TITLE:   GCN CIRCULAR
NUMBER:  41508
SUBJECT: Konus-Wind detection of GRB 250821B
DATE:    25/08/23 16:06:36 GMT
FROM:    Dmitry Frederiks at Ioffe Institute <fred@mail.ioffe.ru>

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

The bright, long-duration GRB 250821B (Ferm-GBM detection:
The Fermi GBM team, GCN 41473; Smith and Meegan, GCN 41477;
SVOM-GRM detection: Tan et al., GCN 41486;
IPN localization Kozyrev et al., GCN 41498)
triggered Konus-Wind (KW) at T0=58069.135 s UT (16:07:49.135).

The burst light curve shows a single emission pulse with a total duration of ~2.5 s.
The emission is seen up to ~5 MeV.

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB250821_T58069/

As observed by Konus-Wind, the burst had
a fluence of (1.34 ± 0.13)x10^-5 erg/cm^2 and
a 64-ms peak energy flux, measured from T0 + 0.512 s,
of (1.77 ± 0.17)x10^-5 erg/cm^2/s (both in the 20 keV - 10 MeV energy range).

The time-integrated spectrum (measured from T0 to T0+8.448 s)
is best fit in the 20 keV - 15 MeV range by a GRB (Band) function
with the following model parameters:
the low-energy photon index alpha = -0.60 (-0.14,+0.15),
the high energy photon index beta = -2.24 (-0.13,+0.10),
the peak energy Ep = 193(-18,+21) keV,
chi2 = 70/96 dof.

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