TITLE: GCN CIRCULAR NUMBER: 30905 SUBJECT: Swift Trigger 1076121 is not a GRB DATE: 21/10/01 01:06:32 GMT FROM: David Palmer at LANL <palmer@lanl.gov> E. Ambrosi (INAF-IASFPA), A. D'Ai (INAF-IASFPA), V. D'Elia (SSDC & INAF-OAR), J.D. Gropp (PSU), S. Laha (GSFC/UMBC), F. E. Marshall (NASA/GSFC), D. M. Palmer (LANL) and M. H. Siegel (PSU) report on behalf of the Neil Gehrels Swift Observatory Team: At 00:43:19 UT, the Swift Burst Alert Telescope (BAT) triggered and located a marginal peak (trigger=1076121). Swift slewed immediately to the location. The BAT on-board calculated location is RA, Dec 71.470, +12.040 which is RA(J2000) = 04h 45m 53s Dec(J2000) = +12d 02' 26" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve does not show any significant activity. The XRT began observing the field at 00:44:28.8 UT, 69.2 seconds after the BAT trigger. No source was detected in 461 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 250 seconds with the U filter starting 284 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 24% 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 0.411. Due to the marginal significance of the image peak (6.54 sigma), the lack of a peak in the light curve, the existence of a noisy pixel in the BAT detector plane histogram, and the lack of a detection of a counterpart by XRT, we believe that this trigger is not due to an astrophysical event.