GRB 250804B

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,

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 250804B
(SVOM-GRM detection: Wang et al., GCN 41229
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 ;
Insight-HXMT detection: Wang et al., GCN 41284
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 )
was detected by Konus-Wind, Swift (BAT), SVOM (GRM), Insight (HXMT),
and Mars-Odyssey (HEND) at about 17092 s UT (04:44:52).
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:
   40.607 (02h 42m 26s) -46.768 (-46d 46' 04")
 Corners:
   40.397 (02h 41m 35s) -46.987 (-46d 59' 12")
   40.852 (02h 43m 24s) -46.590 (-46d 35' 25")
   40.815 (02h 43m 16s) -46.548 (-46d 32' 52")
   40.360 (02h 41m 26s) -46.944 (-46d 56' 39")
 ---------------------------------------------
The error box area is 84 sq. arcmin, and its maximum
dimension is 31 arcmin (the minimum one is 3 arcmin).
The Sun distance was 105 deg.

This localization may be improved.

A Swift ToO has been requested for this burst (GCN 41231
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 , 41239
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 ).
Only source #8 detected by Swift-XRT (https://www.swift.ac.uk/xrt_products/TILED_GRB00138/) 
is inside the IPN box.

A triangulation map and HEALPix FITS file are posted at
http://www.ioffe.ru/LEA/GRBs/GRB250804_T17091/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.

Chen-Wei Wang, Shao-Lin Xiong, and Chao Zheng report on behalf of the Insight-HXMT team:

At 2025-08-04T04:44:51.600 (T0), Insight-HXMT/HE detected the short burst GRB 250804B, which is also detected by SVOM/GRM (Wang et al., GCN #41229
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 ).

The Insight-HXMT/HE light curve mainly consists of a weak precursor and a single main pulse with a T90 of 0.47 +0.14/-0.18 s. The total counts from this burst is 2193 counts.

The HXMT/HE light curve can be found here:
https://www.bursthub.cn//admin/static/hxmtgrb250804B.png

All measurements above are made with the CsI detectors operating in the regular mode with the energy range of about 60-900 keV (deposited energy). Only gamma-rays with energy greater than about 200 keV can penetrate the spacecraft and leave signals in the CsI detectors installed inside of the telescope. 

Insight-HXMT is the first Chinese space X-ray telescope, which was funded jointly by the China National Space Administration (CNSA) and the Chinese Academy of Sciences (CAS). More information about it could be found at: http://www.hxmt.org.

S. Dichiara (PSU), J.A. Kennea (PSU), K.L. Page (U. Leicester), A.P.
Beardmore (U. Leicester), S. Campana (INAF-OAB), A. Melandri
(INAF-OAR), T. Sbarrato (INAF-OAB), D.N. Burrows (PSU) and P.A. Evans
(U. Leicester) reports on behalf of the Swift-XRT team:

Swift-XRT has performed follow-up observations of the IPN-detected
burst GRB 250804B in a series of observations tiled on the sky.  The
total exposure time is	3.3 ks, distributed over 3 tiles; the maximum
exposure at a single sky location in the tiling was 2.2 ks. The data
were collected between T0+79.5 ks and T0+92.2 ks, and are entirely in
Photon Counting (PC) mode. 

Three uncatalogued X-ray sources have been detected, however none of
them is above the RASS limit or shows definitive signs of fading.
Therefore, at the present time we cannot identify which, if any, is the
afterglow. Details of these sources are given below:

Source 5:
  RA (J2000.0):  40.9795  =  02:43:55.09
  Dec (J2000.0): -46.5917  =  -46:35:30.2
  Error: 6.8 arcsec (radius, 90% conf.)
  Count-rate: (6.1 [+3.1, -2.3])e-3 ct s^-1   
  Distance: 1117 arcsec from IPN position.
  Flux: (1.60 [+0.81, -0.62])e-13 erg cm^-2 s^-1 (observed, 0.3-10 keV)

