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

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 (${\theta }_{\mathrm{core}}=0.0147$ 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.

Accepted for publication in ApJ; 14 pages, 5 figures and 3 tables

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