GCN Circular 6745

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/