{
  "bibcode": "2022GCN.32762....1K",
  "body": "D. A. Kann (Goethe Univ.) and J. F. Agui Fernandez (IAA) report:\n\nThe ultra-bright, nearby (de Ugarte Postigo et al., GCN #32648) GRB \n221009A (Swift (afterglow) discovery: Dichiara et al., GCN #32632) was \nso intense, it saturated multiple sensitive satellite detectors that \nregistered the prompt emission (Fermi GBM: Veres et al., GCN #32636, \nLesage et al., GCN #32642; Konus-Wind: Frederiks et al., GCN #32668; \nFermi LAT: Omodei et al. GCN #32760; Agile/MCAL: Ursi et al. GCN #32650; \nINTEGRAL SPI/ACS, Gotz et al., GCN #32660; Insight-HXMT, Tan et al., \nATel #15660).\n\nSome missions detected GRB 221009A without saturation effects, because \nthey are either smaller and less sensitive (GRBAlpha: Ripa et al., GCN \n#32685; SIRI-2: Mitchell et al., GCN #32746) or the burst was off-axis \nand passed through the spacecraft body (SRG ART-XC, Lapshov et al., GCN \n#32663). An especially interesting detection was made by HEBS (Liu et \nal., GCN #32751), which was also not saturated owing to its orbital \nposition and environment.\n\nKonus-Wind determined the energetics and spectrum of the first (\"onset\") \npulse of the main emission episode (unsaturated, preceding the two \nbrightest pulses), from T_0+180 - 200 s, finding a fluence of 8.8E-04 \nerg cm^-2 (even this episode alone would be one of the brightest GRBs \never detected) and a peak energy of ~1 MeV. Using this spectrum and the \nraw, preliminarily corrected count rates of the other episodes, they \ndetermined a total fluence of 5.2E-02 erg cm^-2 (Frederiks et al., GCN \n#32668).\n\nWe take the count-rate light curve linked in the Konus-Wind GCN and \ndetermine the counts of the \"onset\" pulse, thereby determining a \n\"fluence per count\" conversion. With this value, and assuming the third \nepisode of the GRB (from 380 to 610 s) is unsaturated (or corrected \nsuccessfully) and has the same spectrum, we find a fluence of 1.28E-02 \nerg cm^-2 for this episode. This is probably an overestimate, as the \nfinal episode of the GRB is likely softer (see the similar GRB 160625B, \ne.g., B.-B. Zhang et al. 2018, Nature Astronomy, 2, 69). For the HEBS \ndata, we derive a significantly lower fluence of ~3E-03 erg cm^-2. A \npotential explanation is that the high background during the time of the \nGRB led to the softer bands being discarded, losing a lot of counts for \nthe softer episode.\n\nFor the main episode (200 - 300 s), we derive a fluence of 7.65E-02 erg \ncm^-2 from the HEBS light curve, a value somewhat higher than the one \nderived for the entire burst from Konus-Wind. Again, this is dependent \non the spectrum being the same as during the \"onset\" pulse. If it is \neven harder, the fluence would increase correspondingly.\n\nSumming all together (the precursor is negligible), we derive, in a \nbroad bolometric band from 0.1 keV to 100 MeV (see Agui Fernandez et \nal., 2021, MNRAS, submitted, arXiv:2109.13838), an isotropic energy \nrelease of log E_iso = 54.77, a value in perfect agreement with GRB \n160625B in the same band. This places GRB 221009A within the very \nhighest isotropic energy releases measured so far. On the one hand, this \nimplies the GRB is extreme but not an outlier, whereas, on the other \nhand, combined with the very low distance, it makes it an even rarer \nevent.",
  "circularId": 32762,
  "createdOn": 1665841926000,
  "email": "kann@iaa.es",
  "subject": "GRB 221009A: Armchair Energetics",
  "submitter": "Alexander Kann at IAA-CSIC  <kann@iaa.es>",
  "eventId": "GRB 221009A"
}