TITLE: GCN CIRCULAR NUMBER: 11047 SUBJECT: GRB 100805A: Swift detection of a burst with an optical counterpart DATE: 10/08/05 04:27:27 GMT FROM: David Palmer at LANL E. A. Hoversten (PSU), S. D. Barthelmy (GSFC), W. H. Baumgartner (GSFC/UMBC), J. M. Gelbord (PSU), C. Gronwall (PSU), S. T. Holland (CRESST/USRA/GSFC), D. M. Palmer (LANL), M. H. Siegel (PSU), M. C. Stroh (PSU) and T. N. Ukwatta (GSFC/GWU) report on behalf of the Swift Team: At 04:12:42 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 100805A (trigger=430957). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 299.874, +52.614 which is RA(J2000) = 19h 59m 30s Dec(J2000) = +52d 36' 52" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve shows a peak with a duration of about 20 sec. The peak count rate was ~600 counts/sec (15-350 keV), at ~0 sec after the trigger. The XRT began observing the field at 04:14:48.7 UT, 126.4 seconds after the BAT trigger. Using promptly downlinked data we find a fading, uncatalogued X-ray source located at RA, Dec 299.87559, 52.62758 which is equivalent to: RA(J2000) = 19h 59m 30.14s Dec(J2000) = +52d 37' 39.3" with an uncertainty of 3.8 arcseconds (radius, 90% containment). This location is 47 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 consistent with the Galactic value of 1.69e+21 cm^-2 (Kalberla et al. 2005). UVOT took a finding chart exposure of 150 seconds with the White filter starting 129 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 19:59:30.52 = 299.87716 DEC(J2000) = +52:37:40.3 = 52.62787 with a 90%-confidence error radius of about 0.62 arc sec. This position is 3.6 arc sec. from the center of the XRT error circle. The estimated magnitude is 18.00 with a 1-sigma error of about 0.14. No correction has been made for the expected extinction corresponding to E(B-V) of 0.19. Burst Advocate for this burst is E. A. Hoversten (hoversten AT astro.psu.edu). 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.)