TITLE:   GCN CIRCULAR
NUMBER:  45722
SUBJECT: Konus-Wind detection of GRB 260921A (possible merger-origin)
DATE:    26/09/24 13:39:33 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 260921A
(Fermi-GBM detection: Fermi GBM Team, GCN 45674;
Sonawane, GCN 45684;
Swift-BAT detection: Page et al., GCN 45676;
Parsotan et al., GCN 45686;
AstroSat-CZTI detection: Goyal et al., GCN 45687)
triggered Konus-Wind at T0=34147.045 s UT (09:29:07.045).

The burst light curve shows a multipeaked structure
which starts at ~T0-2.8 s and has a total duration of ~8.9 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/GRB260921_T34147/

As observed by Konus-Wind, the burst
had the total fluence of 7.93(-0.60,+0.68)x10^-6 erg/cm2,
and the 64-ms peak flux, measured from T0+2.880 s,
of 3.71(-0.70,+0.73)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+8.448 s)
is best fit in the 20 keV - 10 MeV range
by a power law with exponential cutoff model:
dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep)
with alpha = -0.92(-0.18,+0.21)
and Ep = 334(-43,+59) keV (chi2 = 78/82 dof).
Fitting by a GRB (Band) model yields the same alpha and Ep,
and an upper limit on the high energy photon index: beta < -2.3
(chi2 = 79/81 dof).

The spectrum near the maximum count rate
(measured from T0+0.256 to T0+8.448 s)
is best fit in the 20 keV - 10 MeV range
by a power law with exponential cutoff model:
dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep)
with alpha = -0.90(-0.21,+0.24)
and Ep = 308(-41,+59) keV (chi2 = 80/83 dof).
Fitting by a GRB (Band) model yields the same alpha and Ep,
and an upper limit on the high energy photon index: beta < -2.3
(chi2 = 80/82 dof).

Assuming the redshift z=1.300 (Segura et al., GCN 45678)
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 3.65(-0.27,+0.31)x10^52 erg,
the peak luminosity L_iso is 3.93(-0.74,+0.77)x10^52 erg/s,
the rest-frame peak energy of the time-averaged spectrum
Ep,i,z is 768(-99,+136) keV and the spectrum near the maximum count rate
Ep,p,z is 708(-95,+135) keV.

With the obtained estimates, GRB 260921A is a clear outlier
in the 'Amati' relation and nearly an outlier in the 'Yonetoku'
relation derived for the sample of >300 long KW GRBs with known redshifts
(Tsvetkova et al., 2017; Tsvetkova et al., 2021), but is generally consistent 
with the Type I (merger-origin) population,
see http://www.ioffe.ru/LEA/GRBs/GRB260921_T34147/GRB260921A_rest_frame.pdf

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