TITLE: GCN CIRCULAR NUMBER: 5707 SUBJECT: GRB 061007: Swift detection of a burst with an optical counterpart DATE: 06/10/07 10:32:54 GMT FROM: Patricia Schady at MSSL/Swift P. Schady (MSSL-UCL), J. R. Cummings (NASA/UMBC), C. Guidorzi (Univ Bicocca&INAF-OAB), C. Pagani (PSU), D. M. Palmer (LANL), S. B. Pandey (UCL-MSSL), P. Romano (Univ. Bicocca & INAF-OAB), G. Sato (GSFC/ISAS), M. Stamatikos (NASA/ORAU), L. Vetere (PSU) and H. Ziaeepour (UCL-MSSL) report on behalf of the Swift Team: At 10:08:08 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 061007 (trigger=232683). Swift slewed immediately to the burst. The BAT on-board calculated location is RA,Dec 46.318, -50.488 {03h 05m 16s, -50d 29' 15"} (J2000) with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a multi-peaked structure with a duration of about 100 sec. The peak count rate was ~25000 counts/sec (15-350 keV), at ~54 sec after the trigger. The XRT began observing the field at 10:09:29 UT, 80 seconds after the BAT trigger. XRT found a bright, fading, uncatalogued X-ray source located at RA(J2000) = 03h 05m 19.5s, Dec(J2000) = -50d 30' 02.1", with an estimated uncertainty of 7.5 arcseconds (90% confidence radius). This location is 55 arcseconds from the BAT on-board position, within the BAT error circle. The initial flux in the 0.1s image was 3.4e-08 erg/cm2/s (0.2-10 keV). UVOT took a finding chart exposure of 400 seconds with the V filter starting 195 seconds after the BAT trigger. There is a very bright source in the 2.7' x 2.7' sub-image not present in DSS at RA(J2000) = 03:05:19.51 = 46.3313 DEC(J2000) = -50:30:02.5 = -50.5007 with a 1-sigma error radius of about 0.5 arc sec. This position is 0.7 arc sec. from the center of the XRT error circle. The estimated magnitude is 12.8 with a 1-sigma error of about 0.5 mag. No correction has been made for the expected extinction of about 0.1 magnitudes.