TITLE:   GCN CIRCULAR
NUMBER:  39978
SUBJECT:  Konus-Wind detection of GRB 250327B
DATE:    25/03/31 16:40:41 GMT
FROM:    Dmitry Svinkin at Ioffe Institute <svinkin@mail.ioffe.ru>

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

The long-duration GRB 250327B
(SVOM-ECLAIRs detection: Bouchet et al., GCN 39888)
was detected by Konus-Wind (KW) in the waiting mode
at T0 = T0(ECLAIRs) = 76287 s UT (21:11:27).

A Bayesian block analysis of the KW  waiting mode data
in the 20-1200 keV band reveals a ~20 sigma count rate
increase in the interval from ~T0-12 s to ~T0+424 s.

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB250327B/

The total burst fluence is 2.31(-0.22,+0.26)x10^-5 erg/cm^2,
and the 2.944 s peak energy flux, measured from T0+85 s,
is 2.43(-0.52,+0.55)x10^-7 erg/cm^2.
(both in the 20 keV - 10 MeV energy range).

The time-integrated spectrum of the burst,
measured from T0-12 s to T0+424 s,
can be described by a power law with exponential cutoff (CPL) model
dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep)
with alpha = -1.11(-0.17,+0.21) and Ep = 212(-38,+48) keV.

The spectrum near the peak count rate, measured from T0+76 s to T0+91 s,
can be described by a CPL model with
alpha = -0.82(-0.31,+0.43) and Ep = 203(-49,+64) keV.

Assuming the redshift z=3.035 (Malesani et al., GCN 39893)
and a standard cosmology with H_0 = 67.3 km/s/Mpc, Omega_M = 0.315,
and Omega_Lambda = 0.685 (Planck Collaboration, 2014),
we estimate the following rest-frame parameters:
the isotropic energy release E_iso is 4.78(-0.45,+0.54)x10^53 erg,
the peak luminosity L_iso is 2.03(-0.44,+0.46)x10^52 erg/s,
the rest-frame peak energy of the time-averaged spectrum Ep,i,z is 856(-152,+195) keV and 
the spectrum near the maximum count rate Ep,p,z is 818(-198,+257) keV.

With the obtained estimates, GRB 250327B is inside 68% prediction band for the 'Amati' and 
90% prediction band for the 'Yonetoku' relations derived for the sample of >300 long KW GRBs 
with known redshifts (Tsvetkova et al., 2017; Tsvetkova et al., 2021),
see http://www.ioffe.ru/LEA/GRBs/GRB250327B/GRB250327B_rest_frame.pdf

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