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
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