TITLE:   GCN CIRCULAR
NUMBER:  40243
SUBJECT: Konus-Wind detection of GRB 250424A
DATE:    25/04/24 22:12:37 GMT
FROM:    Anna Ridnaia at Ioffe Institute <ridnaia@mail.ioffe.ru>

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

The long-duration GRB GRB 250424A
(Swift-BAT detection: Cenko et al., GCN 40224;
AstroSat CZTI detection: Harsha et al., GCN 40231)
triggered Konus-Wind at T0=24733.431 s UT (06:52:13.431).

The burst light curve shows a single pulse
which starts at ~T0-17.9 s and has a total duration of ~40.5 s.
The emission is seen up to ~2 MeV.

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB250424_T24733/

As observed by Konus-Wind, the burst
had a fluence of 5.83(-0.16,+0.16)x10^-5 erg/cm2,
and a 64-ms peak flux, measured from T0+4.192 s,
of 8.88(-0.94,+0.95)x10^-6 erg/cm2/s
(both in the 20 keV - 10 MeV energy range).

The time-averaged spectrum of the burst
(measured from T0 to T0+21.760 s)
is best fit in the 20 keV - 10 MeV range
by the GRB (Band) model with the following parameters:
the low-energy photon index alpha = -1.30(-0.06,+0.06),
the high energy photon index beta = -2.87(-0.15,+0.11),
the peak energy Ep = 104(-3,+3) keV
(chi2 = 94/82 dof).

The spectrum near the maximum count rate
(measured from T0+0.256 to T0+5.376 s)
is best fit in the 20 keV - 10 MeV range
by the GRB (Band) model with the following parameters:
the low-energy photon index alpha = -1.09(-0.06,+0.07),
the high energy photon index beta = -2.91(-0.15,+0.12),
the peak energy Ep = 129(-4,+5) keV
(chi2 = 72/66 dof).

Assuming the redshift z=0.310 (Saccardi et al., GCN 40228)
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 1.49(-0.04,+0.04)x10^52 erg,
the peak luminosity L_iso is 2.97(-0.31,+0.32)x10^51 erg/s,
the rest-frame peak energy of the time-averaged spectrum
Ep,i,z is 137(-4,+4) keV and the spectrum near the maximum count rate
Ep,p,z is 169(-5,+7) keV.

With the obtained estimates, GRB 250424A is inside 68% prediction bands for
both 'Amati' and '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/GRB250424_T24733/GRB250424A_rest_frame.pdf

All the quoted errors are at the 68% confidence level.
All the quoted values are preliminary.