TITLE: GCN CIRCULAR NUMBER: 14699 SUBJECT: Swift Trigger 556533 is probably not an astrophysical event DATE: 13/05/24 04:23:05 GMT FROM: David Palmer at LANL A. P. Beardmore (U Leicester), H. A. Krimm (CRESST/GSFC/USRA), C. J. Mountford (U Leicester), K. L. Page (U Leicester), D. M. Palmer (LANL) and M. H. Siegel (PSU) report on behalf of the Swift Team: At 04:03:55 UT, the Swift Burst Alert Telescope (BAT) triggered and located trigger 556533. Swift slewed immediately to the location. The BAT on-board calculated location is RA, Dec 118.580, -28.134 which is RA(J2000) = 07h 54m 19s Dec(J2000) = -28d 08' 03" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed no significant structure. The peak count rate was ~1400 counts/sec (15-350 keV), at ~4 sec after the trigger. The XRT began observing the field at 04:05:29.5 UT, 94.7 seconds after the BAT trigger. No source was detected in 574 s of promptly downlinked data, which covered 92% of the BAT error circle. We are waiting for the full dataset to detect and localise the XRT counterpart. UVOT took a finding chart exposure of nominal 250.000 seconds with the U filter starting 154 seconds after the BAT trigger. No credible afterglow candidate has been found in the initial data products. The 2.7'x2.7' sub-image covers 25% of the BAT error circle. The typical 3-sigma upper limit has been about 19.6 mag. The 8'x8' region for the list of sources generated on-board covers 100% of the BAT error circle. The list of sources is typically complete to about 18 mag. No correction has been made for the large, but uncertain extinction expected. This was a marginal detection (5.85 sigma) in an image at a location which was almost 12 arcminutes from the known source A0753-2802. Under these circumstances, Swift attempts to verify or refute the reality of the possible event using the narrow field instruments. In this case, we believe that this was due to a statistical fluctuation and not an astrophysical event.