TITLE: GCN GRB OBSERVATION REPORT NUMBER: 5212 SUBJECT: GRB 060604: Swift detection of a burst with possible optical counterpart DATE: 06/06/04 18:46:18 GMT FROM: Jamie A. Kennea at PSU/Swift-XRT M. J. Page (UCL-MSSL), A. J. Blustin (UCL-MSSL), J. R. Cummings (NASA/ORAU), N. Gehrels (NASA/GSFC), O. Godet (U Leicester), J. A. Kennea (PSU), K. M. McLean (LANL/UTD), K. L. Page (U Leicester), G. Sato (GSFC/JSPS/USRA) and M. Stamatikos (NASA/ORAU) report on behalf of the Swift Team: At 18:19:00 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 060604 (trigger=213486). Swift slewed immediately to the burst. The BAT on-board calculated location is RA,Dec 337.242, -10.905 {22h 28m 58s, -10d 54' 17"} (J2000) with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a single FRED peak with a duration of about 8 sec. The peak count rate was ~1000 counts/sec (15-350 keV), at ~3 sec after the trigger. The XRT began observing the field at 18:20:48 UT, 109 seconds after the BAT trigger. XRT found a variable, uncatalogued X-ray source located at RA(J2000) = 22h 28m 54.9s, Dec(J2000) = -10d 55' 03.0", with an estimated uncertainty of 4.7 arcseconds (90% confidence radius). During the first 100s of data, the source shows bright flaring activity. This location is 65 arcseconds from the BAT on-board position, within the BAT error circle. The initial flux in the 2.5s image was 2.9e-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 117 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at (RA,DEC) (J2000) of (337.2292,-10.9155) or (22h28m55.01s,-10o54'55.8") with a 1-sigma error radius of about 0.6 arc sec. This position is 7.2 arc sec. from the center of the XRT error circle. The estimated magnitude is 18.0 with a 1-sigma error of about 0.5 mag. No correction has been made for the expected extinction corresponding to E(B-V) of 0.04.