TITLE: GCN CIRCULAR NUMBER: 41905 SUBJECT: Konus-Wind detection of GRB 250919A DATE: 25/09/20 15:50:37 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 GRB 250919A (Fermi-GBM detection: The Fermi GBM team, GCN 41874; Mukherjee & Meegan, GCN 41890; EP-WXT detection: Liang et al., GCN 41879; SVOM-GRM observaion: Wang et al., GCN 41882), Fermi-LAT detection: Holzmann Airasca et al., GCN 41884; Glowbug detection: Cheung et al., GCN 41891; AstroSat CZTI detection: Harsha et al., GCN 41899) triggered Konus-Wind (KW) at T0=1741.403 s UT (00:29:01.403). The burst consists of two separated emission pulses and had the total duration of ~240 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/GRB250919_T01741/ As observed by Konus-Wind, the burst had a fluence of (4.24 ± 0.25)x10^-4 erg/cm^2 and a 64-ms peak energy flux, measured from T0 + 23.168 s, of (1.33 ± 0.09)x10^-4 erg/cm^2/s (both in the 20 keV - 10 MeV energy range). The time-integrated spectrum (measured from T0 to T0+218 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 = -0.94 (-0.05,+0.05), the high energy photon index beta = -2.32 (-0.17,+0.11), the peak energy Ep = 452 (-33,+39) keV, chi2 = 101/97 dof. The spectrum near the peak count rate (measured from T0+23.04 to T0+23.296 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 = -0.07 (-0.09,+0.10), the high energy photon index beta = -2.15 (-0.07,+0.06), the peak energy Ep = 515 (-38,+42) keV, chi2 = 63/62 dof. Assuming the redshift z=1.145 (Levan et al., GCN 41883) 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 (1.53 ± 0.09)x10^54 erg, the isotropic peak luminosity L_iso to (1.03 ± 0.07)x10^54 erg/s, and the rest-frame peak spectral energy Ep,z to (970 ± 80) keV. With the obtained estimates, GRB 250919A 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/GRB250919_T01741/GRB250919A_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/41905. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...