The peak-flux of GRB 221009A measured with GRBAlpha

Jakub Ripa, Hiromitsu Takahashi, Yasushi Fukazawa, Norbert Werner, Filip Munz, Andras Pal, Masanori Ohno, Marianna Dafcikova, Laszlo Meszaros, Balazs Csak, Nikola Husarikova, Martin Kolar, Gabor Galgoczi, Jean-Paul Breuer, Filip Hroch, Jan Hudec, Jakub Kapus, Marcel Frajt, Maksim Rezenov, Robert Laszlo, Martin Koleda, Miroslav Smelko, Peter Hanak, Pavol Lipovsky, Tomas Urbanec, Miroslav Kasal, Ales Povalac, Yuusuke Uchida, Helen Poon, Hiroto Matake, Kazuhiro Nakazawa, Nagomi Uchida, Tamas Bozoki, Gergely Dalya, Teruaki Enoto, Zsolt Frei, Gergely Friss, Yuto Ichinohe, Kornel Kapas, Laszlo L. Kiss, Tsunefumi Mizuno, Hirokazu Odaka, Janos Takatsy, Martin Topinka, Kento Torigoe

Published 2023-02-20Version 1

On 2022 October 9 the brightest gamma-ray burst (GRB) ever observed lit up the high-energy sky. It was detected by a multitude of instruments, attracting the close attention of the GRB community, and saturated many detectors. GRBAlpha, a nano-satellite with a form factor of a 1U CubeSat, has detected this extraordinarily bright long-duration GRB 221009A without strong saturation. We present light curves of the prompt emission in 13 energy bands, from 80 keV to 950 keV, and perform a spectral analysis to calculate the peak flux and peak isotropic-equivalent luminosity. Since the satellite's attitude information is not available for the time of this GRB, more than 200 incident directions were probed in order to find the median luminosity and its systematic uncertainty. We found that the peak flux in the $80-800$ keV range (observer frame) was $F_{\rm{ph}}^{\rm{p}}=1300_{-200}^{+1200}$ ph cm$^{-2}$s$^{-1}$ or $F_{\rm{erg}}^{\rm{p}}=5.7_{-0.7}^{+3.7}\times10^{-4}$ erg cm$^{-2}$s$^{-1}$ and the fluence in the same energy range of the first GRB episide which was observable by GRBAlpha was $S=2.2_{-0.3}^{+1.4}\times10^{-2}$ erg cm$^{-2}$. The peak isotropic-equivalent luminosity in the $92-920$ keV range (rest frame) was $L_{\rm{iso}}^{\rm{p}}=3.7_{-0.5}^{+2.5}\times10^{52}$ erg s$^{-1}$ and the bolometric peak isotropic-equivalent luminosity was $L_{\rm{iso}}^{\rm{p,bol}}=8.4_{-0.9}^{+1.4}\times10^{52}$ erg s$^{-1}$ in the $1-10,000$ keV range (rest frame). The peak emitted energy is $E_p^\ast=E_p(1+z)=1120\pm470$ keV. Our measurement of $L_{\rm{iso}}^{\rm{p,bol}}$ is consistent with the Yonetoku relation.