Brightest GRB flare observed in GRB 221009A: bridge the last gap between flare and prompt emission in GRB

TL;DR

This paper reports the observation of the brightest gamma-ray flare in GRB 221009A, bridging the gap between prompt emission and flares, with unprecedented energy, peak, and temporal resolution, revealing new insights into GRB emission mechanisms.

Contribution

It presents the first high-resolution gamma-ray observation of a GRB flare, linking prompt emission and flare phenomena with detailed spectral and temporal analysis.

Findings

Record-breaking isotropic energy of $1.82 imes 10^{53}$ erg.

Highest peak energy detected in GRB flares, ~300 keV.

Rapid rise and decay times comparable to prompt emission.

Abstract

Flares are usually observed during the afterglow phase of Gamma-Ray Bursts (GRBs) in soft X-ray, optical and radio bands, but rarely in gamma-ray band. Despite the extraordinary brightness, GECAM-C has accurately measured both the bright prompt emission and flare emission of GRB 221009A without instrumental effects, offering a good opportunity to study the relation between them. In this work, we present a comprehensive analysis of flare emission of GRB 221009A, which is composed of a series of flares. Among them, we identify an exceptionally bright flare with a record-breaking isotropic energy $E_{\rm iso} = 1.82 \times 10^{53}$ erg of GRB flares. It exhibits the highest peak energy ever detected in GRB flares, $E_{\rm peak} \sim 300$ keV, making it a genuine gamma-ray flare. It also shows rapid rise and decay timescales, significantly shorter than those of typical X-ray flares observed in soft X-ray or optical band, but comparable to those observed in prompt emissions. Despite these exceptional properties, the flare shares several common properties with typical GRB flares. We note that this is the first observation of a GRB flare in the keV-MeV band with sufficiently high temporal resolution and high statistics, which bridges the last gap between prompt emission and flare.