TITLE:   GCN CIRCULAR
NUMBER:  40367
SUBJECT: GRB 250430A: Konus-Wind detection and joint Konus-Wind+Swift-BAT spectral analysis
DATE:    25/05/06 16:17:44 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 250430A
(Swift-BAT detection: Parsotan et al., GCN 40292;
Moss et al., GCN 40356;
EP-WXT detection: Wang et al., GCN 40299)
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 ~16 sigma count-rate increase in the interval
from T0-0.581 s to T0+5.307 s, where T0 = T0(BAT) = 17:31:00.28 UT.

The KW light curve of this burst is available
at http://www.ioffe.ru/LEA/GRBs/GRB250430A/

To derive broad-band spectral parameters of this burst,
we performed a joint spectral analysis of the Swift/BAT data
(15-150 keV) and the KW 3-channel spectral data (20-1600 keV).

A fit to the time-averaged spectrum, measured from T0-0.581 s
to T0+5.307 s, by the Band GRB function results in
alpha = -0.99(-0.31,+0.70),
beta = -2.01(-0.39,+0.19), and
Ep = 107(-44,+86) keV (chi2/dof= 57.7/59).

The total burst fluence is 3.21(-0.92,+1.09)x10^-6 erg/cm^2,
and the 64-ms peak energy flux, measured from T0+0.446 s,
is 1.63(-0.50,+0.58)x10^-6 erg/cm^2.
(both in the 20 keV - 10 MeV energy range).

Assuming the redshift z=0.767 (Garnichey et al., GCN 40301)
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 5.26(-1.50,+1.78)x10^51 erg,
the isotropic peak luminosity L_iso to 2.35(-0.72,+0.84)x10^51 erg/s,
and the rest-frame peak spectral energy Ep,z to 189(-77,+151) keV.

With the obtained estimates, GRB 250430A is inside 68% prediction bands for both 
the 'Amati' and 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/GRB250430A/GRB250430A_rest_frame.pdf

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