TITLE: GCN CIRCULAR NUMBER: 10256 SUBJECT: GRB 091208B: Swift detection of a burst with an optical afterglow DATE: 09/12/08 10:13:59 GMT FROM: Claudio Pagani at PSU/Swift-XRT C. Pagani (PSU), A. P. Beardmore (U Leicester), P. A. Evans (U Leicester), C. Gronwall (PSU), S. T. Holland (CRESST/USRA/GSFC), N. P. M. Kuin (MSSL), C. B. Markwardt (CRESST/GSFC/UMD), P. T. O'Brien (U Leicester), K. L. Page (U Leicester), A. Rowlinson (U Leicester), M. Stamatikos (OSU/NASA/GSFC), G. Tagliaferri (INAF-OAB) and L. Vetere (PSU) report on behalf of the Swift Team: At 09:49:57 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 091208B (trigger=378559). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 29.410, +16.883 which is RA(J2000) = 01h 57m 39s Dec(J2000) = +16d 52' 57" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a double-peaked structure with a duration of about 15 sec. The peak count rate was ~9100 counts/sec (15-350 keV), at ~8 sec after the trigger. The XRT began observing the field at 09:51:53.1 UT, 115.2 seconds after the BAT trigger. Using promptly downlinked data we find a fading, uncatalogued X-ray source with an enhanced position: RA, Dec 29.3920, 16.8899 which is equivalent to: RA(J2000) = 01h 57m 34.09s Dec(J2000) = +16d 53' 23.6" with an uncertainty of 2.1 arcseconds (radius, 90% containment). This location is 66 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 (4.85e+20 cm^-2, Kalberla et al. 2005), with an excess column of 3.3 (+1.52/-1.28) x 10^21 cm^-2 (90% confidence). UVOT took a finding chart exposure of 150 seconds with the White filter starting 119 seconds after the BAT trigger. There is an afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 01:57:34.06 = 29.39193 DEC(J2000) = +16:53:22.7 = 16.88963 with a 90%-confidence error radius of about 0.6 arc sec. This position is 0.67 arc sec. from the center of the XRT error circle. The estimated magnitude is 17.9. No correction has been made for the expected extinction corresponding to E(B-V) of 0.05. There is a bright afterglow candidate consistent with the XRT position. Burst Advocate for this burst is C. Pagani (pagani 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.)