TITLE: GCN CIRCULAR NUMBER: 6415 SUBJECT: GRB 070518: Swift detection of a burst with optical afterglow DATE: 07/05/18 14:50:19 GMT FROM: David Palmer at LANL C. Guidorzi (Univ Bicocca&INAF-OAB), S. D. Barthelmy (GSFC), A. P. Beardmore (U Leicester), D. N. Burrows (PSU), S. Campana (INAF-OAB), M. M. Chester (PSU), G. Cusumano (INAF-IASFPA), P. A. Evans (U Leicester), N. Gehrels (NASA/GSFC), C. Gronwall (PSU), S. T. Holland (CRESST/GSFC/USRA), J. A. Kennea (PSU), H. A. Krimm (GSFC/USRA), F. E. Marshall (NASA/GSFC), A. Moretti (INAF-OAB), P. T. O'Brien (U Leicester), D. M. Palmer (LANL), J. L. Racusin (PSU), P. Romano (Univ. Bicocca & INAF-OAB), T. Sakamoto (NASA/ORAU), G. Sato (GSFC/ISAS), B. Sbarufatti (INAF-IASFPA), M. Stamatikos (NASA/ORAU), R. L. C. Starling (U Leicester), E. Troja (INAF-IASFPA), D. E. Vanden Berk (PSU) and S. D. Vergani (DIAS-DCU) report on behalf of the Swift Team: At 14:26:21 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 070518 (trigger=279592). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 254.223, +55.297 which is RA(J2000) = 16h 56m 54s Dec(J2000) = +55d 17'47" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve shows a single (FRED-like) peak with a duration of about 7 sec. The peak count rate was ~1500 counts/sec (15-350 keV), at ~0 sec after the trigger. The XRT began observing the field at 14:27:31 UT, 70 seconds after the BAT trigger. XRT found a variable, uncatalogued X-ray source located at RA, Dec 254.1979, +55.2974 which is RA(J2000) = 16h 56m 47.5s Dec(J2000) = 55d 17' 42.5" with an uncertainty of 4.7 arcseconds (radius, 90% containment). This location is 52 arcseconds from the BAT on-board position, within the BAT error circle. The initial flux in the 2.5s image was 5.8e-10 erg/cm2/s (0.2-10 keV). UVOT took a finding chart exposure of 100 seconds with the White (160-650 nm) filter starting 82 seconds after the BAT trigger. We see a fading afterglow located at RA, Dec 254.1986, 55.2951 which is RA(J2000) = 16h 56m 47.7s Dec(J2000) = 55d 17' 42.3" with an uncertainty of 1 arcsecond (radius, 90% containment) with an estimated magnitude of 18. No correction has been made for the expected extinction corresponding to E(B-V) of 0.02.