TITLE:   GCN CIRCULAR
NUMBER:  45923
SUBJECT: Konus-Wind detection of GRB 261007A/EP261007a
DATE:    26/10/12 13:28:08 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 261007A (
SVOM detection: Goto et al., GCN 45824;
Einstein Probe detection of an X-ray transient: Wen et al., GCN 45828;
Fermi-GBM observation: Malacaria, GCN 45830;
NuSTAR detectuin: Waratkar & Grefenstette, GCN 45846;
AstroSat-CZTI detection: Harsha et al., GCN 45859;
SVOM-GRM observation: Ren et al., GCN 45863;
Glowbug detection: Cheung et al., GCN 45867)
triggered Konus-Wind (KW) at T0=20393.695 s UT (05:39:53.695).

The burst light curve shows a bright multipeaked structure
in the interval from ~T0-1.2 s to ~T0+32 s.
The emission is seen up to ~10 MeV.

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB261007_T20393/

As observed by Konus-Wind, the burst had
the fluence of (9.36 ± 0.30)x10^-5 erg/cm^2 and
the 64-ms peak energy flux, measured from T0 + 21.996 s,
of (1.41 ± 0.09)x10^-5 erg/cm^2/s (both in the 20 keV - 10 MeV energy range).

The time-integrated spectrum of the burst (measured from T0 to T0+32 s)
is best fit in the 20 keV - 15 MeV range by a GRB (Band) function
with the following model parameters:
the low-energy photon index alpha = -0.53 (-0.04,+0.04),
the high energy photon index beta = -2.87 (-0.15,+0.12),
the peak energy Ep = 273 (-8,+8) keV,
chi2 = 89/97 dof.

The spectrum near the peak count rate (measured from T0+19.712 s to T0+23.808 s)
is best fit in the 20 keV - 15 MeV range by a GRB (Band) function
with the following model parameters:
the low-energy photon index alpha = -0.52 (-0.07,+0.07),
the high energy photon index beta = -2.73 (-0.15,+0.12),
the peak energy Ep = 245 (-11,+11) keV,
chi2 = 61/73 dof.

Assuming the redshift z=2.101 (Saccardi et al., GCN 45829)
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 (1.04 ± 0.03)x10^54 erg,
the isotropic peak luminosity L_iso to (8.84 ± 0.31)x10^53 erg/s, and
the rest-frame peak spectral energy Ep,z to (847 ± 25) keV.
With the obtained estimates, GRB 261007A is consistent with 68% prediction bands
for both '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/GRB261007_T20393/GRB261007A_rest_frame.pdf

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