Source 6:
  RA (J2000.0):  40.8694  =  02:43:28.64
  Dec (J2000.0): -46.4818  =  -46:28:54.4
  Error: 7.1 arcsec (radius, 90% conf.)
  Count-rate: (9.6 [+4.0, -3.2])e-3 ct s^-1   
  Distance: 1217 arcsec from IPN position.
  Flux: (1.78 [+0.75, -0.59])e-13 erg cm^-2 s^-1 (observed, 0.3-10 keV)

Source 7:
  RA (J2000.0):  40.5489  =  02:42:11.74
  Dec (J2000.0): -46.5143  =  -46:30:51.5
  Error: 8.8 arcsec (radius, 90% conf.)
  Count-rate: (8.8 [+4.8, -3.6])e-3 ct s^-1   
  Distance: 924 arcsec from IPN position.
  Flux: (3.0 [+1.6, -1.2])e-13 erg cm^-2 s^-1 (observed, 0.3-10 keV)

Four catalogued sources were also detected.

The results of the XRT-team automatic analysis of the tiled XRT
observations, including a position-specific upper limit calculator, are
available at https://www.swift.ac.uk/xrt_products/TILED_GRB00138.

This circular is an official product of the Swift-XRT team.

P. A. Evans (U. Leicester) reports on behalf of the Swift team:

Swift has initiated a series of observations, tiled on the sky, of the
IPN GRB 250804B. Automated analysis of the XRT data will
be presented online at https://www.swift.ac.uk/xrt_products/TILED_GRB00138

Any uncatalogued X-ray sources detected in this analysis will be
reported on this website and via GCN COUNTERPART notices. The probability of finding
serendipitous sources, unrelated to the IPN event is high: any X-ray source
considered to be a probable afterglow candidate will be reported via a GCN Circular
after manual consideration.

Details of the XRT automated analysis methods are detailed in Evans et
al. (2007, A&A, 469, 379; 2009, MNRAS, 397, 1177 and 2014, ApJS, 210, 8).

This circular is an official product of the Swift-XRT team.

SVOM/GRM team: Chen-Wei Wang, Shi-Jie Zheng, Yue Huang, Shao-Lin Xiong, Shuang-Nan Zhang (IHEP)

SVOM/ECLAIRs team: Jean-Luc Atteia, Sebastien Guillot (IRAP)

Report on behalf of the SVOM team:

SVOM/GRM was triggered in-flight by GRB 250804B (SVOM trigger reference: sb25080401) at 2025-08-04T04:44:51.600 UTC (T0).

With the event-by-event data downloaded through the X-band ground station, the GRM light curve shows that this burst consists of a single pulse with a T90 of 0.37 +0.09/-0.15 s in the 15-5000 keV band.

The SVOM/GRM light curve can be found here:
https://www.bursthub.cn//admin/static/svgrb250804B.png

The SVOM/GRM on-ground localization of this burst is (J2000):
RA: 36.3 deg
DEC: -42.1 deg
Error: 4.3 deg (1sigma, statistical only)
We caution that the calibration of SVOM/GRM is undergoing and this localization is subject to systematic errors. 

In addition, the position of this burst, as determined by GRM, is located at about 56 degrees from the SVOM optical axis, which is outside the ECLAIRs field of view. Nonetheless, the burst is seen (via analysis of X-band data) in the ECLAIRs data above 50 keV at the T0 given by GRM, likely through the ECLAIRs shielding.

With this localization, the time-averaged spectrum from T0-0.5 to T0+0.4 s is best fitted by a power law function with an exponential high-energy cutoff. The power law index is -0.78 +0.07/-0.08 and the cutoff energy, parameterized as Epeak, is 1650 +38/-28 keV. The event fluence (10-1000 keV) in this time interval is (2.63 +/-0.13)E-06 erg/cm^2. Thus GRB 250804B is consistent with Type I GRBs in the 'Amati' relation diagram, as shown at:  
https://www.bursthub.cn//admin/static/grb250804B_amati.png

The Space Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA, China), National Center for Space Studies (CNES, France) and the Chinese Academy of Sciences (CAS, China), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. GRM is developed by the Institute of High Energy Physics (IHEP) of CAS.

The SVOM point of contact for this burst is: Chen-Wei Wang (IHEP)(cwwang@ihep.ac.cn)