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

TL;DR

This study analyzes the exceptionally bright flare in GRB 221009A using high-time-resolution GECAM-C data in the keV–MeV band, revealing a multi-pulse structure and a gamma-ray peak energy around 300 keV that rivals prompt-emission characteristics. Time-resolved spectroscopy (down to 0.1 s bins) shows a Band/CPL-like spectrum with hard-to-soft evolution and no dominant thermal component, while the brightest episode achieves $E_{ m iso} oughly 1.82 imes10^{53}$ erg and a peak luminosity near $4.5 imes10^{52}$ erg s$^{-1}$, indicating an extraordinary, prompt-like flare superimposed on afterglow. The rapid variability (MVT $ ightarrow$ 0.018 s) and multipulse structure strongly support a common physical mechanism with the prompt emission, suggesting late-time central-engine activity within a magnetically dominated, highly collimated jet (potentially a two-component Blandford–Znajek scenario). These findings bridge the last gap between prompt emission and flare in GRBs and have significant implications for jet composition, engine activity, and flare energetics. The results reinforce the view that some gamma-ray flares are direct extensions of prompt emission and are governed by the same central-engine processes.

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.

Figures (6)

• Figure 1: Light curve of GRB 221009A from prompt emission to afterglow. The light blue line shows the GECAM-C/GRD01 flux (20-200 keV) from $T_{\rm AG}$ to $T_{\rm AG}+415$ s (data from 1_09A_Gecam). The dark blue dotted line fits entire flares and afterglow phase. The blue data points denote the flux (20-200 keV) during the afterglow phase obtained from joint observations by GECAM-C and $Fermi$/GBM from $T_{\rm AG}+435$ s to $T_{\rm AG}+1635$ s. The light purple dotted line extends afterglow into the flare time period (data from B9_2024_09A_afterglow ). Dark Blue vertical line marks the jet break time $T_{\rm AG} + 1021^{+27}_{-26}$ s B9_2024_09A_afterglow.
• Figure 2: Panel (a): Blue dots represent typical X-ray flares fluence calculated in 0.2-10 keV (data from A4_2007_flare_statis2), red stars mark the Brightest Flare fluence calculated in 1-10000 keV, the yellow line indicates the average fluence ratio (flare to prompt emission), and the red line shows the specific ratio for the Brightest Flare. Panel (b): Comparison of $E_{\text{peak}}$ between the Brightest Flare and typical X-ray flares (data from A3_2007_flare_statis1C7_2014_falre_sampleA14_2015_Mev_GeV). The blue histogram represents the statistical distribution of $E_{\text{peak}}$ values for typical X-ray flares; the yellow histogram indicates the Brightest Flare.
• Figure 5: Panel (a): Main: The purple line represents the net light curve from $T_0 + 350$ s to $T_0 + 600$ s, with the yellow-highlighted region indicating Brightest Flare. Inset: The net light curve of the Brightest Flare (from $T_0 + 500$ s to $T_0 + 520$ s). The time bin for net light curve is 0.1 s. Panel (b): Evolution of the MVT is shown. The light and dark blue curves represent the light curves of the flare and afterglow phases observed by GECAM-C/GRD01 and GECAM-C/GRD05, respectively. The light purple data points indicate the calculated MVT values. Panel (c): Light curves for different energy bands and the evolution of spectral hardness of the Brightest Flare as observed by GECAM-C/GRD01. Panels (1) to (6): The purple curves represent the light curves for the respective energy bands, while the blue and green lines indicate the background light curves derived from the revisited orbits. The time bin for all light curves is 0.05 s. Panel (7): The blue line represents the light curve across the full energy band, with the red data points depicting the spectral hardness evolution.
• Figure 9: Panel (a) and Panel (b): The spectra of one time interval of the Brightest Flare, along with their corresponding corner plots, are presented. The time periods analyzed, $T_0 + 511$ s to $T_0 + 512$ s. The spectra for this time interval includes the $\nu F_{\nu}$ spectra and the residual maps. The Band function was employed to fit this time interval. Panel (c): Evolution of the spectral parameters from the spectrum fit to S-3 over time. The three subplots, arranged from top to bottom, correspond to the evolution of flux, $E_{\text{peak}}$, and $\alpha$. The gray curve represents the light curve of the BOAT flare. Panel (d): Main flux transition for the wider time bins of S-4, spanning $T_0 + 350$ s to $T_0 + 600$ s, with the Brightest Flare highlighted by a light yellow background. The two subplots in the smaller panel on the left display the flux transitions during the main phases of S-4 and S-3, arranged from top to bottom.
• Figure 14: This figure presents the statistical distributions of the parameters for the Brightest Flare in comparison to those of typical X-ray flares. Panels (a) to (f) show the distributions for $L_{\text{peak}}$--Duration, $L_{\text{iso}}$--Duration, $L_{\text{peak}}$--$L_{\text{iso}}$, $L_{\text{peak}}$--$E_{\text{iso}}$, $T_{\text{decay}}$--$T_{\text{rise}}$ and Duration--$T_{\text{peak}}$ respectively. The histograms on the upper and right sides of each subplot represent the statistical distributions of the parameters corresponding to the horizontal and vertical axes, respectively. The blue data points denote typical X-ray flares, while the red pentagrams mark the Brightest Flare and the orange pentagrams mark the brightest pulse of the Brightest Flare. The dark blue straight line indicates the results of the statistical analysis A8_2016_Yi_flare_sample. The blue region is the 3 $\sigma$ uncertainty regions for the best fit line.