TITLE: GCN CIRCULAR NUMBER: 41957 SUBJECT: Konus-Wind detection of GRB 250920C DATE: 25/09/23 19:12:38 GMT FROM: Valentina Panteleeva at Ioffe Institute <panteleevav228@gmail.com> V. Panteleeva, 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 250920C (Fermi-GBM detection: The Fermi GBM team, GCN 41903; Mailyan & Meegan, GCN 41917; Swift-BAT detection: Gupta et al., GCN 41904; Gupta et al., GCN 41924; SVOM-GRM observaion: Wang et al., GCN 41929) triggered Konus-Wind at T0=55519.312 s UT (15:25:19.312). The burst light curve shows a multipeaked structure which starts at ~T0-0.4 s and has a total duration of ~41 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/GRB250920_T55519/ As observed by Konus-Wind, the burst had the total fluence of 2.91(-0.21,+0.29)x10^-5 erg/cm2, and the 64-ms peak flux, measured from T0+25.904 s, of 8.27(-0.78,+0.78)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+41.216 s) is best fit in the 20 keV - 10 MeV range by the GRB (Band) model with the following parameters: the low-energy photon index alpha = -1.22(-0.07,+0.09), the high energy photon index beta = -3.19(-6.81,+0.51), the peak energy Ep = 160(-12,+9) keV (chi2 = 104/97 dof). The spectrum near the maximum count rate (measured from T0+24.832 to T0+33.024 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 = -1.14(-0.07,+0.07) and Ep = 220(-14,+16) keV (chi2 = 96/81 dof). Fitting by a GRB (Band) model yields the same alpha and Ep, and an upper limit on the high energy photon index: beta < -3.15 (chi2 = 97/80 dof). Assuming the redshift z=1.399 (Sanchez-Ramirez et al., GCN 41928; Izzo et al., GCN 41955) 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 1.54(-0.11,+0.16)x10^53 erg, the peak luminosity L_iso is 1.05(-0.10,+0.10)x10^53 erg/s, the rest-frame peak energy of the time-averaged spectrum Ep,i,z is 383(-30,+22) keV and the spectrum near the maximum count rate Ep,p,z is 528(-33,+38) keV. With the obtained estimates, GRB 250920C is inside 68% prediction bands for both the 'Amati' and 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/GRB250920_T55519/GRB250920C_rest_frame.pdf All the quoted errors are at the 68% confidence level. All the quoted values are preliminary. View this GCN Circular online at https://gcn.nasa.gov/circulars/41957. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...