GCN Circular 39019

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 250118B
(IPN triangulation: Kozyrev et al., GCN 39017
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triggered Konus-Wind at T0=13107.725 s UT (03:38:27.725).

The burst light curve shows a multipeaked structure,
which starts at ~T0-5.8 s and has a total duration of ~9.4 s.
We note a weaker and softer pulse seen in the same KW detector
before the main pulse, from ~T0-58.8 s to ~T0-55.9 s.
Since this pulse was detected by KW only, its relation
to GRB 250118B cannot be currently confirmed.
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/GRB250118_T13107/

As observed by Konus-Wind, the burst
had a fluence of 3.49(-0.35,+1.30)x10^-6 erg/cm^2
and a 64-ms peak flux, measured from T0-0.244 s,
of 2.97(-0.60,+1.22)x10^-6 erg/cm^2/s
(both in the 20 keV - 10 MeV energy range).

Since a major part of the burst emission
was detected before the trigger time, the spectral analysis
was performed using the KW 3-channel light curve data.

Modelling the KW 3-channel time-integrated spectrum
(measured from T0-5.837 s to T0+3.552 s)
by a power law with exponential cutoff (CPL) model:
dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep),
yields alpha = -1.55(-0.14,+0.18) and Ep = 287(-95,+256) keV.

All the quoted errors are estimated at the 68% confidence level.
All the presented results are preliminary.