TITLE: GCN GRB OBSERVATION REPORT NUMBER: 5505 SUBJECT: GRB 060904B: Swift detection of a burst with optical counterpart DATE: 06/09/04 02:51:23 GMT FROM: Scott Barthelmy at NASA/GSFC D. Grupe (PSU), S. D. Barthelmy (GSFC), M. M. Chester (PSU), J. R. Cummings (NASA/UMD), S. T. Holland (GSFC/USRA), S. D. Hunsberger (PSU), J. A. Kennea (PSU), H. A. Krimm (GSFC/USRA), C. B. Markwardt (GSFC/UMD), F. E. Marshall (NASA/GSFC), D. M. Palmer (LANL), M. Stamatikos (NASA/ORAU) and M. C. Stroh (PSU) report on behalf of the Swift Team: At 02:31:03 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 060904B (trigger=228006). Swift slewed immediately to the burst. The BAT on-board calculated location is RA,Dec 58.213, -0.720 {03h 52m 51s, -00d 43' 11"} (J2000) with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve shows an initial 5-sec wide peak at T+1 sec followed by an ~30-sec wide weaker peak at T+150 sec. The total duration is about 180 sec. The peak count rate was ~3000 counts/sec (15-350 keV), at ~1 sec after the trigger. The XRT began observing the field at 02:32:12 UT, 69 seconds after the BAT trigger. XRT found a variable, uncatalogued X-ray source located at RA(J2000) = 03h 52m 50.0s, Dec(J2000) = -00d 43' 32.8", with an estimated uncertainty of 6.5 arcseconds (90% confidence radius). This location is 26 arcseconds from the BAT on-board position, within the BAT error circle. The initial flux in the 2.5s image was 4.3e-10 erg/cm2/s (0.2-10 keV). The source shows a rapid increase in count rate indicating a bright flare that is possibly correlated with the second bright peak in the BAT. UVOT took a finding chart exposure of 246 seconds with the V filter starting 70 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 (58.2106,-0.7253) or (03h52m50.54s,-00o43'31.1") with a 1-sigma error radius of about 0.7 arc sec. This position is 7.7 arc sec. from the center of the XRT error circle. The estimated magnitude is 18.6 with a 1-sigma error of about 0.5 mag. No correction has been made for the expected extinction of about 0.6 magnitudes.