TITLE: GCN CIRCULAR NUMBER: 15765 SUBJECT: GRB 140129B: Swift detection of a burst with an optical afterglow DATE: 14/01/29 13:07:18 GMT FROM: David Palmer at LANL M. G. Bernardini (INAF-OAB), P. D'Avanzo (INAF-OAB), V. D'Elia (ASDC), P. A. Evans (U Leicester), J. A. Kennea (PSU), A. Y. Lien (NASA/GSFC/ORAU), F. E. Marshall (NASA/GSFC), A. Melandri (INAF-OAB), K. L. Page (U Leicester), D. M. Palmer (LANL), M. H. Siegel (PSU) and C. A. Swenson (PSU) report on behalf of the Swift Team: At 12:51:09 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 140129B (trigger=585149). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 326.763, +26.207 which is RA(J2000) = 21h 47m 03s Dec(J2000) = +26d 12' 24" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a single-peaked structure with a duration of about 2 sec. The peak count rate was ~1200 counts/sec (15-350 keV), at ~1 sec after the trigger. The XRT began observing the field at 12:52:16.5 UT, 67.1 seconds after the BAT trigger. Using promptly downlinked data we find an uncatalogued X-ray source with an enhanced position: RA, Dec 326.7574, 26.2065 which is equivalent to: RA(J2000) = 21h 47m 01.77s Dec(J2000) = +26d 12' 23.3" with an uncertainty of 2.1 arcseconds (radius, 90% containment). This location is 18 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. 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 in excess of the Galactic value (5.58 x 10^20 cm^-2, Kalberla et al. 2005), with an excess column of 2.7 (+2.52/-2.09) x 10^21 cm^-2 (90% confidence). UVOT took a finding chart exposure of 150 seconds with the White filter starting 356 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 21:47:01.67 = 326.75695 DEC(J2000) = +26:12:22.9 = 26.20637 with a 90%-confidence error radius of about 0.61 arc sec. This position is 1.5 arc sec. from the center of the XRT error circle. The estimated magnitude is 16.92 with a 1-sigma error of about 0.14. No correction has been made for the expected extinction corresponding to E(B-V) of 0.09. Burst Advocate for this burst is M. G. Bernardini (grazia.bernardini AT brera.inaf.it). 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.)