The peak-flux of GRB 221009A measured with GRBAlpha

Abstract

The brightest gamma-ray burst ever observed, long-duration GRB 221009A, was detected by GRBAlpha nano-satellite without 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 $F_{\mathrm{ph}}^{\mathrm{p}}=1300_{-200}^{+1200}$ ph cm${}^{-2}$s${}^{-1}$ or $F_{\mathrm{erg}}^{\mathrm{p}}=5.7_{-0.7}^{+3.7}\times 10^{-4}$ erg cm${}^{-2}$s${}^{-1}$ and the fluence in the same energy range of the first GRB episode lasting 300 s, which was observable by GRBAlpha, was $S=2.2_{-0.3}^{+1.4}\times 10^{-2}$ erg cm${}^{-2}$ or $S^{\mathrm{bol}}=4.9_{-0.5}^{+0.8}\times 10^{-2}$ erg cm${}^{-2}$ for the extrapolated range of $0.9-8,690$ keV. We infer the isotropic-equivalent released energy of the first GRB episode to be $E_{\mathrm{iso}}^{\mathrm{bol}}=2.8_{-0.5}^{+0.8}\times 10^{54}$ 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 $L_{\mathrm{iso}}^{\mathrm{p}}=3.7_{-0.5}^{+2.5}\times 10^{52}$ erg s${}^{-1}$ and the bolometric peak isotropic-equivalent luminosity was $L_{\mathrm{iso}}^{\mathrm{p},\mathrm{bol}}=8.4_{-1.5}^{+2.5}\times 10^{52}$ erg s${}^{-1}$ (4 s scale) in the $1-10,000$ keV range (rest frame). The peak emitted energy is $E_p^{\ast }=E_p(1+z)=1120\pm 470$ keV. Our measurement of $L_{\mathrm{iso}}^{\mathrm{p},\mathrm{bol}}$ is consistent with the Yonetoku relation. It is possible that, due to the spectral evolution of this GRB and orientation of GRBAlpha at the peak time, the true values of peak flux, fluence, $L_{\mathrm{iso}}$, and $E_{\mathrm{iso}}$ are even higher. [abridged]

7 pages, 7 figures, 1 table, accepted for publication in Astronomy & Astrophysics

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• Properties of the extremely energetic GRB~221009A from Konus-WIND and SRG/ART-XC observations
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• The power of the rings: the GRB 221009A soft X-ray emission from its dust-scattering halo
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• Carpet-3 detection of a photon-like air shower with estimated primary energy above 100 TeV in a spatial and temporal coincidence with GRB 221009A
• The brightest GRB ever detected: GRB 221009A as a highly luminous event at z = 0.151
• Swift/UVOT: 18 Years of Long GRB Discoveries and Advances
• GRB 221009A and the Apparently Most Energetic Gamma-Ray Bursts
• Design and Performance of a Universal SiPM Readout System for X- and Gamma-Ray Missions
• Probes for String-Inspired Foam, Lorentz, and CPT Violations in Astrophysics
• Polarization Measurements as a Probe of Axion-Photon Coupling: a Study of GRB 221009A
• Characterization of more than three years of in-orbit radiation damage of SiPMs on GRBAlpha and VZLUSAT-2 CubeSats
• A photohadronic interpretation of the H.E.S.S. afterglow observations from GRB 221009A