TITLE: GCN CIRCULAR NUMBER: 23105 SUBJECT: GRB 180809B: Swift detection of a burst with an optical afterglow DATE: 18/08/09 20:40:30 GMT FROM: Scott Barthelmy at NASA/GSFC M. J. Moss (George Washington University), S. D. Barthelmy (GSFC), V. D'Elia (ASDC), C. Gronwall (PSU), J.D. Gropp (PSU), J. A. Kennea (PSU), A. Y. Lien (GSFC/UMBC), D. M. Palmer (LANL) and A. Tohuvavohu (PSU) report on behalf of the Neil Gehrels Swift Observatory Team: At 20:28:48 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 180809B (trigger=852553). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 299.712, -15.302, which is RA(J2000) = 19h 58m 51s Dec(J2000) = -15d 18' 06" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve shows multiple bright peaks structure with a total duration of about 80 sec. The peak count rate was ~35,000 counts/sec (15-350 keV), at ~35 sec after the trigger. XRT found a bright, uncatalogued X-ray source located at RA, Dec 299.6998, -15.2988 which is equivalent to: RA(J2000) = +19h 58m 47.95s Dec(J2000) = -15d 17' 55.7" with an uncertainty of 6.1 arcseconds (radius, 90% containment). No event data are yet available to determine the column density using X-ray spectroscopy. The initial flux in the 0.1 s image was 1.51e-08 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of 150 seconds with the White filter starting 81 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 19:58:47.96 = 299.69983 DEC(J2000) = -15:17:56.9 = -15.29915 with a 90%-confidence error radius of about 0.64 arc sec. This position is 1.3 arc sec. from the center of the XRT error circle. The estimated magnitude is 18.91 with a 1-sigma error of about 0.15. No correction has been made for the expected extinction corresponding to E(B-V) of 0.16. Burst Advocate for this burst is M. J. Moss (mikejmoss3 AT gmail.com). 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.)