Diagnosing GRB Prompt Emission Site with Spectral Cut-Off Energy

TL;DR

This paper proposes a method to determine the emission site of GRB prompt emission by analyzing the spectral cutoff energy, which depends on the Lorentz factor and emission radius, aiding understanding of GRB physics.

Contribution

It introduces a diagnostic approach using spectral cutoff energy to identify the GRB emission site, considering various possible locations like the photosphere or deceleration radius.

Findings

Spectral cutoff energy depends on Lorentz factor and emission radius.

Measuring Lorentz factor allows diagnosing the emission site.

Method can distinguish between different GRB emission models.

Abstract

The site and mechanism of gamma-ray burst (GRB) prompt emission is still unknown. Although internal shocks have been widely discussed as the emission site of GRBs, evidence supporting other emission sites have been also suggested recently, including the closer-in photosphere where the fireball becomes transparent and further-out radii near the fireball deceleration radius where magnetic dissipation may be important. With the successful operation of the GLAST experiment, prompt high energy emission spectra from many GRBs would be detected in the near future. We suggest that the cut-off energy of the prompt emission spectrum from a GRB depends on both the fireball bulk Lorentz factor and the unknown emission radius from the central engine. If the bulk Lorentz factor could be independently measured (e.g. from early afterglow observations), the observed spectral cutoff energy can be used to diagnose the emission site of gamma-rays. This would provide valuable information to understand the physical origin of the GRB promp emission.