GRB 240531A

J. Joshi (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 short-duration GRB 240531A.

The source was clearly detected in the CZT detectors in the 20-200 keV energy range. The light curve showed a single burst of emission that peaked at 2024-05-31 18:54:12.0 UTC. The measured peak count rate associated with the burst is 1140 (+248, -251) counts/s above the background in the combined data of all quadrants, with a total of 202 (+40, -41) counts. The local mean background count rate was 335 (+11, -16) counts/s. Using cumulative rates, we measure a T90 of 0.6 (+0.1, -0.2) s.

It was also clearly detected in the CsI anticoincidence (Veto) detector in the 100-500 keV energy range. The light curve showed a single burst of emission that peaked at 2024-05-31 18:54:11.2 UTC. The measured peak count rate is 292 (+72, -67) counts/s above the background in the combined Veto data of all quadrants, with a total of 264 (+155, -128) counts. The local mean background count rate was 1421 (+8, -14) counts/s. Due to the intrinsic 1 s binning of veto data, we cannot reliably estimate a T90 from it.

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

D. Svinkin, D. Frederiks, A. Ridnaia, A. Lysenko,
and T. Cline on behalf of the Konus-Wind team,

E. Burns, on behalf of the IPN,

S. Barthelmy, J. Cummings, H. Krimm, D. Palmer, and A. Tohuvavohu
on behalf of the Swift-BAT team,

and

G. Waratkar (IITB), J.Joshi (IUCAA), A. Vibhute (IUCAA),
V. Bhalerao (IITB), D. Bhattacharya (Ashoka University/IUCAA),
A. R. Rao (IUCAA/TIFR), and S. Vadawale (PRL)
on behalf of the Astrosat-CZTI team, report:

The short-duration GRB 240531A
(Astrosat-CZTI detection: Wang et al., GCN 36586
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was detected by Astrosat (CZTI), Konus-Wind (KW), in the waiting mode,
and Swift (BAT) at about 68052 s UT (18:54:12).
The burst occurred during Swift slewing interval
(when normal BAT triggering is disabled).

We have triangulated it to a BAT-Konus annulus centered at
RA(2000)=91.405 deg (06h 05m 37s) Dec(2000)=+25.128 deg (+25d 07' 40"),
whose radius is 84.474 +/- 16.042 deg (3 sigma).

The annulus combined with Swift and Astrosat Earth occultation information, and the KW ecliptic latitude response, gives 3848 sq. deg (3 sigma) localization region.

A triangulation map and HEALPix FITS file are posted at
http://www.ioffe.ru/LEA/GRBs/GRB240531_T68052/IPN
The HEALPix triangulation map is the multi-order HEALPix in units of probability density.