TITLE: GCN CIRCULAR NUMBER: 9879 SUBJECT: GRB 090904: Swift detection of a burst DATE: 09/09/04 01:18:05 GMT FROM: Scott Barthelmy at NASA/GSFC M. Perri (ASDC), S. D. Barthelmy (GSFC), W. H. Baumgartner (GSFC/UMBC), D. N. Burrows (PSU), V. D'Elia (ASDC), S. T. Holland (CRESST/USRA/GSFC), J. A. Kennea (PSU), F. E. Marshall (NASA/GSFC), M. H. Siegel (PSU), M. Stamatikos (OSU/NASA/GSFC), M. A. Stark (PSU) and T. N. Ukwatta (GSFC/GWU) report on behalf of the Swift Team: At 01:01:06 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 090904 (trigger=361830). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 100.882, +50.206 which is RA(J2000) = 06h 43m 32s Dec(J2000) = +50d 12' 23" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve shows a small peak at T_zero and then two large peaks at T+130 and T+175 sec with a duration of at least 200 sec. The peak count rate was ~2500 counts/sec (15-350 keV), at ~T+130 sec after the trigger. The XRT began observing the field at 01:05:38.6 UT, 271.7 seconds after the BAT trigger. XRT found a bright, uncatalogued, fading X-ray source located at RA, Dec 100.8816, +50.2040 which is equivalent to: RA(J2000) = 06h 43m 31.58s Dec(J2000) = +50d 12' 14.4" with an uncertainty of 5.2 arcseconds (radius, 90% containment). This location is 7.3 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. The XRT prompt light curve shows a large flare peaking about 30s after the start of the XRT observations. The initial flux in the 2.5 s image was 1.71e-09 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of 150 seconds with the White filter starting 141 seconds after the BAT trigger. No credible afterglow candidate has been found in the initial data products. Because part of the 2.7'x2.7' sub-image was not received, the overlap with the XRT error circle is uncertain. The 8'x8' region for the list of sources generated on-board covers 100% of the XRT error circle. The list of sources is typically complete to about 18 mag. No correction has been made for the expected extinction corresponding to E(B-V) of 0.10. Burst Advocate for this burst is M. Perri (perri AT asdc.asi.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.)