GRB 261029C

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

SVOM/ECLAIRs team: Olivier Godet, Marius Brunet (IRAP)

Report on behalf of the SVOM team:

SVOM/GRM was triggered by GRB 261029C in-flight at 2026-10-29T23:38:37.000 (T0) (SVOM trigger reference: sb26102904), which is also detected by AstroSat CZTI (Harsha K. H. et al, GCN #46129
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 ), Konus-Wind (D. Svinkin et al., GCN #46171
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 ) and Psyche (D. Svinkin et al., GCN #46159
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 ).

With the event-by-event data downloaded through the X-band ground station, the GRM light curve shows that this burst mainly consists of one pulse with a T90 of 0.25 +0.15/-0.05 s in the 15-5000 keV band.

The SVOM/GRM light curve can be found here:
https://www.bursthub.cn/gcn/2026/svgrb261029C.png

In addition, the position of this burst, according to IPN localization (RA = 336.252, Dec = -1.683, D. Svinkin et al., GCN #46159
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 ), is located at about 95.6 degrees from the SVOM optical axis, which is outside the ECLAIRs field of view. ECLAIRs saw it through the shield above 50 keV.

With this localization, the time-averaged spectrum from T0-0.2 to T0+0.2 s is best fitted by a power law function with an exponential high-energy cutoff. The power law index is -0.73 +0.14/-0.12 and the cutoff energy, parameterized as Epeak, is (1.33 +0.35/-0.26)E3 keV. The event fluence (10-1000 keV) in this time interval is (2.78 +0.17/-0.18)E-6 erg/cm^2.

The localization of GRB 261029C in the 'Amati' relation diagram is shown at: 
https://www.bursthub.cn/gcn/2026/svgrb261029C_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/GRM point of contact for this burst is: Jin-Peng Zhang (IHEP) (zhangjinpeng@ihep.ac.cn)

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

The bright, short-duration GRB 261029C
(AstroSat-CZTI detection: Harsha et al., GCN 46129
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 ;
IPN triangulation: Svinkin et al., GCN 46159
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 )
triggered Konus-Wind at T0=85117.559 s UT (23:38:37.559).

The burst light curve shows a single bright peak
which starts at ~T0-10 ms and has a duration of ~30 ms,
followed by a weaker emission seen up to ~T0+350 ms.
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/GRB261029_T85117/

As observed by Konus-Wind, the burst
had the total fluence of 8.27(-0.76,+0.80)x10^-6 erg/cm2,
and the 16-ms peak flux, measured from T0-0.006 s,
of 6.57(-1.08,+1.65)x10^-5 erg/cm2/s
(both in the 20 keV - 10 MeV energy range).

The time-averaged spectrum of the burst
(measured from T0 to T0+0.192 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 = -0.72(-0.14,+0.16),
the high energy photon index beta = -2.20(-0.59,+0.27),
the peak energy Ep = 1432(-361,+482) keV
(PG-stat = 87/78 dof).

Since a significant fraction of the main peak of the burst light curve
was detected before the trigger, we performed spectral analysis
of the burst peak using the KW 3-channel light curve data.

Modelling the KW 3-channel time-integrated spectrum
(measured from T0-0.010 s to T0+0.018 s)
by a power law with exponential cutoff (CPL) model:
dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep),
yields alpha = -0.70(-0.24,+0.26) and Ep = 645(-140,+300) keV.

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

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

E. Burns on behalf of the IPN,

D.J. Lawrence, J.O. Goldsten, P.N. Peplowski 
on behalf of the Psyche/GRNS team,

A. Arya (IITB), G. Waratkar (Caltech/IITB), V. Bhalerao (IITB),
on behalf of the Astrosat-CZTI team,

and

C. Wang (IHEP), S. Xiong (IHEP), S. Zhang (IHEP), J. Wei (NAOC),
and B. Cordier (CEA) on behalf of the SVOM-GRM team, report:

The bright, short-duration GRB 261024C
(AstroSat-CZTI detection: Harsha et al., GCN 46129
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 )
was detected by SVOM (GRM), Konus-Wind, AstroSat (CZTI),
and Psyche (GRNS) at about 85117 s UT (23:38:37).

We have triangulated it to two preliminary, 3 sigma error boxes
whose coordinates are:

Box 1:
 ---------------------------------------------
  RA(2000), deg                 Dec(2000), deg
 ---------------------------------------------
 Center:
  336.252 (22h 25m 01s)  -1.683 ( -1d 41' 00")
 Corners:
  336.366 (22h 25m 28s)  -2.174 ( -2d 10' 27")
  335.918 (22h 23m 40s)  -0.938 ( +0d 56' 16")
  336.151 (22h 24m 36s)  -1.225 ( -1d 13' 30")
  336.633 (22h 26m 32s)  -2.551 ( -2d 33' 02")
 ---------------------------------------------

Box 2:
 ---------------------------------------------
  RA(2000), deg                 Dec(2000), deg
 ---------------------------------------------
 Center:
  340.324 (22h 41m 18s) -12.314 (-12d 18' 52")
 Corners:
  340.077 (22h 40m 19s) -11.872 (-11d 52' 20")
  340.577 (22h 42m 19s) -13.093 (-13d 05' 35")
  340.559 (22h 42m 14s) -12.724 (-12d 43' 26")
  340.025 (22h 40m 06s) -11.414 (-11d 24' 50")
 ---------------------------------------------
The area of each error box is 596 sq. arcmin, and their maximum
dimension is 1.76 deg (the minimum one is 7.3 arcmin).
The Sun distance for each box was ~120 deg.

This localization may be improved.

A triangulation map and HEALPix FITS files are posted at
http://www.ioffe.ru/LEA/GRBs/GRB261029_T85117/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. 

Harsha K. H. (IUCAA), S. Salunke (IUCAA), S. Puri (IITB), A. Goyal (IITB), A. Arya (IITB), G. Waratkar (Caltech/IITB), V. Bhalerao (IITB),  A. Vibhute (IUCAA), 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 GRB 261029C which was also detected by KONUS-Wind (via IPN notices) and SVOM/GRM (via notices).

The source was detected in the CZT detectors in the 20-200 keV energy range. The light curve peaks at 2026-10-29 23:38:36.92 UTC. The measured peak count rate associated with the burst is 2429 (+322, -332) counts/s above the background in the combined data of all four quadrants, with a total of 351 (+46, -51) counts. The local mean background count rate was 304 (+11, -15) counts/s. We measure a T90 of 0.17 (+0.06, -0.03) s from the cumulative CZT light curve.

The source was also detected in the CsI anticoincidence (Veto) detector in the 100-500 keV energy range.

CZTI data products like interactive and downloadable light curves for this GRB can be found at: 
https://astrosat.iucaa.in/cift/cift_products/ET20261029T233836/ET20261029T233836_details.html

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