TITLE: GCN CIRCULAR NUMBER: 6383 SUBJECT: GRB 070508: Swift detection of an intense burst with an optical counterpart DATE: 07/05/08 04:47:27 GMT FROM: Scott Barthelmy at NASA/GSFC D. Grupe (PSU), S. D. Barthelmy (GSFC), M. M. Chester (PSU), G. Cusumano (INAF-IASFPA), C. B. Markwardt (GSFC/UMD), F. E. Marshall (NASA/GSFC), C. Pagani (PSU), D. M. Palmer (LANL), J. L. Racusin (PSU), M. Stamatikos (NASA/ORAU) and L. Vetere (PSU) report on behalf of the Swift Team: At 04:18:17 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 070508 (trigger=278854). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 312.783, -78.368 which is RA(J2000) = 20h 51m 08s Dec(J2000) = -78d 22' 04" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve shows a complex multi-peaked structure with a duration of at least 40 sec. The peak count rate was ~45000 counts/sec (15-350 keV), at ~15 sec after the trigger. The XRT began observing the field at 04:19:33 UT, 76 seconds after the BAT trigger. XRT found a bright, uncatalogued X-ray source located at RA, Dec 312.7959, -78.3850 which is RA(J2000) = 20h 51m 11.0s Dec(J2000) = -78d 23' 06.0" with an uncertainty of 5.0 arcseconds (radius, 90% containment). This location is 62 arcseconds from the BAT on-board position, within the BAT error circle. The initial flux in the 2.5s image was 1.6e-09 erg/cm2/s (0.2-10 keV). UVOT took a finding chart exposure of 100 seconds with the White (160-650 nm) filter starting 85 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 20:51:11.81 = 312.7992 DEC(J2000) = -78:23:04.9 = -78.3847 with a 1-sigma error radius of about 0.5 arc sec. This position is 2.6 arc sec. from the center of the XRT error circle. The estimated magnitude is 19.4 with a 1-sigma error of about 0.5 mag. No correction has been made for the expected reddening of E(B-V) of 0.14.