GCN Circular 41508
Subject
Konus-Wind detection of GRB 250821B
Event
Date
2025-08-23T16:06:36Z (2 days ago)
From
Dmitry Frederiks at Ioffe Institute <fred@mail.ioffe.ru>
Via
email
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.