GRB 221009A: Detection by GRBAlpha

The extraordinarily bright long-duration GRB 221009A (Swift/BAT detection: Kennea et al., GCN 32635; Fermi-GBM detection: Veres et al., GCN 32636; Fermi-LAT detection: Pillera et al., GCN 32658; INTEGRAL SPI/ACS detection: Gotz et al., GCN 32660; Konus-Wind detection: Frederiks et al., GCN 32668; IPN triangulation: Svinkin et al., GCN 32641; LHAASO detection: Huang et al., GCN 32677) at a redshift of z = 0.151 (de Ugarte Postigo et al., GCN 32648) was detected by the GRBAlpha 1U CubeSat (Pal et al. Proc. SPIE 2020). The GRB did not saturate our detector and the peak count rate reached ~22 000 count/s in the ~70-890 keV energy band (for a 50 cm^2 detector) at 2022-10-09 13:20:52 UTC. The duration of the GRB was >250s. GRBAlpha was flying above the northern polar region with elevated background levels. The end part of the GRB was recorded while passing the outer Van Allen radiation belt. The light curve obtained by GRBAlpha is available here: https://grbalpha.konkoly.hu/static/share/GRB221009A_GCN_GRBAlpha.pdf GRBAlpha, launched on 2021 March 22, is a demonstration mission for a future CubeSat constellation (Werner et al. Proc. SPIE 2018). Its detector consists of a 75 x 75 x 5 mm^3 CsI(Tl) scintillator read out by a SiPM array, covering the energy range from ~50 keV to ~1000 keV. To increase the duty cycle and the downlink rate, we are continuously upgrading the on-board data acquisition software stack. The ground segment is also supported by the radio amateur community and it takes advantage of the SatNOGS network for increased data downlink volume.