TITLE: GCN CIRCULAR NUMBER: 13622 SUBJECT: GRB 120811C: Swift detection of a burst with an optical counterpart DATE: 12/08/11 15:52:41 GMT FROM: Scott Barthelmy at NASA/GSFC B. N. Barlow (PSU), S. D. Barthelmy (GSFC), W. H. Baumgartner (GSFC/UMBC), D. Grupe (PSU), F. E. Marshall (NASA/GSFC), D. M. Palmer (LANL) and B.-B. Zhang (PSU) report on behalf of the Swift Team: At 15:34:52 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 120811C (trigger=530689). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 199.709, +62.291 which is RA(J2000) = 13h 18m 50s Dec(J2000) = +62d 17' 29" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve shows a single bright peak with a duration of about 35 sec. The peak count rate was ~7000 counts/sec (15-350 keV), at ~6 sec after the trigger. The XRT began observing the field at 15:36:00.8 UT, 68.7 seconds after the BAT trigger. XRT found a bright, uncatalogued X-ray source located at RA, Dec 199.6833, 62.3027 which is equivalent to: RA(J2000) = +13h 18m 43.99s Dec(J2000) = +62d 18' 09.7" with an uncertainty of 5.0 arcseconds (radius, 90% containment). This location is 60 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 initial flux in the 2.5 s image was 2.97e-09 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of 150 seconds with the White filter starting 75 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 13:18:43.81 = 199.68253 DEC(J2000) = +62:18:02.7 = 62.30076 with a 90%-confidence error radius of about 0.75 arc sec. This position is 7.1 arc sec. from the center of the XRT error circle. The estimated magnitude is 18.40 with a 1-sigma error of about 0.15. No correction has been made for the expected extinction corresponding to E(B-V) of 0.03. Burst Advocate for this burst is B. N. Barlow (bnb2 AT 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.)