GCN Circular 32762

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
Not found
 
 
), 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.