GRB 250113A

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

report on behalf of the SVOM team:

During the commissioning phase, SVOM/GRM was triggered in-flight by a long burst GRB 250113A (SVOM trigger reference: sb25011303) at 2025-01-13T06:24:36.900 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 multi-pulses with a T90 of 42 +2/-3 s.

The SVOM/GRM light curve can be found here:
https://www.bursthub.cn//admin/static/svgrb250113A.png

The SVOM/GRM on-ground localization of this burst is (J2000):
RA: 321.4 deg
DEC: -21.6 deg
Error: 2.7 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 159 degrees from the SVOM optical axis. 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)

Patrizia Barria(a,b), Giulia Gianfagna(a), James Craig Rodi(a), Aishwarya Linesh Thakur(a), Luigi Piro(a), Lorenzo Natalucci(a,b) report:

GRB 250113A was discovered by SVOM/GRM (GCN 38923
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 ) at time 2025-01-13T06:24:36.900 (UTC) with a duration of 42 s. We searched for any corresponding counterpart in the INTEGRAL/SPI-ACS data.

In a light curve above 80 keV, we find a pulse temporally coincident with the GRM detection having an approximate duration of ~ 3 sec.

The approximate peak count rate is 73000 cts/s over a median background rate of 62500 cts/s.

This work is based on observations with INTEGRAL, an ESA project with instruments and a science data centre funded by ESA member states (especially the PI countries: Denmark, France, Germany, Italy, Switzerland, and Spain), and with the participation of Russia and the USA. The SPI-ACS detector system has been provided by MPE Garching/Germany.

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(a) INAF/IAPS-Rome
(b) ICSC National Research Centre for High-Performance Computing

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,

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

E. Burns on behalf of the IPN,

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

S. Barthelmy, J. Cummings, H. Krimm, D. Palmer, and A. Tohuvavohu
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 long-duration GRB 250113A
(SVOM-GRM detection: Wang et al., GCN 38923
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 ;
INTEGRAL-SPI-ACS detection: Barria et al., GCN 38935
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 )
was detected by SVOM (GRM), Konus-Wind, INTEGRAL (SPI-ACS),
Swift (BAT), and Mars-Odyssey(HEND), at about 23077 s UT (06:24:37).
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
 -------------------------------
 Corners:
   236.558        16.588
   236.711        16.218
   220.529       -15.743
   220.199       -15.607
 -------------------------------
The error box area is 12.2 sq. deg, and its maximum
dimension is 36.0 deg (the minimum one is 20 arcmin).
The Sun distance was 68 deg.

This localization may be improved.

A triangulation map and HEALPix FITS files are posted at
http://www.ioffe.ru/LEA/GRBs/GRB250113_T23073/IPN
The HEALPix triangulation maps are in units of probability density.

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

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

The long-duration GRB 250113A
(SVOM/GRM observation: Wang et al., GCN 38923
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 ;
INTEGRAL/SPI-ACS detection: Barria et al., GCN 38935
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 ;
IPN triangulation: Kozyrev et al., GCN 38953
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 )
triggered Konus-Wind at T0=23073.686 s UT (06:24:33.686).

The burst light curve shows a multipeaked structure
which starts at ~T0-3.0 s and has a total duration of ~46.8 s.
The emission is seen up to ~2 MeV.

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB250113_T23073/

As observed by Konus-Wind, the burst
had a fluence of 3.88(-0.13,+0.14)x10^-5 erg/cm2,
and a 64-ms peak flux, measured from T0+0.656 s,
of 6.12(-1.01,+1.03)x10^-6 erg/cm2/s
(both in the 20 keV - 10 MeV energy range).

The time-averaged spectrum of the burst
(measured from T0 to T0+49.408 s)
is best fit in the 20 keV - 10 MeV range
by a power law with exponential cutoff model:
dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep)
with  alpha = -1.42(-0.05,+0.05)
and Ep = 251(-18,+21) keV (chi2 = 75/88 dof).
Fitting by a GRB (Band) model yields the same alpha and Ep,
and an upper limit on the high energy photon index: beta < -2.5
(chi2 = 75/87 dof).

The spectrum near the maximum count rate
(measured from T0+0.256 to T0+8.448 s)
is best fit in the 20 keV - 10 MeV range
by the GRB (Band) model with the following parameters:
the low-energy photon index alpha = -0.19(-0.16,+0.19),
the high energy photon index beta = -2.49(-0.27,+0.18),
the peak energy Ep = 321(-30,+31) keV
(chi2 = 59/74 dof).

All the quoted errors are at the 68% confidence level.
All the quoted values are preliminary.