substantially during a single burst, and the model can easily account for the diversity of high-frequency spectra observed by BATSE. Our model works in baryonic as well as in magnetically dominated outflows, as long as the magnetic field component is not overwhelmingly dominant.