Observation of GRB 221009A Early Afterglow in X-Ray/Gamma-Ray Energy Bands

Abstract

The early afterglow of a gamma-ray burst (GRB) can provide critical information on the jet and progenitor of the GRB. The extreme brightness of GRB 221009A allows us to probe its early afterglow in unprecedented detail. In this Letter, we report comprehensive observation results of the early afterglow of GRB 221009A (from T 0+660 s to T 0+1860 s, where T 0 is the Insight-HXMT/HE trigger time) in X-ray/gamma-ray energy band (from 20 keV to 20 MeV) by Insight-HXMT High Energy X-ray Telescope, GECAM-C, and Fermi/Gamma-ray Burst Monitor. We find that the spectrum of the early afterglow in 20 keV–20 MeV can be well described by a cutoff power law with an extra power law that dominates the low- and high-energy bands, respectively. The cutoff power law E peak is ∼30 keV, and the power-law photon index is ∼1.8 throughout the early afterglow phase. By fitting the light curves in different energy bands, we find that a significant achromatic break (from keV to TeV) is required at T 0 + 1246 − 26 + 27 s (i.e., 1021 s since the afterglow starting time T AG = T 0+225 s), providing compelling evidence of a jet break. Interestingly, both the pre-break and post-break decay slopes vary with energy, and these two slopes become closer in the lower energy band, making the break less identifiable. Intriguingly, the spectrum of the early afterglow experienced a slight hardening before the break and a softening after the break. These results provide new insights into the physics of this remarkable GRB.

Camelidae on the BOAT: observation of a second spectral component in GRB 221009A

“…Given the absence of variability in the TeV light curve as observed by LHAASO, the MeV and GeV-TeV spectral components are not produced from the same emission region of the early epochs (< 500 s). In contrast, the late MeV emission (> 600 s) is temporally featurless and can be assigned to the afterglow emission component, as also considered by other authors [18,8,19,20]. We thus consider the latest (1000 − 1350 s) MeV-TeV spectrum to rise from the synchrotron and synchrotron self-Compton (SSC) radiation of the forward shock-accelerated electrons of the circum-burst medium.…”

Spectra of GRB 221009A at Low Energies Derived from Ground-based Very Low-frequency Measurements

“…It consisted of a precursor of ∼10 s, a quiescent period of ∼170 s, a main emission stage of ∼60 s, and a flaring episode of ∼250 s (An et al 2023;Williams et al 2023). A jet break in the early afterglow was also observed by both GECAM-C, Insight-HXMT, and Fermi/GBM at ∼1246 s after the trigger time of Insight-HXMT (An et al 2023;Zheng et al 2024). Optical observations indicated that this burst was located at a galaxy with the redshift z = 0.151 (Castro-Tirado et al 2022).…”