GRB 251108C

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 251108C
(AstroSat-CZTI detection: Salunke et al., GCN 42664
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IPN triangulation: Kozyrev et al., GCN 42680
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triggered Konus-Wind at T0=83283.415 s UT (23:08:03.415).

The burst light curve shows a multipeaked structure
which starts at ~T0-0.2 s and has a total duration of ~8.5 s.
The emission is seen up to ~6 MeV.

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB251108_T83283/

As observed by Konus-Wind, the burst
had a fluence of 1.51(-0.19,+0.23)x10^-5 erg/cm2,
and a 64-ms peak flux, measured from T0-0.172 s,
of 1.23(-0.23,+0.26)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+8.448 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 = -1.05(-0.10,+0.11)
and Ep = 1415(-316,+464) keV (chi2 = 55/69 dof).
Fitting by a GRB (Band) model yields the same alpha and Ep,
and an upper limit on the high energy photon index: beta < -1.7
(chi2 = 53/68 dof).

The spectrum near the maximum count rate
(measured from T0 to T0+0.256 s)
is best fit in the 20 keV - 10 MeV range
by a power law with exponential cutoff model
with  alpha = -0.96(-0.17,+0.22)
and Ep = 2202(-834,+1554) keV (chi2 = 26/30 dof).
Fitting by a GRB (Band) model yields the same alpha and Ep,
and an upper limit on the high energy photon index: beta < -1.4
(chi2 = 26/29 dof).

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

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,

G. Waratkar, J.Joshi, V. Bhalerao, D. Bhattacharya, A. Arya,
and S. Vadawale, on behalf of the Astrosat-CZTI 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 251108C
(AstroSat-CZTI detection: Salunke et al., GCN 42664
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was detected by Konus-Wind, AstroSat (CZTI), and Mars-Odyssey (HEND),
at about 83275 s UT (23:07:55).

We have triangulated it to a preliminary, 3 sigma error box
whose area is 2.13 sq. deg, and its maximum
dimension is 40 deg (the minimum one is 5 arcmin).
The Sun distance was ~155 deg.

This localization may be improved.

A triangulation map and HEALPix FITS file are posted at
http://www.ioffe.ru/LEA/GRBs/GRB251108_T83283/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.

S. Salunke (IUCAA), M. Tembhurnikar (IUCAA), Harsha K. H. (IUCAA), A. Goyal (IITB), A. Arya (IITB), G. Waratkar (Caltech/IITB), A. Vibhute (IUCAA), V. Bhalerao (IITB), 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 long-duration GRB 251108C. This burst was also detected by the Konus-Wind team (priv. comm.) T0(KW) = 23:08:03.415 UT.

The source was clearly detected in the CZT detectors in the 20-200 keV energy range. The light curve peaks at 2025-11-08 23:07:57.38 UTC. The measured peak count rate associated with the burst is 250 (+36, -38) counts/s above the background in the combined data of all quadrants, with a total of 630 (+121, -124) counts. We caution that there is a 0.3 s readout dead time in CZT data during the burst which affects the calculated total counts. The local mean background count rate was 257 (+3, -4)   counts/s. Using cumulative rates, we measure a T90 of 7.0 (+1.8, -1.6) s. 

The source was also clearly detected in the CsI anticoincidence (Veto) detector in the 100-500 keV energy range. The light curve peaks at 2025-11-08 23:07:56.96 UTC The measured peak count rate associated with the burst is 660 (+49, -53) counts/s above the background in the combined data of all quadrants, with a total of 3568 (+326, -374) counts. The local mean background count rate was 1128 (+7, -7) counts/s.  Using cumulative rates, we measure a T90 of 10.9 (+3.2, -3.0) s. 

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