TITLE: GCN CIRCULAR NUMBER: 6375 SUBJECT: GRB 070506: Swift detection of a burst with optical afterglow DATE: 07/05/06 06:05:32 GMT FROM: David Palmer at LANL C. Pagani (PSU), M. M. Chester (PSU), C. Guidorzi (Univ Bicocca&INAF-OAB), S. Immler (GSFC/USRA), C. B. Markwardt (GSFC/UMD), D. M. Palmer (LANL), J. L. Racusin (PSU), T. Sakamoto (NASA/ORAU), M. Stamatikos (NASA/ORAU) and L. Vetere (PSU) report on behalf of the Swift Team: At 05:35:58 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 070506 (trigger=278693). Swift slewed to the burst immediately after the end of the 64 second image trigger. The BAT on-board calculated location is RA, Dec 347.217, +10.728 which is RA(J2000) = 23h 08m 52s Dec(J2000) = +10d 43' 42" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a double-peaked structure with a duration of about 6 sec. The peak count rate was ~1700 counts/sec (15-350 keV), at ~6 sec after the trigger. The XRT began observing the field at 05:38:05 UT, 127 seconds after the BAT trigger. Using ground-processed quick-look data, XRT found an uncatalogued X-ray source located at RA, Dec 347.2192, +10.7224 which is RA(J2000) = 23h 08m 52.6s Dec(J2000) = 10d 43' 20.7" with an uncertainty of 9.0 arcseconds (radius, 90% containment). This location is 22 arcseconds from the BAT on-board position, within the BAT error circle. The initial flux in the 2.5s image was 2.5e-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 459 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 23h 08m 52.39s (= 347.2183d) DEC(J2000) = +10d 43' 20.3" (= 10.7223d) with a 1-sigma error radius of about 0.5 arc sec. This position is 3.0 arcseconds 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 galactic reddening of E(B-V) = 0.040.