TITLE: GCN CIRCULAR NUMBER: 41027 SUBJECT: GRB 250706B/C: Konus-Wind analysis of the prompt emission DATE: 25/07/08 13:55:05 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: We report on a further analysis of the KW detection of the very bright, long-duration GRB 250706C (Frederiks et al., GCN 41013), which common origin with the SVOM/ECLAIRs GRB 250706B (Palmerio et al., GCN 40989) was confirmed from the FERMI/LAT observation (Longo et al., GCN 41019). GRB 250706C triggered Konus-Wind (KW) at T0=60322.626 s UT (16:45:22.626), ~1200 s before the SVOM/ECLAIRs trigger on GRB 250706B. The burst light curve shows a very bright, multi-peaked emission pulse with the duration of ~50 s (20-1500 keV). This pulse is followed by a weaker, smoothly decaying emission tail, visible up to the end of the KW triggered data record (~T0+250 s). The emission in the main pulse is seen up to ~15 MeV. The KW light curve is available at http://www.ioffe.ru/LEA/GRBs/GRB250706_T60322/ As observed by Konus-Wind, the burst had the total fluence of (5.12 ± 0.06)x10^-4 erg/cm^2 and a 16-ms peak energy flux, measured from T0 + 36.800 s, of (4.16 ± 0.22)x10^-5 erg/cm^2/s (both in the 20 keV - 10 MeV energy range). The time-integrated spectrum (measured from T0 to T0+51.968 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 = -1.00 (-0.01,+0.01), the high energy photon index beta = -3.65 (-0.44,+0.23), the peak energy Ep = 850(-15,+15) keV, chi2 = 125/96 dof. The spectrum near the peak count rate (measured from T0+36.608 s to T0+37.120 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 = -1.10 (-0.04,+0.04), the high energy photon index beta = -2.91 (-0.60,+0.30), the peak energy Ep = 1005(-91,+104) keV, chi2 = 69/58 dof. The spectrum of the decaying tail (measured from T0+51.968 s to T0+232.192) is best fit by a simple power law (PL) function with the PL photon index of (-2.0 ± 0.06), chi2 = 92/99 dof. Assuming the redshift z=0.942 (Palmerio et al., GCN 41022) 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 (1.26 ± 0.02)x10^54 erg, the isotropic peak luminosity L_iso to (2.00 ± 0.11)x10^53 erg/s, and the rest-frame peak spectral energy Epi,z to 1650(-30,+30) keV. With the obtained estimates, GRB 250706B/C is consistent with 68% prediction bands of 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/GRB250706_T60322/GRB250706BC_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/41027. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...