TITLE: GCN CIRCULAR NUMBER: 22217 SUBJECT: Swift Trigger 796115 is probably not a GRB DATE: 17/12/09 18:19:49 GMT FROM: Scott Barthelmy at NASA/GSFC S. D. Barthelmy (GSFC), P. D'Avanzo (INAF-OAB), A. Deich (PSU), S. W. K Emery (UCL-MSSL), J.D. Gropp (PSU), J. A. Kennea (PSU), N. P. M. Kuin (UCL-MSSL), S. J. LaPorte (PSU), A. Y. Lien (GSFC/UMBC), F. E. Marshall (NASA/GSFC), A. Melandri (INAF-OAB), D. M. Palmer (LANL), M. H. Siegel (PSU) and A. Tohuvavohu (PSU) report on behalf of the Swift Team: At 17:57:15 UT, the Swift Burst Alert Telescope (BAT) triggered (trigger=796115). Swift slewed immediately to the location. The BAT on-board calculated location is RA, Dec 134.821, -55.226, which is RA(J2000) = 08h 59m 17s Dec(J2000) = -55d 13' 32" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). As is typical for image triggers, the light curve does not show anything significant. The XRT began observing the field at 17:59:30.5 UT, 134.6 seconds after the BAT trigger. No source was detected in 556 s of promptly downlinked data. 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 137 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. Because of the density of catalogued stars, further analysis is required to report an upper limit for any afterglow in the sub-image. The 8'x8' region for the list of sources generated on-board covers 100% of the BAT error circle. Because of the density of catalogued stars, further analysis is required to report an upper limit for any afterglow in the region. No correction has been made for the expected extinction corresponding to E(B-V) of 0.35. Due to the marginal detection (7.23 sigma) by BAT, which is reduced further in ground analysis, the lack of a rate trigger, and the non-detection of an afterglow by XRT, we believe that this is probably a statistical fluctuation in the image plane and not a real astrophysical source. There will be no further observations unless a source is detected in analysis of the full downlinked data.