GCN Circular 39498

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 250225B
(Swift-BAT detection: Williams et al., GCN 39473
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 ;
SVOM/GRM detection: Zhang et al., GCN 39493
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 )
triggered Konus-Wind at T0=70750.895 s UT (19:39:10.895).

The burst light curve shows a bright initial pulse
which starts at ~T0-2.2 s and has a total duration of ~3.3 s,
followed by two weaker emission episodes centered
around ~T0+60 s and T0+108 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/GRB250225_T70750/

As observed by Konus-Wind, the burst
had a fluence of 1.54(-0.18,+0.28)x10^-5 erg/cm2,
and a 64-ms peak flux, measured from T0+0.262 s,
of 1.14(-0.24,+0.35)x10^-5 erg/cm2/s
(both in the 20 keV - 10 MeV energy range).

The time-averaged spectrum of the initial pulse
(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 = -1.14(-0.16,+0.18)
and Ep = 634(-181,+393) keV (chi2 = 66/84 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 = 66/83 dof).

The time-averaged spectrum of the second emission episode
(measured from T0+57.600 to T0+65.792 s)
is best fit in the 20 keV - 10 MeV range
by a power law with exponential cutoff model
with alpha = -1.23(-0.21,+0.28)
and Ep = 361(-118,+292) keV (chi2 = 97/99 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 = 95/98 dof).

The time-averaged spectrum of the third emission episode
(measured from T0+106.752 to T0+114.944 s)
is best fit in the 20 keV - 10 MeV range
by a power law with exponential cutoff model
with alpha = -0.91(-0.69,+1.12)
and Ep = 187(-74,+228) keV (chi2 = 110/99 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 = 110/98 dof).

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