GRB 250404B

S. Salunke (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 250404B.

The source was clearly detected in the CZT detectors in the 20-200 keV energy range. The light curve showed multiple peaks of emission with the strongest peak at 2025-04-04 18:36:09.15 UTC. The measured peak count rate associated with the burst is 720 (+181, -101) counts/s above the background in the combined data of three quadrants (out of four), with a total of 375 (+52, -54) counts. The local mean background count rate was 292 (+10, -13) counts/s. Using cumulative rates, we measure a T90 of 0.79 (+0.04, -0.05) s. 

The source was also clearly detected in the CsI anticoincidence (Veto) detector in the 100-500 keV energy range. The light curve peaked at 2025-04-04 18:36:08.40 UTC. The measured peak count rate associated with the burst is 665 (+73, -81) counts/s above the background in the combined data of all quadrants, with a total of 635 (+154, -167) counts. The local mean background count rate was 1266 (+14, -15) 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

A.S. Kozyrev, D.V. Golovin, M.L. Litvak, I.G. Mitrofanov, and A.B. Sanin
on behalf of the HEND/Mars Odyssey team,

D. Svinkin, D. Frederiks, A. Lysenko, A. Ridnaia,
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, A. Tohuvavohu,
and J. DeLaunay on behalf of the Swift-BAT team,

G. Waratkar, J.Joshi, V. Bhalerao, D. Bhattacharya,
and S. Vadawale, on behalf of the Astrosat-CZTI team,

and

W. Boynton, C. Fellows, K. Harshman, H. Enos, R. Starr,
and A.S. Gardner on on behalf of the GRS-Odyssey GRB team,
report:

The short-duration GRB 250404B
(AstroSat CZTI detection: Salunke et al., GCN Circ. 40078
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was detected by Konus-Wind, Swift (BAT), AstroSat (CZTI),
and Mars-Odyssey (HEND) at about 66970 s UT (18:36:10).
The burst was outside the coded field of view of the BAT.

We have triangulated it to a preliminary, 3 sigma error box
whose coordinates are:

 ---------------------------------------------
  RA(2000), deg                 Dec(2000), deg
 ---------------------------------------------
 Center:
  273.629 (18h 14m 31s)  -2.251 ( -2d 15' 03")
 Corners:
  273.108 (18h 12m 26s)  -2.676 ( -2d 40' 35")
  274.127 (18h 16m 31s)  -1.659 ( -1d 39' 32")
  274.151 (18h 16m 36s)  -1.842 ( -1d 50' 30")
  273.131 (18h 12m 31s)  -2.866 ( -2d 51' 58")
 ---------------------------------------------
The error box area is 768 sq. arcmin, and its maximum
dimension is 1.6 deg (the minimum one is 9 arcmin).
The Sun distance was 100 deg.

This localization may be improved.

A triangulation map and HEALPix FITS file are posted at
http://www.ioffe.ru/LEA/GRBs/GRB250404_T66970/IPN/
The HEALPix triangulation map is the multi-order HEALPix in units of
probability density.

The Konus-Wind time history and spectrum will be given
in a forthcoming GCN Circular.