The peak flux of GRB 221009A measured with GRBAlpha

Context. 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. Aims. GRBAlpha, a nano-satellite with a form factor of a 1U CubeSat, detected this extraordinarily bright long-duration GRB, GRB 221009A, without saturation but affected by pile-up. We present light curves of the prompt emission in 13 energy bands, from 80 keV to 950 keV, and performed a spectral analysis to calculate the peak flux and peak isotropic-equivalent luminosity. Methods. 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. Results. We find that the peak flux in the 80 − 800 keV range (observer frame) was F ph p = 1300 −200 +1200 ph cm −2 s −1 , or F erg p = 5.7 −0.7 +3.7 × 10 −4 erg cm −2 s −1 , and the fluence in the same energy range of the first GRB episode, which lasted 300 s and was observable by GRBAlpha, was S = 2.2 −0.3 +1.4 × 10 −2 erg cm −2 , or S bol = 4.9 −0.5 +0.8 × 10 −2 erg cm −2 for the extrapolated range of 0.9 − 8690 keV. We infer the isotropic-equivalent released energy of the first GRB episode to be E iso bol = 2.8 −0.5 +0.8 × 10 54 erg in the 1 − 10 000 keV band (rest frame at z = 0.15). The peak isotropic-equivalent luminosity in the 92 − 920 keV range (rest frame) was L iso p = 3.7 −0.5 +2.5 × 10 52 erg s −1 , and the bolometric peak isotropic-equivalent luminosity was L iso p,bol = 8.4 −1.5 +2.5 × 10 52 erg s −1 (4 s scale) in the 1 − 10 000 keV range (rest frame). The peak emitted energy is E p ∗ = E p (1+ z ) = 1120 ± 470 keV. Our measurement of L iso p,bol is consistent with the Yonetoku relation. It is possible that, due to the spectral evolution of this GRB and the orientation of GRBAlpha at the peak time, the true values of peak flux, fluence, L iso , and E iso are even higher.