Insight-HXMT observation on the prompt emission and afterglow of GRB 221009A

ATel #15703; M. Y. Ge, Y. P. Chen, J. Y. Liao, S. N. Zhang, S. L. Xiong, S. Zhang, C. Z. Liu, Y. Chen, X. B. Li, F. J. Lu, L. M. Song, C. K. Li, W. J. Tan, Y. Q. Zhang (IHEP) report on behalf of the Insight-HXMT team
on 20 Oct 2022; 09:00 UT
Credential Certification: Yu-Peng Chen (chenyp@ihep.ac.cn)

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We report the Insight-HXMT observations on both the prompt emission and the afterglow of the record-breaking GRB 221009A (GCN 32632, ATel #15650).

Insight-HXMT is mainly composed of three X-ray telescopes: the High Energy X-ray telescope (HE, Liu et al. 2022, SCPMA, 63, 249503), the Medium Energy X-ray telescope (ME, Cao et al. 2022, SCPMA, 63, 249504) and the Low Energy X-ray telescope (LE, Chen et al. 2022, SCPMA, 63, 249505). As a subsystem of the HE, the Anti-coincidence Detector (ACD, Liu et al. 2022, SCPMA, 63, 249503) is composed of 18 detector units with large-area plastic scintillators, which are primarily sensitive to the charged particles and used as particle veto for the main detector (NaI/CsI) of HE. There are also three Particle Monitor (PM) detectors around the HE, which are mainly used to monitor particle flux in the orbit to indicate whether the satellite is in the SAA area and thus the telescopes should be turned on and off accordingly.

In principle, all these detectors mentioned above could measure gamma-rays (with different efficiency though) which could penetrate the shielding material or structures of the satellite, especially the CsI of HE (Zhang et al. 2022 SCPMA, 63, 249502) which has been used to monitor gamma-ray bursts routinely. The Insight-HXMT trigger time given by the ground search of CsI light curve is 2022-10-09T13:17:00.050 (T0, ATel #15660).

Although some detectors (e.g. ACD and PM) have very low detection efficiency to gamma-rays, GRB 221009A is so bright that all detectors mentioned above recorded the prompt emission. However, there are some data gaps in HE, ME and ACD during the most bright part of the burst as they all went into SAA mode (i.e. turned off) which was triggered by the large count rate of PM. Fortunately, the lightcurves of LE and PM are well recovered or recorded for the peaks of main burst (T0+200s to T0+300s).

With all these detectors, Insight-HXMT has observed the full history of the prompt emission of GRB 221009A. Please note that the lightcurves of HE, ME LE, ACD and PM are scaled for clarity. The spectral and timing analyses of the prompt emission are ongoing and will be reported. The prompt emission measured by Insight-HXMT: http://twiki.ihep.ac.cn/pub/HXMT/GRBList/GRB221009A_mainemission.png

Moreover, Insight-HXMT also made observations on the afterglow phase of GRB 221009A, which include one scan observation from T0+10 ks to T0+30 ks and 20 pointing observations from T0+70 ks to T0+309 ks.

From a preliminary analysis, the 1.5-10 keV spectra of these observations are well fitted by an absorbed power law with a photon index of ~2.5 and a column density of ~1.0×10²² cm^-2. The unabsorbed flux in 1.5-10 keV band declined from 1.0×10^-8 (2×10^-10) at T0+11006 s to 3.4×10^-11 (2×10^-11) erg/s/cm^-2 at T0+253790.11 s, following a power law with an index ~-1.66. The data above can be found at http://hxmten.ihep.ac.cn/grb/735.jhtml

Combining the observations of prompt emission and afterglow, the Insight-HXMT light curve of the full burst of GRB 221009A could be found at: http://twiki.ihep.ac.cn/pub/HXMT/GRBList/GRB221009A_afterglow.png . The flux (in 0.25-3 MeV) for prompt emission is derived from HE CsI data, while the flux (in 1.5-10 keV) of afterglow is from LE data. All these results are preliminary.

In summary, Insight-HXMT have made a comprehensive observation of GRB 221009A from the very beginning of the burst to the mid-late afterglow in the X-ray and gamma-ray range, providing an important and unique data set to characterize this historical burst.