TITLE:   GCN CIRCULAR
NUMBER:  45751
SUBJECT: Konus-Wind detection of GRB 260924A
DATE:    26/09/25 20:37:17 GMT
FROM:    Anna Ridnaia at Ioffe Institute <ridnaia@mail.ioffe.ru>

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

The long-duration GRB 260924A
(Fermi-GBM detection: Fermi GBM team, GCN 45712;
McDermott & Scotton, GCN 45723;
Swift detection: D'Elia et al., GCN 45713;
Gupta et al., GCN 45733;
AstroSat-CZTI detection: Salunke et al., GCN 45720;
Fermi-LAT detection: Airasca et al., GCN 45721;
Nustar detection: Waratkar & Grefenstette, GCN 45732;
SVOM-GRM detection: Yu et al., GCN 45741)
triggered Konus-Wind at T0=04406.079 s UT (01:13:26.079).

The burst light curve shows two separated multipeaked emission episodes.
The first episode starts at ~T0-3.5 s and has a total duration of ~212 s,
the second starts at ~T0+328 s and lasts up to ~T0+361 s.
The emission is seen up to ~8 MeV.

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB260924_T04406/

As observed by Konus-Wind, the burst
had the total fluence of 3.83(-0.40,+0.43)x10^-4 erg/cm2,
and the 64-ms peak flux, measured from T0+108.288 s,
of 1.10(-0.16,+0.16)x10^-5 erg/cm2/s
(both in the 20 keV - 10 MeV energy range).

The time-averaged spectrum of the burst
(measured from T0 to T0+360.704 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 = -0.62(-0.11,+0.14),
the high energy photon index beta = -2.37(-0.31,+0.20),
the peak energy Ep = 497(-61,+59) keV
(chi2 = 91/97 dof).

The spectrum near the maximum count rate
(measured from T0+106.752 to T0+114.944 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 = -0.62(-0.09,+0.11),
the high energy photon index beta = -2.41(-0.23,+0.17),
the peak energy Ep = 596(-60,+63) keV
(chi2 = 82/86 dof).

Assuming the redshift z=3.510 (Castro-Tirado et al., GCN 45740)
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 1.00(-0.11,+0.11)x10^55 erg
(on a par with the most energetic to date GRB 221009A, Frederiks et al., 2023),
the peak luminosity L_iso is 1.30(-0.19,+0.19)x10^54 erg/s (one of the most 
luminous GRBs to date), the rest-frame peak energy of the time-averaged spectrum
Ep,i,z is 2242(-275,+267) keV and the spectrum near the maximum count rate
Ep,p,z is 2690(-270,+285) keV.

With the obtained estimates, GRB 260924A 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/GRB260924_T04406/GRB260924A_rest_frame.pdf

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