TITLE: GCN CIRCULAR NUMBER: 32762 SUBJECT: GRB 221009A: Armchair Energetics DATE: 22/10/15 13:52:06 GMT FROM: Alexander Kann at IAA-CSIC D. A. Kann (Goethe Univ.) and J. F. Agui Fernandez (IAA) report: The ultra-bright, nearby (de Ugarte Postigo et al., GCN #32648) GRB 221009A (Swift (afterglow) discovery: Dichiara et al., GCN #32632) was so intense, it saturated multiple sensitive satellite detectors that registered the prompt emission (Fermi GBM: Veres et al., GCN #32636, Lesage et al., GCN #32642; Konus-Wind: Frederiks et al., GCN #32668; Fermi LAT: Omodei et al. GCN #32760; Agile/MCAL: Ursi et al. GCN #32650; INTEGRAL SPI/ACS, Gotz et al., GCN #32660; Insight-HXMT, Tan et al., ATel #15660). Some missions detected GRB 221009A without saturation effects, because they are either smaller and less sensitive (GRBAlpha: Ripa et al., GCN #32685; SIRI-2: Mitchell et al., GCN #32746) or the burst was off-axis and passed through the spacecraft body (SRG ART-XC, Lapshov et al., GCN #32663). An especially interesting detection was made by HEBS (Liu et al., GCN #32751), which was also not saturated owing to its orbital position and environment. Konus-Wind determined the energetics and spectrum of the first ("onset") pulse of the main emission episode (unsaturated, preceding the two brightest pulses), from T_0+180 - 200 s, finding a fluence of 8.8E-04 erg cm^-2 (even this episode alone would be one of the brightest GRBs ever detected) and a peak energy of ~1 MeV. Using this spectrum and the raw, preliminarily corrected count rates of the other episodes, they determined a total fluence of 5.2E-02 erg cm^-2 (Frederiks et al., GCN #32668). We take the count-rate light curve linked in the Konus-Wind GCN and determine the counts of the "onset" pulse, thereby determining a "fluence per count" conversion. With this value, and assuming the third episode of the GRB (from 380 to 610 s) is unsaturated (or corrected successfully) and has the same spectrum, we find a fluence of 1.28E-02 erg cm^-2 for this episode. This is probably an overestimate, as the final episode of the GRB is likely softer (see the similar GRB 160625B, e.g., B.-B. Zhang et al. 2018, Nature Astronomy, 2, 69). For the HEBS data, we derive a significantly lower fluence of ~3E-03 erg cm^-2. A potential explanation is that the high background during the time of the GRB led to the softer bands being discarded, losing a lot of counts for the softer episode. For the main episode (200 - 300 s), we derive a fluence of 7.65E-02 erg cm^-2 from the HEBS light curve, a value somewhat higher than the one derived for the entire burst from Konus-Wind. Again, this is dependent on the spectrum being the same as during the "onset" pulse. If it is even harder, the fluence would increase correspondingly. Summing all together (the precursor is negligible), we derive, in a broad bolometric band from 0.1 keV to 100 MeV (see Agui Fernandez et al., 2021, MNRAS, submitted, arXiv:2109.13838), an isotropic energy release of log E_iso = 54.77, a value in perfect agreement with GRB 160625B in the same band. This places GRB 221009A within the very highest isotropic energy releases measured so far. On the one hand, this implies the GRB is extreme but not an outlier, whereas, on the other hand, combined with the very low distance, it makes it an even rarer event.