TITLE: GCN CIRCULAR NUMBER: 3999 SUBJECT: Swift-BAT trigger 155764 is probably not a GRB DATE: 05/09/18 20:29:07 GMT FROM: Scott Barthelmy at NASA/GSFC D. Grupe (PSU), S. Barthelmy (GSFC), D. Burrows (PSU), N. Gehrels (GSFC), H. Krimm (GSFC/USRA), J. Kennea (PSU), C. Markwardt (GSFC/UMD), D. Palmer (LANL), P. Roming (PSU) on behalf of the Swift team: The Swift-BAT trigger (trigger=155764) at 19:42:13 UT on 18 Sep 2005 is probably not due to a GRB or any other astrophysical source. We will have the full data set at 23:45 UT, after which we will issue a final determination. The spacecraft slewed immediately and the XRT began observing at 19:43:11 (at T+58s). The XRT generated an on-board centroid on a cosmic ray near the edge of the field of view. This position is invalid (GCN 3998). The spectrum and light curve are ambiguous, and we cannot rule out the possibility of a weak afterglow in the XRT field of view. Further analysis will require downloaded data following the next ground station pass. [GCN OPS NOTE(18sep05): Per author's request, the trigger number in the subject line was corrected.]