TITLE: GCN CIRCULAR NUMBER: 18522 SUBJECT: GRB 151029A: Swift detection of a burst with an optical counterpart DATE: 15/10/29 08:49:18 GMT FROM: Jamie A. Kennea at PSU/Swift-XRT T. N. Ukwatta (LANL), A. D'Ai (INAF-IASFPA), J. A. Kennea (PSU), A. Maselli (INAF-IASFPA), P. T. O'Brien (U Leicester) and D. M. Palmer (LANL) report on behalf of the Swift Team: At 07:49:54 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 151029A (trigger=662086). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 38.529, -35.354, which is RA(J2000) = 02h 34m 07s Dec(J2000) = -35d 21' 14.4" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). Lightcurve data is not immediately available. Using promptly downlinked data we find an uncatalogued X-ray source with an enhanced position: RA, Dec 38.5280, -35.3855 which is equivalent to: RA(J2000) = 02h 34m 06.72s Dec(J2000) = -35d 23' 07.7" with an uncertainty of 2.3 arcseconds (radius, 90% containment). This position is 113.4 arcsec from the BAT position. This position may be improved as more data are received; the latest position is available at http://www.swift.ac.uk/sper. We cannot determine whether the source is fading at the present time. A power-law fit to a spectrum formed from promptly downlinked event data gives a column density consistent with the Galactic value of 3.41 x 10^20 cm^-2 (Willingale et al. 2013). UVOT took a finding chart exposure of 150 seconds with the White filter starting 209 seconds after the BAT trigger. The initial 2.7'x2.7' sub-image does not cover the XRT error circle. There is a candidate afterglow in the UVOT srclist at: RA(J2000) = 02:34:08.08 = 38.527964 DEC(J2000) = +35:23:08.8 = -35.385784 with a 90%-confidence error radius of about 0.61 arc sec. This position is 1.06 arc sec. from the center of the XRT error circle. The estimated magnitude is 17.34 with a 1-sigma error of about 0.04. No correction has been made for the expected extinction. Burst Advocate for this burst is T. N. Ukwatta (tilan.ukwatta 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.)