Jet-Structure Imprint on the Curvature Tail of Gamma-Ray Burst Prompt Emission

TL;DR

This paper develops a numerical model to analyze the curvature effect in gamma-ray burst prompt emission, revealing that a power-law wing jet structure best explains observed late-time light-curve breaks.

Contribution

The authors introduce a new numerical model for interpreting GRB prompt emission light curves to infer jet structure, emphasizing the significance of the curvature effect.

Findings

Simple spherical and top-hat jet models are inadequate.

A power-law wing jet with a uniform core fits the observed data.

Late-time light-curve breaks are effective diagnostics of jet structure.

Abstract

Even though the prompt emission of gamma-ray bursts (GRBs) is highly beamed, high-latitude emission still produces a distinct light curve break after the intrinsic emission ceases and the edge of the jet comes into view. This curvature effect offers a direct probe of the jet structure during the prompt phase. To uncover the geometric structure of the GRB jet encoded in the prompt light-curve evolution, we develop a numerical model that calculates synchrotron light curves from structured jets to interpret the observed break. We apply this model to the prompt emission of GRB 230307A, which displays a rare late-time break. Our analysis demonstrates that simple spherical outflow and top-hat jet models are inadequate to reproduce the light curve. Instead, the observations are best described by a power-law wing jet with a uniform core ($\theta_{\rm core}=0.0147$ rad) and a surrounding power-law wing. Our results demonstrate that the break in late-time prompt emission can be a powerful diagnostic of GRB jet structure.