TITLE: GCN CIRCULAR NUMBER: 31937 SUBJECT: GRB 220421B: Detection by VZLUSAT-2 DATE: 22/04/23 15:55:15 GMT FROM: Jakub Ripa at Masaryk University J. Ripa (Masaryk U.), A. Pal (Konkoly Observatory), M. Ohno (Eotvos U./Hiroshima U.),  N. Werner  (Masaryk U.),  L. Meszaros (Konkoly Observatory), H. Takahashi (Hiroshima U.), B. Csak (Konkoly Observatory), M. Topinka, F. Munz, F. Hroch (Masaryk U.), J. Hudec, J. Kapus, M. Frajt, M. Rezenov (Spacemanic s.r.o), R. Laszlo (Needronix), G. Galgoczi (Wigner Research Center/Eotvos U.), N. Uchida (ISAS/JAXA), T. Enoto (Kyoto U.), Zs. Frei (Eotvos U.), Y. Fukazawa, K. Hirose, H. Matake (Hiroshima U.), S. Hisadomi (Nagoya U.), Y. Ichinohe (Rikkyo U.), L. L. Kiss (Konkoly Observatory),  T. Mizuno (Hiroshima U.), K. Nakazawa (Nagoya U.), H. Odaka (Univ of Tokyo), K. Torigoe (Hiroshima U.), P. Svoboda, V. Daniel, J. Dudas, M. Junas, J. Gromes (VZLU), I. Vertat (FEL ZCU)  -- the VZLUSAT-2/GRB payload collaboration. A GRB detected by GECAM (trigger at 2022-04-21 21:03:20.6 observation ID 00035461090, https://www.swift.psu.edu/guano/) was also detected by both GRB detector units on board of the VZLUSAT-2 3U CubeSat (https://www.vzlusat2.cz/en/). Note that the reported GECAM trigger is ~15s earlier than the onset of the GRB seen by VZLUSAT-2. The data acquisition was performed by GRB detector units no. 0 and no. 1. The 38 sigma detection significance was confirmed at 2022-04-21 21:03:47 UTC. The long-duration GRB has the T90 duration of 62 s. The light curve obtained by VZLUSAT-2 is available here: https://vzlusat2.konkoly.hu/static/share/GRB220421B_GCN_VZLUSAT2.pdf GRB detectors on VZLUSAT-2 are a demonstration payload for a future CubeSats constellation (Werner et al. Proc. SPIE 2018). Two GRB modules of VZLUSAT-2 are placed in a perpendicular manner and each consists of a 75 x 75 x 5 mm3 CsI scintillator read out by a SiPM array, covering the energy range from ~30 keV to ~1000 keV. VZLUSAT-2 was launched on 2022 January 13 from Cape Canaveral.