Brightest Gamma-Ray-burst Flare Observed in GRB 221009A: Bridging the Last Gap between the Prompt Emission and Flare in Gamma-Ray Bursts

Abstract Flares are usually observed during the afterglow phases of Gamma-ray bursts (GRBs) in the soft-X-ray, optical, and radio bands—but rarely in the gamma-ray band. Despite its 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 the 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 iso = 1.82 × 10 53 erg for GRB flares. It exhibits the highest peak energy ever detected in GRB flares, E peak ∼ 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 the soft-X-ray or optical bands 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, bridging the last gap between the prompt emission and flare.