Letter to the Editor

The peak flux of GRB 221009A measured with GRBAlpha

¹ Department of Theoretical Physics and Astrophysics, Faculty of Science, Masaryk University, Kotlářská 267/2, Brno 611 37, Czech Republic
e-mail: ripa.jakub@gmail.com
² Department of Physics, Graduate School of Advanced Science and Engineering, Hiroshima University, Higashi-Hiroshima, Japan
³ Konkoly Observatory, Research Centre for Astronomy and Earth Sciences, Budapest, Hungary
⁴ Eötvös Loránd University, Egyetem tér 1-3, Budapest, Hungary
⁵ Wigner Research Centre for Physics, Konkoly-Thege Miklós út 29-33, Budapest, Hungary
⁶ Spacemanic Ltd, Jablonec 110, 900 86 Jablonec, Slovakia
⁷ Needronix Ltd, Ilkovičová 3, 812 19 Bratislava, Slovakia
⁸ EDIS vvd., 04001 Košice, Slovakia
⁹ Faculty of Aeronautics, Technical University of Košice, Košice, Slovakia
¹⁰ Department of Radio Electronics, Faculty of Electrical Engineering and Communication, Brno University of Technology, Brno, Czech Republic
¹¹ Tokyo University of Science, Noda, Chiba, Japan
¹² Department of Physics, Nagoya University, Nagoya, Aichi, Japan
¹³ Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Kanagawa, Japan
¹⁴ Institute of Earth Physics and Space Science (EPSS), Csatkai E. u. 6-8., 9400 Sopron, Hungary
¹⁵ Department of Physics and Astronomy, Universiteit Gent, 9000 Ghent, Belgium
¹⁶ School of Science, Kyoto University, Kyoto, Japan
¹⁷ Department of Physics, Rikkyo University, Tokyo, Japan
¹⁸ Department of Theoretical Physics, Institute of Physics, Budapest University of Technology and Economics, Műegyetem rkp. 3, 1111 Budapest, Hungary
¹⁹ MTA-BME Quantum Dynamics and Correlations Research Group, Budapest University of Technology and Economics, Műegyetem rkp. 3, 1111 Budapest, Hungary
²⁰ Department of Earth and Space Science, Osaka University, Toyonaka, Osaka, Japan
²¹ INAF – Istituto di Astrofisica Spaziale e Fisica Cosmica, Via A. Corti 12, 20133 Milano, Italy

Received: 12 February 2023
Accepted: 14 March 2023

Abstract

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 × 10⁵⁴ 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 × 10⁵² erg s⁻¹, and the bolometric peak isotropic-equivalent luminosity was L_iso^p,bol = 8.4_−1.5^+2.5 × 10⁵² 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.