GRB 240926A

SVOM/GRM team: Wen-Long Zhang, Yue Huang, Yan-Ting Zhang, Jia-Cong Liu, Yong-Wei Dong, Jiang-Tao Liu, Shi-Jie Zheng, Jian-Chao Sun, Wen-Jun Tan, Jiang He, Chen-Wei Wang, Min Gao, Hao-Xuan Guo, Lu Li, Yong-Ye Li, Hong-Wei Liu, Xin Liu, Hao-Li Shi, Li-Ming Song, You-Li Tuo, Hao-Xi Wang, Jin Wang, Jin-Zhou Wang, Ping Wang, Rui-Jie Wang, Yu-Xi Wang, Bo-Bing Wu, Shao-Lin Xiong, Jian-Ying Ye, Yi-Tao Yin, Wen-Hui Yu, Fan Zhang, Li Zhang, Peng Zhang, Shuang-Nan Zhang, Shu-Min Zhao, Xiao-Yun Zhao, Chao Zheng (IHEP), Maria-Grazia Bernardini (LUPM/INAF-OAB), Laurent Bouchet (IRAP), David Corre (CEA), Tais Maiolino (LUPM), Frédéric Piron (LUPM), Stéphane Schanne (CEA), Jingwei Wang (IAP)

SVOM JSWG: Jian-Yan Wei (NAOC), Bertrand Cordier (CEA), Shuang-Nan Zhang (IHEP), Stéphane Basa (LAM), JeanLuc Attéia (IRAP), Arnaud Claret (CEA), Zi-Gao Dai (USTC), Frédéric Daigne (IAP), Jin-Song Deng (NAOC), Andrea Goldwurm (APC), Diego Götz (CEA), Xu-Hui Han (NAOC), Cyril Lachaud (APC), En-Wei Liang (GXU), Yu-Lei Qiu (NAOC), Susanna Vergani (Obs.Paris), Jing Wang (NAOC), Chao Wu (NAOC), Li-Ping Xin (NAOC), Bing Zhang (UNLV)

report on behalf of the SVOM team:

During the commissioning phase, the SVOM/GRM was triggered in-flight by GRB 240926A (SVOM trigger reference: sb24092601) at 2024-09-26T11:42:45.000 UT (T0).

The real-time alert data and light curves of SVOM/GRM were downlinked to the ground through the VHF system with low latency. The light curves show that this burst consists of multiple pulses with a T90 of 55.54 s -0.39/+0.48 s.

The SVOM/GRM light curve can be found here:
https://www.bursthub.cn//admin/static/svgrb240926A.png

The SVOM/GRM on-ground localization of this burst is (J2000):
RA: 241.92 deg
DEC: 40.31 deg
Error: 3.78 deg (1 sigma, statistical only)
We caution that the calibration of SVOM/GRM is undergoing and this localization is subject to systematic errors. The localization may be improved in the refined analysis.

At the time of the burst ECLAIRs was not in operational mode.

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: Wen-Long Zhang (IHEP)(zhangwl@ihep.ac.cn)

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

E. Burns, on behalf of the IPN,

E. Bozzo and C. Ferrigno, on behalf of the INTEGRAL SPI-ACS GRB 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 240926A
(SVOM-GRM detection: Zhang et al., GCN 37616
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 )
was detected by SVOM (GRM), Konus-Wind, INTEGRAL (SPI-ACS),
and Mars-Odyssey (HEND) at about 42164 s UT (11:42:44).

We have triangulated it to a preliminary, 3 sigma error box
whose coordinates are:
 ---------------------------------------------
  RA(2000), deg                 Dec(2000), deg
 ---------------------------------------------
 Center:
  240.361 (16h 01m 27s) +45.364 (+45d 21' 52")
 Corners:
  241.263 (16h 05m 03s) +45.816 (+45d 48' 59")
  241.214 (16h 04m 51s) +45.898 (+45d 53' 53")
  239.475 (15h 57m 54s) +44.903 (+44d 54' 12")
  239.526 (15h 58m 06s) +44.822 (+44d 49' 21")
 ---------------------------------------------
The error box area is 483 sq. arcmin, and its maximum
dimension is 1.6 deg (the minimum one is 5.1 arcmin).
The Sun distance was 68 deg.

The IPN localization is consistent with, but reduces the area of,
the SVOM-GRM localization (GCN 37616
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 ).

This localization may be improved.

A triangulation map and HEALPix FITS file are posted at
http://www.ioffe.ru/LEA/GRBs/GRB240926_T42164/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 240926A
(SVOM/GRM observation: Zhang et al., GCN 37616
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 ;
IPN triangulation: Kozyrev et al., GCN 37639
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triggered Konus-Wind (KW) at T0=42164.378 s UT (11:42:44.378).

The burst light curve shows a multi-peaked structure,
which starts at ~T0-16 s, peaks at ~T0+31 s,
and has a total duration of ~75 s.
The emission is seen up to ~3 MeV.

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB240926_T42164/

As observed by Konus-Wind, the burst had
a fluence of (5.89 ± 0.59)x10^-5 erg/cm^2 and
a 64-ms peak energy flux, measured from T0 + 31.424 s,
of (9.03 ± 0.85)x10^-6 erg/cm^2/s (both in the 20 keV - 10 MeV energy range).

The time-integrated spectrum (measured from T0 to T0+62.720 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 = -1.65 (-0.05,+0.06),
the high energy photon index beta = -2.60 (-0.55,+0.21),
the peak energy Ep = 118 (-11,+12) keV,
chi2 = 119/97 dof.

The spectrum near the peak count rate (measured from T0+31.232 s to T0+38.144 s)
is best fit in the 20 keV - 15 MeV range by a power law with exponential
cutoff (CPL) model: dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep)
with  alpha = -1.55(-0.07,+0.07) and Ep = 151(-14,+17) keV (chi2 = 98/80 dof).
Fitting this spectrum by a Band function yields the same values of alpha and Ep,
and an upper limit on the high energy photon index beta of -2.6 (chi2 = 98/79 dof).

All the quoted errors are estimated at the 90% confidence level.
All the presented results are preliminary.