TITLE:   GCN CIRCULAR
NUMBER:  40164
SUBJECT: Konus-Wind detection of GRB 250403A
DATE:    25/04/17 17:43:37 GMT
FROM:    Dmitry Frederiks at Ioffe Institute <fred@mail.ioffe.ru>

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

The long GRB 250403A (Fermi GBM detection: Fermi GBM team, GCN 40025;
Veres et al., GCN 40060; SVOM detection: Julacanti et al., GCN 40026;
AstroSat CZTI detection: Tembhurnikar et al., GCN 40077)
was detected by Konus-Wind (KW) in the waiting mode.

A Bayesian block analysis of the KW waiting mode data in the 20-400 keV band
reveals a ~12 sigma count-rate increase in the interval
from T0-0.4 s to T0+11.5 s where T0 = 15:15:22 UT.

The KW light curve of this burst is available
at http://www.ioffe.ru/LEA/GRBs/GRB250403A/

Modeling the time-integrated spectrum of the burst
by a power law with exponential cutoff (CPL) model
dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep)
yields alpha = -1.14 (-0.23, + 0.23) and Ep = 83(-6,+6) keV.
In the 10 keV -10 MeV band, standard for the KW analysis,
the burst fluence is (2.55 ± 0.18)x10^-6 erg/cm^2
and the 2.944 s peak energy flux is (2.51 ± 0.22)x10^-7 erg/cm^2/s.

Assuming the redshift z=1.847 (de Ugarte Postigo et al., GCN 40162)
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 burst isotropic energy release E_iso to (2.2 ± 0.2)x10^52 erg,
the isotropic peak luminosity L_iso to (6.3 ± 0.6)x10^51 erg/s, and
the rest-frame peak spectral energy Ep,z to (236 ± 11) keV.
With the obtained estimates, GRB 250403A is consistent (inside 68% prediction bands)
of both 'Amati' and 'Yonetoku' relations 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/GRB250403A/GRB250403A_rest_frame.pdf

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