TITLE: GCN CIRCULAR NUMBER: 11315 SUBJECT: Swift Trigger 435520 is probably not a GRB DATE: 10/10/03 09:41:04 GMT FROM: Jamie A. Kennea at PSU/Swift-XRT J. R. Cummings (NASA/UMBC), D. N. Burrows (PSU), O. Godet (U Leicester), C. Guidorzi (U Ferrara), S. T. Holland (CRESST/USRA/GSFC), J. A. Kennea (PSU), W.B Landsman (GSFC), V. Mangano (INAF-IASFPA), R. Margutti (INAF-OAB), C. B. Markwardt (NASA/GSFC), A. Melandri (INAF-OAB), K. L. Page (U Leicester), B. Sbarufatti (INAF-OAB/IASFPA) and C. A. Swenson (PSU) report on behalf of the Swift Team: At 09:18:01 UT, the Swift Burst Alert Telescope (BAT) triggered and located trigger 435520. Swift slewed immediately to the location. The BAT on-board calculated location is RA, Dec 300.937, +32.386 which is RA(J2000) = 20h 03m 45s Dec(J2000) = +32d 23' 11" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve does not show any obvious signal, which is typical for image triggers. Based on ground processing of flight detector image data, the significance is 6.2 sigma, which would not normally exceed the detection criteria. We would need additional event-by-event data processed on the ground to clarify the significance. The XRT began observing the field at 09:20:24.85 UT, 143.1 seconds after the BAT trigger. No source was detected in the promptly available XRT 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 147 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 coverage of the BAT error circle by the 8'x8' region for the list of sources generated on-board is uncertain because the large number of sources filled the available telemetry. 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 large, but uncertain extinction expected.