TITLE: GCN CIRCULAR NUMBER: 45046 SUBJECT: Konus-Wind detection of GRB 260623B / EP260623a DATE: 26/06/26 15:11:51 GMT FROM: Dmitry Svinkin at Ioffe Institute <svinkin@mail.ioffe.ru> 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; Wang et al., GCN 45035) 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) 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. View this GCN Circular online at https://gcn.nasa.gov/circulars/45046. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...