TITLE: GCN CIRCULAR NUMBER: 21409 SUBJECT: Swift Trigger 766161 is not a GRB DATE: 17/08/04 05:40:10 GMT FROM: David Palmer at LANL S. J. LaPorte (PSU), A. Y. Lien (GSFC/UMBC), K. L. Page (U Leicester), D. M. Palmer (LANL) and B. Sbarufatti (INAF-OAB/PSU) report on behalf of the Swift Team: At 05:08:36 UT, the Swift Burst Alert Telescope (BAT) detected a marginal peak in the image domain (trigger=766161). Swift slewed immediately to the location. The BAT on-board calculated location is RA, Dec 230.833, -19.547 which is RA(J2000) = 15h 23m 20s Dec(J2000) = -19d 32' 49" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). As is usual with an image trigger, the available BAT light curve shows no significant structure. The XRT began observing the field at 05:11:04.0 UT, 147.5 seconds after the BAT trigger. No source was detected in 1.4 ks of promptly downlinked data, which covered 98% 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 150 seconds with the White filter starting 150 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 expected extinction corresponding to E(B-V) of 0.14. Ground re-analysis of the detector histogram shows a greatly reduced significance (from 7.3 to 4.2 sigma). This, combined with the lack of a rate trigger and the non-detection of a counterpart by XRT, leads us to conclude that this is probably a statistical fluctuation in the image plane and not an astrophysical source.