TITLE: GCN CIRCULAR NUMBER: 17514 SUBJECT: Swift Trigger 633173 is probably not an astrophysical source DATE: 15/03/01 18:25:04 GMT FROM: Scott Barthelmy at NASA/GSFC S. D. Barthelmy (GSFC), M. M. Chester (PSU), M. De Pasquale (INAF-IASFPA), N. Gehrels (NASA/GSFC), A. Y. Lien (GSFC/UMBC), F. E. Marshall (NASA/GSFC), A. Maselli (INAF-IASFPA), D. M. Palmer (LANL) and M. H. Siegel (PSU) report on behalf of the Swift Team: At 17:58:50 UT, the Swift Burst Alert Telescope (BAT) triggered and located a likely noise event (trigger=633173). Swift slewed immediately to the location. The BAT on-board calculated location is RA, Dec 139.480, -22.437, which is RA(J2000) = 09h 17m 55s Dec(J2000) = -22d 26' 13" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). As is typical for image triggers, nothing shows up in the real-time TDRSS light curve. No XRT counterpart was detected in 980 s of promptly downlinked data. UVOT took a finding chart exposure of 250 seconds with the U filter starting 341 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 21% of the BAT error circle. The typical 3-sigma upper limit has been about 19.2 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 18.0 mag. No correction has been made for the expected extinction corresponding to E(B-V) of 0.10. This event alerted as part of a program to follow up low-significance possible detections in the direction of nearby galaxies. Due to the marginal significance of the detection (6.16 sigma) the distance of the event from the putative host galaxy (5 arcminutes, about twice the galaxy's nominal radius) and the lack of a detection by the XRT and UVOT, we believe that this is not an astrophysical detection.