GRB 250222A

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

SVOM/ECLAIRs team: Frédéric Piron (LUPM), Nicolas Dagoneau (CEA), Maria-Grazia Bernardini (INAF-OAB), Jean-Luc Atteia, Sébastien Guillot (IRAP)

report on behalf of the SVOM team:

SVOM/GRM was triggered in-flight by a long burst GRB 250222A (SVOM trigger reference: sb25022201) at 2025-02-22T14:24:26.100 UTC (T0).

With the event-by-event data downloaded through the X-band ground station, the GRM light curve of 15 to 600 keV shows that this burst consists of multi-pulses with a T90 of 4.3 +0.6/-0.5 s.

The SVOM/GRM light curve can be found here:
https://www.bursthub.cn//admin/static/svgrb250222A.png

The SVOM/GRM on-ground localization of this burst is (J2000):
RA: 284.2 deg
DEC: -61.0 deg
Error: 1.6 deg (1sigma, statistical only)
We caution that the calibration of SVOM/GRM is undergoing and this localization is subject to systematic errors. 

In addition, this burst was detected by the Count-Rate Trigger onboard ECLAIRs, as an increase in counts over background, but it was not localized by the coded-mask imaging process, which confirms that the burst occurred outside the ECLAIRs field of view.

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

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

Y. Zhang, C. Wang, S. Xiong, J. Wei, and B. Cordier
on behalf of the SVOM-GRM team,

E. Bozzo and C. Ferrigno, on behalf of the INTEGRAL SPI-ACS GRB team,

A. Goldstein, M. S. Briggs, C. Wilson-Hodge,
and E. Burns on behalf of the Fermi GBM team,

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

The long-duration GRB 250222A
(SVOM-GRM detection: Wang et al., GCN 39454
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 )
was detected by SVOM (GRM), Konus-Wind, and INTEGRAL (SPI-ACS)
at about 51859 s UT (14:24:19).

Fermi (GBM) and Swift (BAT) were taking data, but shows 
no rate increase at the burst time.

We have triangulated it to a preliminary, 3 sigma error box
whose coordinates are:
 ---------------------------------------------
  RA(2000), deg                 Dec(2000), deg
 ---------------------------------------------
 Center:
  359.336 (23h 57m 21s)  +0.946 ( +0d 56' 47")
 Corners:
  335.308 (22h 21m 14s) +25.783 (+25d 47' 00")
  352.637 (23h 30m 33s) +12.776 (+12d 46' 35")
    6.156 (00h 24m 37s) -19.368 (-19d 22' 04")
    2.915 (00h 11m 40s)  -9.794 ( -9d 47' 40")
 ---------------------------------------------
The error box area is 38.5 sq. deg, and its maximum
dimension is 54 deg (the minimum one is 3 deg).
The Sun distance was 25 deg.

This localization may be improved.

The SVOM (GRM) localization (GCN 39454
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 ) is inconsistent with 
the IPN localization and also with burst non-detection by Fermi (GBM) and Swift (BAT).

A triangulation map and HEALPix FITS file are posted at
http://www.ioffe.ru/LEA/GRBs/GRB250222_T51862/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.

D. Frederiks, A.Lysenko, A. Ridnaia, D. Svinkin,
A. Tsvetkova,  M. Ulanov, and T. Cline,
on behalf of the Konus-Wind team, report:

The long GRB 250222A (SVOM-GRM detection: Wang et al., GCN 39454
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 ;
IPN triangulation: Svinkin et al., GCN 39588
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 )
triggered Konus-Wind (KW) at T0=51862.905 s UT (14:24:22.905).

The burst light curve shows a bright, multi-peaked pulse,
with the duration of ~2.5 s, followed by a weaker, decaying emission.
The total duration of the burst is ~13 s.
The emission is seen up to ~10 MeV.

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB250222_T51862/

As observed by Konus-Wind, the burst had
a fluence of (1.79 ± 0.19)x10^-5 erg/cm^2 and
a 64-ms peak energy flux, measured from T0 + 0.064 s,
of (1.53 ± 0.20)x10^-5 erg/cm^2/s (both in the 20 keV - 10 MeV energy range).

The time-integrated spectrum (measured from T0 to T0+16.640 s)
is best fit in the 20 keV - 15 MeV range
by a GRB (Band) function with the following model parameters:
the low-energy photon index alpha = -0.51 (-0.22,+0.26),
the high energy photon index beta = -2.28 (-0.14,+0.11),
the peak energy Ep = 173 (-21,+23) keV,
chi2 = 98/97 dof.

The spectrum near the peak count rate (measured from T0 to T0+0.256 s)
is best fit in the 20 keV - 15 MeV range
by a GRB (Band) function with the following model parameters:
the low-energy photon index alpha = 0.40 (-0.61,+1.09),
the high energy photon index beta = -1.97 (-0.13,+0.10),
the peak energy Ep = 169 (-39,+46) keV,
chi2 = 42/43 dof.

All the quoted errors are estimated at the 68% confidence level.
All the presented results are preliminary.