GCN Circular 2925

D. Palmer (LANL), S. Barthelmy, L. Barbier (GSFC), S. Campana (OAB),
J. Cummings (GSFC), E. Fenimore (LANL), N. Gehrels (GSFC),
D. Hullinger (GSFC/UMD), K. Hurley (UCB), H. Krimm (GSFC/USRA),
C. Markwardt (GSFC/UMD), K. McLean (LANL), T. Sakamoto (GSFC),
G. Sato (ISAS), M. Suzuki (Saitama), M. Tashiro (Saitama), J. Tueller (GSFC)
on behalf of the Swift-BAT team:

The Burst Alert Telescope (BAT) on Swift triggered on and recorded
the super-flare outburst of SGR1806-20 at 21:30:26 2004-12-27.  We did not
localize the burst because it was outside the coded aperture FOV of the instrument.
The angle from the bore sight was 105 degrees, illuminating the instrument
from slightly underneath the plane of the detectors through the XRT and UVOT
instruments and the body of the spacecraft.  Even with the absorption
from this material, the main peak countrate in the detector array
is saturated at 2.5E6 evts/sec (the maximum count rate the detector electronics
is capable of).  The FWHM of the main peak was ~0.5 sec.

After the main peak, BAT recorded a series of 51 pulsations
with a period of 7.56 sec and with three peaks within each pulsation.

Prior to the super-flare peak, there was a precursor peak at T-143 sec,
The peak count rate (well below the BAT instrumental saturation level)
was 31,000 evts/sec (15-350 keV, with nearly all counts below 100 keV) 
and with a square profile of rise and fall times less than 100 msec and
width of 1.2 sec.