GCN Circular 42685

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.