TITLE:   GCN CIRCULAR
NUMBER:  40087
SUBJECT: Konus-Wind detection of GRB 250404A / EP250404a
DATE:    25/04/06 12:08:53 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 long-duration GRB 250404A (Fermi-GBM detection:
The Fermi GBM team, GCN 40050; Mukherjee & Meegan, GCN 40067),
coincident with the X-ray transient EP250404a (Hu et al., GCN 40051),
triggered Konus-Wind (KW) at T0=51605.773 s UT (14:20:05.773).

The burst light curve shows a multi-peaked structure
which starts at ~T0-21 s and has a total duration of ~105 s.
The emission is seen up to ~1 MeV.

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB250404_T51605/

As observed by Konus-Wind, the burst had
a fluence of (2.83 ± 0.09)x10^-5 erg/cm^2 and
a 64-ms peak energy flux, measured from T0 + 0.832 s,
of (1.78 ± 0.22)x10^-6 erg/cm^2/s (both in the 20 keV - 10 MeV energy range).

The time-integrated spectrum (measured from T0 to T0+81.186 s)
is best fit in the 20 keV - 15 MeV range by a power law with exponential
cutoff (CPL) model: dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep)
with  alpha = -1.38(-0.13,+0.14) and Ep = 56(-5,+4) keV (chi2 = 76/98 dof).
Fitting this spectrum by a Band function yields the same values of alpha and Ep,
and an upper limit on the high energy photon index beta of -3.5 (chi2 = 76/97 dof).

The spectrum near the peak count rate(measured from T0 to T0+8.448 s)
is best fit in the 20 keV - 15 MeV range by a CPL
with  alpha = -1.16(-0.17,+0.19) and Ep = 68(-9,+8) keV (chi2 = 90/96 dof).
Fitting this spectrum by a Band function yields the same values of alpha and Ep,
and an upper limit on the high energy photon index beta of -4.2 (chi2 = 90/95 dof).

Assuming the redshift z ~ 2.627 ( Li et al., GCN 40084) 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 (4.60 ± 0.14)x10^53 erg,
the isotropic peak luminosity L_iso to (1.05 ± 0.13)x10^53 erg/s,
the rest-frame peak energy of the time-integrated spectrum Ep,i,z to (203 ± 18) keV,
and the rest-frame peak energy at the peak of the emission Ep,p,z to (247 ± 30) keV.
With the obtained estimates, GRB 250404A lies at the lower edge, but is consistent
with 90% prediction bands of the '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/GRB250404_T51605/GRB250404A_rest_frame.pdf

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