GCN Circular 45046

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 260623B
was detected by Konus-Wind (KW) in the waiting mode 
at about T0 = T0(EP) = 10445 s UT (02:54:05).
A Bayesian block analysis of the KW waiting mode data in
the 20-300 keV band reveals a ~10 sigma count-rate increase in
the interval from T0-259 s to T0+327 s.

The burst overlaps in time with EP260623a (EP-WXT detection: Wang et al., GCN 45020
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 ; 
Wang et al., GCN 45035
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 ) and the KW ecliptic latitude response is consistent with the EP260623a localization.
The positional and temporal coincidence of GRB 260623B with
the EP260623a supports the conclusion that both events have a common origin.

The KW light curve of this burst is available
at http://www.ioffe.ru/LEA/GRBs/GRB260623B

The total burst fluence is 8.2(-1.2,+1.2)x10^-6 erg/cm^2,
and the 2.944 s peak energy flux, measured from T0+3.366 s,
is 0.7(-0.3,+0.4)x10^-7 erg/cm^2/s.
(both in the 20 keV - 1500 keV energy range).

The time-integrated spectrum of the burst,
measured from T0-258.650 s to T0+327.206 s,
can be described by a power law with exponential cutoff (CPL) model
dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep)
with alpha > -1.16 and Ep = 76(-8,+28) keV.

The spectrum near the maximum count rate
(measured from T0-2.522 to T0+38.694 s)
can be described with the CPL model with alpha > -1.42 and Ep = 87(-15,+83) keV.

Assuming the redshift z=0.703 (Izzo et al., GCN 45022
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 )
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 1.13(-0.16,+0.24)x10^52 erg,
the peak luminosity L_iso is 1.71(-0.72,+0.97)x10^50 erg/s,
the rest-frame peak energy of the time-averaged spectrum
Ep,i,z is 130(-14,+47) keV and the spectrum near the maximum count rate
Ep,p,z is 148(-26,+143) keV.

With the obtained estimates, GRB 260623B is inside the 68% prediction band for
the 'Amati' and inside the 90% prediction band for 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/GRB260623B/GRB260623B_rest_frame.pdf

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