TITLE: GCN CIRCULAR NUMBER: 11181 SUBJECT: GRB 100902A: Swift detection of a burst DATE: 10/09/02 19:51:36 GMT FROM: David Palmer at LANL T. Sakamoto (NASA/UMBC), W. H. Baumgartner (GSFC/UMBC), A. P. Beardmore (U Leicester), J. R. Cummings (NASA/UMBC), P. A. Evans (U Leicester), N. Gehrels (NASA/GSFC), J. M. Gelbord (PSU), C. Guidorzi (U Ferrara), S. T. Holland (CRESST/USRA/GSFC), E. A. Hoversten (PSU), W.B Landsman (GSFC), K. L. Page (U Leicester), D. M. Palmer (LANL), A. Rowlinson (U Leicester), B. Sbarufatti (INAF-OAB/IASFPA), M. H. Siegel (PSU), M. Stamatikos (OSU/NASA/GSFC), R. L. C. Starling (U Leicester) and M. C. Stroh (PSU) report on behalf of the Swift Team: At 19:31:54 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 100902A (trigger=433160). BAT slewed to the location after a brief delay due to observing constraints. The BAT on-board calculated location is RA, Dec 48.646, +30.973 which is RA(J2000) = 03h 14m 35s Dec(J2000) = +30d 58' 21" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a small spike around the trigger time. There are brighter spikes around T+150 sec and T+200 sec with a duration of 50 sec. The peak count rate was ~1200 counts/sec (15-350 keV), at ~210 sec after the trigger. The XRT began observing the field at 19:37:10.1 UT, 316.2 seconds after the BAT trigger. Using promptly downlinked data we find a bright, fading, uncatalogued X-ray source with an enhanced position: RA, Dec 48.6294, 30.9791 which is equivalent to: RA(J2000) = 03h 14m 31.05s Dec(J2000) = +30d 58' 44.8" with an uncertainty of 2.3 arcseconds (radius, 90% containment). This location is 56 arcseconds from the BAT onboard position, within the BAT error circle. This position may be improved as more data are received; the latest position is available at http://www.swift.ac.uk/sper. A power-law fit to a spectrum formed from promptly downlinked event data gives a column density in excess of the Galactic value (1.13e+21 cm^-2, Kalberla et al. 2005), with an excess column of 4.1 (+2.41/-2.03) x 10^21 cm^-2 (90% confidence). The initial flux in the 0.1 s image was 2.20e-07 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of nominal 150 seconds with the White filter starting 325 seconds after the BAT trigger. No credible afterglow candidate has been found in the initial data products. Data from the 2.7'x2.7' sub-image are not available at this time. 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 0.31. Burst Advocate for this burst is T. Sakamoto (Taka.Sakamoto AT nasa.gov). 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.)