TITLE: GCN CIRCULAR NUMBER: 8943 SUBJECT: Swift Trigger 345551 is probably not a GRB DATE: 09/03/07 04:53:54 GMT FROM: Scott Barthelmy at NASA/GSFC P. A. Evans (U Leicester), S. D. Barthelmy (GSFC), A. P. Beardmore (U Leicester), N. Gehrels (NASA/GSFC), E. A. Hoversten (PSU), F. E. Marshall (NASA/GSFC), C. Pagani (PSU), K. L. Page (U Leicester), D. M. Palmer (LANL), A. M. Parsons (GSFC), M. H. Siegel (PSU), T. N. Ukwatta (GSFC/GWU) and L. Vetere (PSU) report on behalf of the Swift Team: At 03:46:37 UT, the Swift Burst Alert Telescope (BAT) triggered and located a marginal peak (trigger=345551). Swift slewed immediately to the location. The BAT on-board calculated location is RA, Dec 244.979, -28.644, which is RA(J2000) = 16h 19m 55s Dec(J2000) = -28d 38' 38" with an uncertainty of 4 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve shows a slight increase above background at T_0, but it is very weak. The XRT began observing the field at 03:48:52.38 UT, 135.5 seconds after the BAT trigger. Using promptly downlinked data we find an uncatalogued X-ray source located at RA, Dec 244.99412, -28.63366 which is equivalent to: RA(J2000) = 16h 19m 58.59s Dec(J2000) = -28d 38' 01.2" with an uncertainty of 3.8 arcseconds (radius, 90% containment). The source does not appear to be fading at this time, with a limit of alpha ~0.5. A power-law fit to a spectrum formed from promptly downlinked event data gives a column density consistent with the Galactic value of 1.14e+21 cm^-2 (Kalberla et al. 2005). UVOT took a finding chart exposure of 150 seconds with the White filter starting 128 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 100% of the XRT 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 XRT 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 0.53. We are currently commissioning a new sub-threshold capability for Swift, allowing the spacecraft to quickly slew to events with marginal statistics (in the 5.5 - 6.5 sigma range) in the BAT image, and using XRT to test whether a source is detected at a consistent position. If automatic ground processing finds a consistent XRT source, the event is automatically promoted to GRB status, and the original BAT position and other TDRSS messages are released as delayed GCN Notices. We believe that this event was most likely to be a low-significance statistical fluctuation in the BAT image, in coincidence with a serendipitous detection of a non-GRB astrophysical source by the XRT. After the XRT source was found, GCN Notices were transmitted, 7 minutes after the original trigger. The association of the initial BAT detection with a real GRB will be determined using Malindi data, including analysis of the XRT data to see if the XRT source exhibits GRB-like temporal and spectral behavior. The currently available XRT data does not indicate fading.