TITLE: GCN CIRCULAR NUMBER: 6745 SUBJECT: A giant outburst from Cygnus X-1 detected by Konus-Wind and Suzaku-WAM DATE: 07/08/10 17:01:31 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, K. Yamaoka, M. Ohno, C. Kira, Y. Fukazawa, T. Takahashi, M. Tashiro, Y. Terada, T. Murakami, and K. Makishima on behalf of the Suzaku WAM team, report: At 2007-08-07 ~23:40 UT, Konus-Wind detected the onset of a very long soft event which had been observed at least until 2007-08-08 ~7:00 UT in the G1 (19-70 keV) and G2 (70-300 keV) bands. The Konus-Wind light curve shows multipeaked structure with the maximum intensity at 2007-08-08 00:16 UT. The Konus-Wind ecliptic latitude response is consistent with the position of Cyg X-1. At the time, Suzaku-WAM detected emission with the patterns similar to those observed by the Konus-Wind and occultation steps which correspond to the set/rise times of Cyg X-1. The WAM angular response is consistent with the Cyg X-1 position. There is also a flaring seen in the XTE-ASM light curve of Cyg X-1 at this time (http://heasarc.gsfc.nasa.gov/cgi-bin/xte_weather/xte_lc?source=CYGX1&action=plot) Thus, we believe this event is a giant outburst from Cyg X-1. Preliminary analysis of the Konus-Wind data shows that the outburst had a fluence of ~1.4x10^-3 erg/cm2 (in the 15-350 keV) range, the time-integrated spectrum (accumulated from 2007-08-07 23:40 to 2007-08-08 07:00 UT) can be described by a power law with exponential cutoff model: dN/dE ~ E^(-alpha)*exp(-E*(2-alpha)/Ep) with alpha ~1.0 and Ep ~63 keV. This is the most powerful gamma-ray outburst from Cyg X-1 ever observed. The most powerful Cyg X-1 outburst detected before (on 2002 Feb. 24) had a fluence of 8.0x10^-4 erg/cm2 in the same energy range (Golenetskii et al., 2003, ApJ, 596, 1113). The Konus-Wind light curve of this outburst is available at http://www.ioffe.rssi.ru/LEA/CygX1/070807/