E. Mazets, S. Golenetskii, R.Aptekar, D. Frederiks, V. Pal'shin, and
T. Cline on behalf of the Konus-Wind team, report:
On December 27, 2004, the Konus-Wind instrument detected a giant flare
from SGR 1806-20. The flare was preceded by a high emission
of recurrent soft bursts. A series of numerous bursts occurred
on October 5. On the analogy of the behaviour of SGR 1900+14 before
the giant flare on 1998 August 27, it was possible to expect,
that SGR 1806-20 is close to generation of a giant flare (GCN 2769).
Another series began on December 21 and lasted until the giant flare.
These bursts were strong in their flux, exhibiting a well pronounced
spectral variability and in some cases unusual spectral features.
A record of the giant outburst was triggered by a recurrent burst,
which was the strongest in the series. The giant outburst occurred
on 142 s after this trigger. The flare started with a very intense
initial pulse, resulting in overloading of the detector.
It was followed by a long pulsating tail lasted more than 380 s.
A measured period of the pulsations is 7.57 � 0.07 s,
indicating its increasing since 2000, when value of 7.49 s
was determined in Chandra observations of X-ray persistent flux
(Kaplan et al., 2002, ApJ, 564, 935) .
Giant outbursts were observed earlier in SGR 0526-66 on 1979 March 5,
in SGR 1627-41 on 1998 June 18, in SGR 1900+14 on 1998 August 27.
The event of December 27 completes this list for all of well known SGRs.
After the outburst, SGR 1806-20 continues to emit recurrent bursts.
A determination of energetic and spectral parameters of the outburst
will take some time because of a huge intensity of the initial pulse.