GCN Circular 3768

J. Cummings (GSFC/NRC), S. Barthelmy (GSFC),  D. Burrows (PSU), 
M. Chester (PSU), N. Gehrels (GSFC),  M. Goad (U. Leicester), 
O. Godet (U. Leicester), J. Kennea (PSU), C. Markwardt (GSFC/UMD), 
A. Morgan (PSU), C. Pagani (PSU), K. Page (U. Leicester), D. Palmer (LANL), 
T. Sakamoto (GSFC/NRC)
on behalf of the Swift team:

At 20:41:26.7 UT, Swift-BAT triggered and located potential GRB 050805B
(trigger=149131).  The spacecraft slewed immediately.  The BAT
on-board calculated location is RA,Dec 306.005d, +37.213d {+20h 24m
01s, +37d 12' 46"} (J2000), with an uncertainty of 3 arcmin (radius,
90% containment, stat+sys).  The trigger duration is 32 milliseconds,
and the image significance was 6.3 sigma.  At that level, we consider
the BAT detection itself to be marginal.

XRT began observing the burst at 20:42:40.8 UT, 74 seconds after the BAT 
trigger.  The XRT was not able to centroid on a point source in the field, 
however the full field lightcurve shows a significant elevation above 
background, suggesting the presence of an X-ray source in the field. A 
catalogue search reveals no known X-ray sources in the field, suggesting 
this X-ray source is the X-ray afterglow.  There is evidence of fading in 
the lightcurve, which will require ground processing to confirm.

UVOT began observing the burst at 20:42:38.6 UT, 72 seconds after the BAT 
trigger.  In a preliminary ground-processed 100-sec image, no new sources 
are found when compared to the DSS catalog down to a 5-sigma limiting 
magnitude of V > 19.0.