TITLE: GCN CIRCULAR NUMBER: 19153 SUBJECT: Swift trigger 677890 is noise DATE: 16/03/05 21:03:09 GMT FROM: Scott Barthelmy at NASA/GSFC S. D. Barthelmy (GSFC), J. A. Kennea (PSU), K. L. Page (U Leicester) and D. M. Palmer (LANL) report on behalf of the Swift Team: At 20:42:32 UT, the Swift Burst Alert Telescope (BAT) triggered on a noise event that was spatially near Ser X-1 (trigger=677890). Swift slewed immediately to the location. The BAT on-board calculated location is RA, Dec 279.868, +5.113, which is RA(J2000) = 18h 39m 28s Dec(J2000) = +05d 06' 45" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). As is typical for images triggers (64 sec in this case) the real-time light curve does not show anything. We believe this trigger is non-astrophysical. The XRT began observing the field at 20:44:52.5 UT, 140.4 seconds after the BAT trigger. The position determined from promptly downlinked data differs significantly from the on-board position, suggesting that the XRT may have centroided on a cosmic ray; the initial XRT position notice should be treated with caution. The source in the promptly downlinked data is only a marginal detection and is likely due to background variations. We do not believe it is related to this trigger. UVOT took a finding chart exposure of 250 seconds with the U filter starting 146 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 none of the BAT error circle. The overlap of the 8'x8' region for the list of sources generated on-board and the XRT error circle is uncertain. No correction has been made for the large, but uncertain extinction expected. This trigger was due to a low-significance image fluctuation in an image without a corresponding rate trigger. Although the derived location of the image peak was close enough to Ser X-1 to trigger automated follow-up observations, its 9 arc minute offset is inconsistent with being a true detection of the source. Therefore, we believe that this event is merely a statistical fluctuation in the image plane, and not an astrophysical event.