TITLE: GCN CIRCULAR NUMBER: 23291 SUBJECT: Konus-Wind observation of GRB 180925A DATE: 18/10/02 12:49:43 GMT FROM: Anna Kozlova at Ioffe Institute A. Kozlova, S. Golenetskii, R. Aptekar, D. Frederiks, M. Ulanov, D. Svinkin, A. Tsvetkova, A. Lysenko, and T. Cline on behalf of the Konus-Wind team, report: The long-duration GRB 180925A (Swift-BAT detection: Gropp et al., GCN 23268; Ukwatta et al., GCN 23273; Insight-HXMT/HE observation: Luo et al., GCN 23275) triggered Konus-Wind at T0=52650.967 s UT (14:37:30.967). The burst light curve shows a multipeaked structure which starts at ~T0 and has a total duration of ~26 s. The emission is seen up to ~6 MeV. As observed by Konus-Wind, the burst had a fluence of 1.44(-0.23,+0.32)x10^-5 erg/cm2, and a 64-ms peak flux, measured from T0+5.088 s, of 4.99(-1.51,+1.66)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+24.832 s) is best fit in the 20 keV - 6 MeV range by a power law with exponential cutoff model: dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep) with alpha = -0.99(-0.17,+0.21) and Ep = 528(-135,+239) keV (chi2 = 77/76 dof). Fitting by a GRB (Band) model yields alpha = -0.84(-0.33,+0.40), Ep = 374(-139,+403) keV, and beta = -2.0(-8.0,+0.3) (chi2 = 77/75 dof). The spectrum near the maximum count rate (measured from T0+0.256 to T0+8.448 s) is best fit in the 20 keV - 6 MeV range by a power law with exponential cutoff model with alpha = -0.85(-0.20,+0.23) and Ep = 544(-125,+200) keV (chi2 = 93/76 dof). Fitting by a GRB (Band) model yields the same alpha and Ep, and an upper limit on the high energy photon index: beta < -2.8 (chi2 = 93/75 dof). The Konus-Wind light curve of this GRB is available at http://www.ioffe.ru/LEA/GRBs/GRB180925_T52650/ All the quoted errors are at the 90% confidence level. All the quoted values are preliminary.