GCN Circular 3381

C. Hurkett, E. Rol, (U Leicester), L. Barbier (GSFC), S. Barthelmy (GSFC),
A. Blustin (MSSL), D. Burrows (PSU),  J. Cummings (GSFC/NRC),
N. Gehrels (GSFC),  J. Kennea (PSU), C. Markwardt (GSFC/UMD), S. Holland
(GSFC/USRA) on behalf of the Swift Team;

At 04:00:19.23 UT, the Swift Burst Alert Telescope (BAT) triggered and
located on-board GRB050509b (trigger #118749).   The BAT on-board
calculated location is RA, dec 189.056, +29.000 (12h 36m 13s, +29d 00' 01'')
(J2000) with an uncertainty of 4 arcmin (radius, 3-sigma, including 
estimated systematic uncertainty). The BAT light curve showed a single short
spike with a duration of less than 128 milliseconds. The peak
count rate measured by BAT was about 2100 counts/sec in the 15 - 
350 keV band.

The Swift spacecraft slewed promptly onto the BAT position.  The XRT
imaged the field at 04:01:12 (53.0 s after the burst), but did not find
any bright source in the field.  An XRT position will be determined by
analysis of the full data following the next ground station pass.

The Swift Ultra Violet/Optical Telescope (UVOT) observations began at
04:01:09.8 UT, 50 seconds after the BAT trigger.  The first
data taken after the spacecraft settled was a 100 sec exposure
using the V filter with the midpoint of the observation at
100 sec after the BAT trigger. Based on a preliminary comparison with the
DSS, there is a possible low significance source at RA=12:36:18, 
DEC=29:01:24 which is not present in the DSS. It has a V-band 
magnitude of 18.8.