GRB 230818B

P. K. Navaneeth (IUCAA), G. Waratkar (IITB), A. Vibhute (IUCAA), V. Bhalerao (IITB), D. Bhattacharya (Ashoka University/IUCAA), A. R. Rao (IUCAA/TIFR), and S. Vadawale (PRL) report on behalf of the AstroSat CZTI collaboration:

Analysis of AstroSat CZTI data with the CIFT framework (Sharma et al., 2021, JApA, 42, 73) showed the detection of a long GRB 230818B which was also detected by Konus-Wind (reported in GCN notices).

The source was clearly detected in the 20-200 keV energy range. The light curve peaks at 2023-08-18 10:12:55.5 UTC. The measured peak count rate associated with the burst is 139 (+41, -20) counts/s above the background in the combined data of all quadrants, with a total of 1336 (+411, -462) counts. The local mean background count rate was 423 (+3, -4) counts/s. Using cumulative rates, we measure a T90 of 20 (+8, -6) s. 

The source was also clearly detected in the CsI anticoincidence (Veto) detector in the 100-500 keV energy range. The light curve peaks at 2023-08-18 10:12:54.3 UTC. The measured peak count rate associated with the burst is 379 (+74, -41) counts/s above the background in the combined data of all quadrants, with a total of 4772 (+696, -879) counts. The local mean background count rate was 1568 (+7, -8) counts/s. We measure a T90 of 26 (+2, -5) s from the cumulative Veto light curve.

CZTI is built by a TIFR-led consortium of institutes across India, including VSSC, URSC, IUCAA, SAC, and PRL. The Indian Space Research Organisation funded, managed, and facilitated the project.

CZTI GRB detections are reported regularly on the payload site at: 
http://astrosat.iucaa.in/czti/?q=grb

M. Dafcikova, J. Ripa (Masaryk U.), A. Pal (Konkoly Observatory),  N. Werner  (Masaryk U.), M. Ohno (Hiroshima U.),  L. Meszaros, B. Csak (Konkoly Observatory), H. Takahashi (Hiroshima U.), F. Munz , M. Topinka, F. Hroch, N. Husarikova, J.-P. Breuer (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.

The long duration GRB 230818B (AstroSat detection: GCN 34524
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 ; Konus/Wind detection at 2023-08-18 10:12:54.913 UT) was detected by the GRB detectors on board of the VZLUSAT-2 3U CubeSat (https://www.vzlusat2.cz/en/).

The data acquisition was performed by the GRB detector units no. 0 and no. 1. The detection was confirmed at the peak time 2023-08-18 10:12:53 (10:13:04) UTC. The T90 duration is 29 s (29 s) and the significance during T90 reaches 14 sigma (12 sigma) for detector unit no. 0 (no. 1).

The light curve obtained by VZLUSAT-2 is available here:
https://vzlusat2.konkoly.hu/static/share/GRB230818B_GCN_VZLUSAT2.pdf

All VZLUSAT-2 detections are listed at: https://monoceros.physics.muni.cz/hea/VZLUSAT-2/
The GRB detectors on VZLUSAT-2 are a demonstration payload for a future CubeSat 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.

Y. Shimizu (Kanagawa U), A. Yoshida, T. Sakamoto, 
S. Sugita (AGU), Y. Kawakubo (LSU),K. Yamaoka (Nagoya U),
S. Nakahira (JAXA), Y. Asaoka (ICRR), S. Torii, Y. Akaike, 
K. Kobayashi (Waseda U), T. Tamura (Kanagawa U), 
N. Cannady (GSFC/UMBC),
M. L. Cherry (LSU), S. Ricciarini (U of Florence),
P. S. Marrocchesi (U of Siena),
and the CALET collaboration:

The long GRB 230818B (AstroSat CZTI detection: 
Navaneeth et al., GCN Circ. 34524
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 ; VZLUSAT-2 detection: 
Dafcikova et al., GCN Circ. 34525
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 ; Konus/Wind detection at 
2023-08-18 10:12:54.913 UT) triggered the CALET Gamma-ray 
Burst Monitor (CGBM) at 10:12:54.02 UTC on 18 August 2023
(http://cgbm.calet.jp/cgbm_trigger/flight/1376388662/).
The burst signal was seen by all CGBM detectors.
Because of a problem in one of the ground alert processing script,
the GCN notice was not distributed automatically for this event.

The burst light curve shows a multi-peaked structure that starts
at T-0.6 sec, peaks at T+0.6 sec, and ends at T+32.0 sec.
The T90 and T50 durations measured by the SGM data are 27.6 +/- 1.2 sec
and 13.3 +/- 0.6 sec (40-1000 keV), respectively.

The ground-processed light curve is available at

http://cgbm.calet.jp/cgbm_trigger/ground/1376388662/index.html

The CALET data used in this analysis are provided by
the Waseda CALET Operation Center located at Waseda University.