Testing High-latitude Curvature Effect of Gamma-Ray Bursts with {\it Fermi} Data: Evidence of Bulk Acceleration in Prompt Emission

TL;DR

This study analyzes Fermi satellite data to detect high-latitude curvature emission in gamma-ray bursts, revealing evidence of bulk acceleration in the prompt emission phase, which supports Poynting-flux-dominated jet models.

Contribution

It provides the first spectral and temporal analysis of high-latitude emission in single-pulse GRBs, constraining emission radii and indicating jet acceleration during prompt emission.

Findings

Identified high-latitude emission in a subset of GRBs.

Constrained emission radius to approximately 10^{15}-10^{16} cm.

Detected faster decay than constant Lorentz factor models, implying jet acceleration.

Abstract

When a gamma-ray burst (GRB) emitter stops emission abruptly, the observer receives rapidly fading emission from high latitudes with respect to the line of sight, known as the ``curvature effect''. Identifying such emission from GRB prompt-emission lightcurves would constrain the radius of prompt emission from the central engine and the composition of GRB jets. We perform a dedicated search of high-latitude emission (HLE) through spectral and temporal analyses of a sample of single-pulse bursts detected by the Gamma-ray Burst Monitor on board the {\it Fermi} satellite. We identify HLE from a subsample of bursts and constrain the emission radius to be $R_{\rm GRB} \sim (10^{15}-10^{16})$ cm from the central engine. Some bursts have the HLE decay faster than predicted by a constant Lorentz factor jet, suggesting that the emission region is undergoing acceleration during prompt emission. This supports the Poynting-flux-dominated jet composition for these bursts. The conclusion is consistent with previous results drawn from spectral-lag modeling of prompt emission and HLE analysis of X-ray flares.