TITLE: GCN CIRCULAR NUMBER: 6459 SUBJECT: Konus-Wind observation of GRB 070521 DATE: 07/05/23 16:45:59 GMT FROM: Valentin Pal'shin at Ioffe Inst S. Golenetskii, R.Aptekar, E. Mazets, V. Pal'shin, D. Frederiks, and T. Cline on behalf of the Konus-Wind team report: The long GRB 070521 (Swift-BAT trigger #279935: Guidorzi et al., GCN 6431; Palmer et al., GCN 6440) triggered Konus-Wind at T0=24691.587 s UT (06:51:31.587). The burst started at T-T0 ~-33 s and had a duration of ~55 s. As observed by Konus-Wind the burst had a fluence of 1.81(-0.31, +0.06)x10^-5 erg/cm2, and a 64-ms peak flux measured from T0+5.312 s 4.12(-1.07, +0.78)x10^-6 erg/cm2/s (both in the 20 keV - 1 MeV energy range). The burst shows strong spectral evolution. The time-integrated spectrum of the burst (from T0 to T0+23.808 s: this interval comprises ~80% of the burst total counts) can be fitted (in the 20 keV - 1 MeV range) by a power law with exponential cutoff model: dN/dE ~ E^(-alpha)*exp(-E*(2-alpha)/Ep) with alpha = 0.93 +/- 0.12 and Ep = 222(-21, +27) keV (chi2 = 43/56 dof). All the quoted errors are at the 90% confidence level. The Konus-Wind light curve of this GRB is available at http://www.ioffe.rssi.ru/LEA/GRBs/GRB070521_T24691/