TITLE:   GCN CIRCULAR
NUMBER:  32819
SUBJECT: GRB 221009A: Analysis of the initial episode using data of GBM/Fermi and SPI-ACS/INTEGRAL
DATE:    22/10/22 23:05:12 GMT
FROM:    Alexei Pozanenko at IKI, Moscow  <apozanen@iki.rssi.ru>

P. Minaev (IKI), A. Pozanenko (IKI), I. Chelovekov (IKI) report on 
behalf of GRB IKI FuN:

We analyze the first episode of GRB 221009A (Dichiara et al., GCN 32632; 
Kennea and Williams, GCN  32635; Veres et al., GCN 32636; Bissaldi et 
al., GCN  32636; Svinkin et al., GCN 32641; Piano et al., GCN 32657; 
Pillera et al., GCN 32658;  Gotz et al., GCN 32660; Xiao et al., GCN 
32661; Frederiks et al., GCN 32668; GCN 32668; Ripa et al., GCN 32685; 
Kozyrev et al., GCN 32805) not affected by dead time and pile-up 
instrumental effects, using publicly available data of GBM/Fermi and 
SPI-ACS/INTEGRAL.

We investigate energy spectrum of the time interval (T0-1, T0+55) s, 
where T0 = 13:16:59 UTC 2022-10-09 is a time of  GBM/Fermi trigger.  The 
spectrum can be fitted by a simple power law model with the spectral 
index of gamma = -1.68 +/- 0.01.

See the spectrum in http://grb.rssi.ru/GRB221009A/GRB221009A_1st_episode.png

It is in a contradiction with CPL (power law with exponential cutoff) 
spectral model for the episode, obtained by Lesage et al., GCN 32642 
using GBM/Fermi data (Epeak = 375 +/- 87 keV) and by Frederiks et al., 
GCN 32668 using Konus/WIND data (Epeak = 975 (-332, +712) keV). The 
value of spectral index is not typical for indexes before the break (in 
CPL or Band model), but it is a rather typical for the index after the 
break in Band model. We hypothesize the actual break to be placed at low 
energies (Epeak < 20 keV). The hypothesis could be supported by placing 
Eiso parameter into Amati correlation diagram.   The Eiso = (3.2 +/- 
0.1)E51 erg is calculated  within our power law model with gamma = -1.68 
and using redshift value of z = 0.151 (de Ugarte Postigo et al., GCN 
32648; Castro-Tirado et al., GCN 32686). The Eiso value corresponds to 2 
sigma range of possible Epeak value of (20, 315) keV for type II (long) 
GRBs (Minaev et al., MNRAS, 492, 2, 1919, 2020).

See the correlation diagram in
http://grb.rssi.ru/GRB221009A/GRB221009_Amati_1st_episode.png

The softness of the spectrum is also confirmed by our analysis of 
SPI-ACS/INTEGRAL data: assuming high values of Epeak (e.g. Epeak = 975 
keV) and using SPI-ACS - GBM cross-calibration method to convert SPI-ACS 
count fluxes to energy units (Minaev et al., in preparation) we obtain 
underestimated value of fluence F_ACS = (1.3 +/- 0.3)E-5 erg/cm^2  for 
Epeak = 975 keV, which is less than reference value of F_GBM = (2.38 +/- 
0.04)E-5 erg/cm^2 in (10, 1000) keV range, calculated within our power 
law model with gamma = -1.68.
Using data of GBM/Fermi we perform cross-correlation analysis of the 
initial episode and find significant spectral lags, typical for type II 
(long) GRBs (e.g. Minaev et al., Astronomy Letters, 40, 5, 235, 2014). 
Spectral lag ��� energy dependence is fitted satisfactory by logarithmic 
function lag ~ A*log(E) with spectral lag index A = 0.11 +/- 0.03.

The spectral lag and its parameter can be found in 
http://grb.rssi.ru/GRB221009A/GRB221009_GBM_lags_1st_episode.png