TITLE: GCN CIRCULAR NUMBER: 6210 SUBJECT: GRB 070318: Swift detection of a burst with an optical afterglow DATE: 07/03/18 07:49:43 GMT FROM: Scott Barthelmy at NASA/GSFC J. R. Cummings (NASA/UMBC), S. D. Barthelmy (GSFC), D. N. Burrows (PSU), G. Cusumano (INAF-IASFPA), N. Gehrels (NASA/GSFC), C. Gronwall (PSU), V. La Parola (INAF-IASFPA), T. Mineo (INAF-IASFPA) and D. M. Palmer (LANL) report on behalf of the Swift Team: At 07:28:56 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 070318 (trigger=271019). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 48.540, -42.962 which is RA(J2000) = 03h 14m 10s Dec(J2000) = -42d 57' 42" 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 70 sec. The peak count rate was ~4000 counts/sec (15-350 keV), at ~2 sec after the trigger. The XRT began observing the field at 07:29:59 UT, 64 seconds after the BAT trigger. XRT found a bright, fading, uncatalogued X-ray source located at RA, Dec 48.4856, -42.9468 which is RA(J2000) = 03h 13m 56.5s Dec(J2000) = -42d 56' 48.4" with an uncertainty of 4.7 arcseconds (radius, 90% containment). This location is 153 arcseconds from the BAT on-board position, within the BAT error circle. The initial flux in the 2.5s image was 2.6e-09 erg/cm2/s (0.2-10 keV). UVOT took a finding chart exposure of 400 seconds with the V filter starting 180 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 03:13:56.83 = 48.4868 DEC(J2000) = -42:56:46.3 = -42.9462 with a 1-sigma error radius of about 0.5 arc sec. This position is 3.8 arc sec. from the center of the XRT error circle. The estimated magnitude is 15.4 with a 1-sigma error of about 0.5 mag. No correction has been made for the expected extinction of about 0.1 magnitudes.