TITLE: GCN CIRCULAR NUMBER: 8762 SUBJECT: GRB 090102: Swift detection of a burst with an optical counterpart DATE: 09/01/02 03:08:41 GMT FROM: David Burrows at PSU/Swift V. Mangano (INAF-IASFPA), S. D. Barthelmy (GSFC), D. N. Burrows (PSU), N. Gehrels (NASA/GSFC), S. T. Holland (CRESST/USRA/GSFC), E. A. Hoversten (PSU), J. A. Kennea (PSU), V. La Parola (INAF-IASFPA), W.B Landsman (GSFC), C. B. Markwardt (CRESST/GSFC/UMD), F. E. Marshall (NASA/GSFC) and B. Sbarufatti (INAF-IASFPA) report on behalf of the Swift Team: At 02:55:45 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 090102 (trigger=338895). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 128.259, +33.092 which is RA(J2000) = 08h 33m 02s Dec(J2000) = +33d 05' 29" 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 30 sec starting about 15 sec before the trigger. The peak count rate was ~7000 counts/sec (15-350 keV), at ~2 sec after the trigger. The XRT began observing the field at 03:02:13.1 UT, 387.2 seconds after the BAT trigger. XRT found a bright, uncatalogued X-ray source located at RA, Dec 128.2423, +33.1142 which is equivalent to: RA(J2000) = 08h 32m 58.15s Dec(J2000) = +33d 06' 51.1" with an uncertainty of 6.1 arcseconds (radius, 90% containment). This location is 94 arcseconds from the BAT onboard position, within the BAT error circle. No event data are yet available to determine the column density using X-ray spectroscopy. UVOT took a finding chart exposure of nominal 150 seconds with the White filter starting 395 seconds after the BAT trigger. There is a candidate afterglow in the list of sources generated on-board at RA(J2000) = 08:32:58.54 = 128.24391 DEC(J2000) = +33:06:51.2 = 33.11421 with a 90%-confidence error radius of about 1.10 arc sec. This position is 4.8 arc sec. from the center of the XRT error circle. The estimated magnitude is 18.11. No correction has been made for the expected extinction corresponding to E(B-V) of 0.05. Burst Advocate for this burst is V. Mangano (vanessa AT ifc.inaf.it). Please contact the BA by email if you require additional information regarding Swift followup of this burst. In extremely urgent cases, after trying the Burst Advocate, you can contact the Swift PI by phone (see Swift TOO web site for information: http://www.swift.psu.edu/too.html.)