TITLE: GCN CIRCULAR NUMBER: 6299 SUBJECT: Trigger 276045: Swift detection of a possible burst DATE: 07/04/18 11:50:45 GMT FROM: David Palmer at LANL C. Guidorzi (Univ Bicocca&INAF-OAB), A. P. Beardmore (U Leicester), D. N. Burrows (PSU), N. Gehrels (NASA/GSFC), S. T. Holland (CRESST/GSFC/USRA), J. A. Kennea (PSU), H. A. Krimm (GSFC/USRA), W. B. Landsman (NASA/GSFC), V. Mangano (INAF-IASFPA), C. B. Markwardt (GSFC/UMD), F. E. Marshall (NASA/GSFC), P. T. O'Brien (U Leicester), J. P. Osborne (U Leicester), K. L. Page (U Leicester), D. M. Palmer (LANL), A. M. Parsons (GSFC), M. Perri (ASDC), P. Romano (Univ. Bicocca & INAF-OAB), B. Sbarufatti (INAF-IASFPA), P. Schady (MSSL-UCL), M. Stamatikos (NASA/ORAU), G. Tagliaferri (INAF-OAB), E. Troja (INAF-IASFPA), D. E. Vanden Berk (PSU) and H. Ziaeepour (UCL-MSSL) report on behalf of the Swift Team: At 11:21:01 UT, the Swift Burst Alert Telescope (BAT) triggered and located a marginal-significance image peak that may be GRB 070418 (trigger=276045). Swift slewed immediately to the location. The BAT on-board calculated location is RA, Dec 228.504, +51.226 which is RA(J2000) = 15h 14m 01s Dec(J2000) = +51d 13' 34" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve shows a single 0.064 second timebin with an excess of ~100 counts above background, primarily in the 100-350 keV energy bin. The very short duration and confinement to the highest energy channel indicate that this is very likely to be a cosmic ray shower, although a short, weak, very hard GRB cannot be eliminated at this time. A final determination of the cause of this event will require data from the next Malindi pass, which will be available around 1300 UT. The XRT began taking data at 11:22:22 UT, 81 seconds after the BAT trigger. The XRT on-board centroid algorithm did not find a source in the image and no prompt position is available. Down-linked data (for a total exposure of 1140s) show no sources in the XRT field. UVOT took a finding chart exposure of 100 seconds with the White (160-650 nm) filter starting 85 seconds after the BAT trigger. No afterglow candidate has been found in the initial data products. The 2.7'x2.7' sub-image covers 25% of the BAT error circle. The typical 3-sigma upper limit has been about 18.5 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 mag. No correction has been made for extinction of E(B-V)=0.02.