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