TITLE: GCN CIRCULAR NUMBER: 11227 SUBJECT: GRB 100906A: Swift detection of a burst with an optical afterglow DATE: 10/09/06 14:07:13 GMT FROM: Scott Barthelmy at NASA/GSFC C. B. Markwardt (NASA/GSFC), S. D. Barthelmy (GSFC), A. P. Beardmore (U Leicester), D. N. Burrows (PSU), V. D'Elia (ASDC), P. A. Evans (U Leicester), N. Gehrels (NASA/GSFC), J. M. Gelbord (PSU), O. Godet (U Leicester), J. A. Kennea (PSU), N. P. M. Kuin (UCL-MSSL), W.B Landsman (GSFC), O. M. Littlejohns (U Leicester), F. E. Marshall (NASA/GSFC), D. C. Morris (GWU/GSFC), P. T. O'Brien (U Leicester), K. L. Page (U Leicester), B. Sbarufatti (INAF-OAB/IASFPA), M. H. Siegel (PSU), R. L. C. Starling (U Leicester) and T. N. Ukwatta (GSFC/GWU) report on behalf of the Swift Team: At 13:49:27 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 100906A (trigger=433509). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 28.709, +55.614, which is RA(J2000) = 01h 54m 50s Dec(J2000) = +55d 36' 50" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve shows multiple bright peaks with a duration of about 130 sec. The peak count rate was ~11000 counts/sec (15-350 keV), at ~10 sec after the trigger. The XRT began observing the field at 13:50:47.6 UT, 80.2 seconds after the BAT trigger. Using promptly downlinked data we find a bright, fading, uncatalogued X-ray source located at RA, Dec 28.68305, 55.62985 which is equivalent to: RA(J2000) = 01h 54m 43.93s Dec(J2000) = +55d 37' 47.5" with an uncertainty of 3.8 arcseconds (radius, 90% containment). This location is 78 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 (2.21 x 10^21 cm^-2, Kalberla et al. 2005), with an excess column of 2.6 (+2.21/-1.90) x 10^21 cm^-2 (90% confidence). The initial flux in the 0.1 s image was 2.34e-08 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of 250 seconds with the U filter starting 146 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 01:54:44.11 = 28.68380 DEC(J2000) = +55:37:49.6 = 55.63045 with a 90%-confidence error radius of about 0.61 arc sec. This position is 1.6 arc sec. from the center of the XRT error circle. The estimated magnitude is 14.90 with a 1-sigma error of about 0.14. No correction has been made for the expected extinction corresponding to E(B-V) of 0.36. A bright XRT flare at ~120 s corresponds to the last peak in the prompt emission. The UVOT bright source is probably also associated in time with the flare. More information to follow. Burst Advocate for this burst is C. B. Markwardt (Craig.Markwardt 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.)