TITLE: GCN CIRCULAR NUMBER: 10045 SUBJECT: Konus-RF observation of GRB 091018 DATE: 09/10/19 21:35:13 GMT FROM: Valentin Pal'shin at Ioffe Inst S. Golenetskii, R.Aptekar, E. Mazets, V. Pal'shin, D. Frederiks, P. Oleynik, M. Ulanov, and D. Svinkin on behalf of the Konus-RF team, report: The soft GRB 091018 (Swift-BAT trigger #373172: Stamatikos et al., GCN 10034, Markwardt et al., GCN 10040) triggered Konus-RF at T0=74900.651 s UT (20:48:20.651). The burst light curve shows a single pulse with a duration of ~5 s. As observed by Konus-RF the burst had a fluence of 1.44(-0.16, +0.19)x10^-6 erg/cm2, and a 256-ms peak flux measured from T0-0.280 s of 4.32(-0.94, +0.97)x10^-7 erg/cm2/s (both in the 20 keV - 1 MeV energy range). The time-integrated spectrum of the burst (from T0-2s to T0+3s) is well fitted (in the 10 keV - 1 MeV range) by a power law with exponential cutoff model: dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep), with alpha = -1.53(-0.39, +0.59), and Ep = 28(-16, +10) keV (chi2 = 83.4/94 dof). Fitting by GRB (Band) model yields the same alpha and Ep, and only an upper limit on the high energy photon index: beta < -2.44 (chi2 = 81.7/93 dof). All the quoted values are preliminary. All the quoted errors are at the 90% confidence level. Assuming z = 0.971 (Chen et al., GCN 10038) and a standard cosmology model with H_0 = 70 km/s/Mpc, Omega_M = 0.27, Omega_Lambda = 0.73, the isotropic energy release E_iso ~3.7x10^51 erg, the peak luminosity (L_iso)_max ~2.2x10^51 erg/s, and Ep_rest ~55 keV. The Konus-RF light curve of this GRB is available at http://www.ioffe.ru/LEA/GRBs/GRB091018_T74900/KRF/