GCN Circular 2853

D. Palmer (LANL), E. Fenimore (LANL), S. Barthelmy (GSFC), G. Chincarini
(OAB), J. Cummings (GSFC), N. Gehrels (GSFC), P. Giommi (ASDC), M. Goad
(U.Leicester), D. Hullinger (UMD), H. Krimm (USRA), C. Markwardt (UMD), K. Mason
(MSSL), K. McLean (LANL), J. Nousek (PSU), J. Osborne (U.Leicester), A. Parsons
(GSFC), G. Sato (ISAS), M. Suzuki (Saitama), J. Tueller (GSFC), A. Wells
(U.Leicester), N. White (GSFC), on behalf of the Swift BAT team.

On Dec 17 2004, 7:28:30 UT, the Swift Burst Alert Telescope (BAT)  
triggered and located on-board an apparent gamma-ray burst. The
spacecraft did not autonomously slew to the burst since automated slewing
is not yet enabled.

The location is RA/Dec 164.79, -17.95 (J2000). This is about 42 degrees off the
bore sight in the partially encoded field of view. The Swift attitude
control system is still undergoing calibration, but we estimate that the
position is within 12 arcmin (radius). The on-orbit location differed from a
location processed on the ground by about 6 arcmin. After 
calibration we expect on-board locations to be within 4 arcmin.

The burst can be characterized as a smooth profile with a rise time of
2 sec and a fall time of 6 sec. The T90 was ~6.7. Although BAT is still
undergoing calibration, ground processing gives a power law spectrum with
a number index of -1.6 and a fluence of 5E-6 erg/cm^2 in the ~20 to 200
keV range. The peak was about 4.4 cts/cm^2/sec in ~15 to 200 keV.

[GCN OPS NOTE (18-19dec04): "UL" was changed to "U.Leicester".  And the words
"(J2000)" and "(radius)" were added to the position estimate.]