TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  3158
SUBJECT: Swift-BAT detection of GRB 050331b
DATE:    05/03/31 19:52:38 GMT
FROM:    Craig Markwardt at NASA/GSFC/UMD  <craigm@milkyway.gsfc.nasa.gov>

C. B. Markwardt (GSFC/UMD), L. Barbier (GSFC), S. Barthelmy (GSFC),
J. Cannizzo (GSFC/UMBC), J. Cummings (GSFC/NRC), M. DePasquale (MSSL),
E. Fenimore (LANL), N. Gehrels (GSFC), D. Hullinger (UMD), 
H. Krimm (GSFC/USRA), D. Palmer (LANL), A. Parsons (GSFC),
T. Sakamoto (GSFC/NRC), G. Sato (ISAS), A. Smale (NASA HQ),
M. Suzuki (Saitama), J. Tueller (GSFC)
on behalf of the Swift team:

At 18:44:04 UT Swift-BAT triggered on GRB 050331b, trigger 112981, a
second burst within ninety minutes. The BAT-derived position is
RA,Dec= 143.840,-42.678 (J2000).  We note this is 81 arcsec from the
XRT Position GCN Notice.  The lightcurve has a series of
low-significance peaks covering a total burst duration of ~30 sec.
The light curve also shows a slow rise from T+60 to T+160 s in the
lowest energy bands.  We believe that this slow variation is due to
Vela X-1, which is within 10 degrees of the GRB position.  While
similar in duration to GRB 050331a, the light curve of GRB 050331b is
significantly different in appearance.  The peak count rate is ~1000
cnts/sec.

NOTE: This is the second verified burst of the day.  The earlier
burst, now called GRB 050331a, trigger 112977, was located 2.6 degrees
away from this burst (trigger 112981).  The wide angular separation,
and dissimilar light curves make it highly unlikely that these two
bursts are a gravitationally lensed pair.  The two bursts GRB 050331a
and GRB 050331b are two distinct bursts.