TITLE: GCN CIRCULAR NUMBER: 13530 SUBJECT: GRB 120729A: Swift detection of a burst with an optical afterglow DATE: 12/07/29 11:17:04 GMT FROM: David Burrows at PSU/Swift T. N. Ukwatta (MSU), D. N. Burrows (PSU), M. M. Chester (PSU), S. T. Holland (STScI), H. A. Krimm (CRESST/GSFC/USRA), V. Mangano (INAF-IASFPA), A. Maselli (INAF-IASFPA), E. Sonbas (NASA/GSFC/Adiyaman Univ.) and M. Stamatikos (OSU/NASA/GSFC) report on behalf of the Swift Team: At 10:56:14 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 120729A (trigger=529095). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 13.056, +49.940 which is RA(J2000) = 00h 52m 13s Dec(J2000) = +49d 56' 23" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a FRED-shaped structure with a total duration of about 25 sec. The peak count rate was ~5700 counts/sec (15-350 keV), at ~0 sec after the trigger. The XRT began observing the field at 10:57:22.2 UT, 68.1 seconds after the BAT trigger. Using promptly downlinked data we find a bright, fading, uncatalogued X-ray source with an enhanced position: RA, Dec 13.0745, 49.9401 which is equivalent to: RA(J2000) = 00h 52m 17.88s Dec(J2000) = +49d 56' 24.5" with an uncertainty of 2.4 arcseconds (radius, 90% containment). This location is 43 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 (1.42 x 10^21 cm^-2, Kalberla et al. 2005), with an excess column of 2.1 (+1.91/-1.68) x 10^21 cm^-2 (90% confidence). The initial flux in the 2.5 s image was 1.75e-09 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of 150 seconds with the White filter starting 77 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 00:52:17.84 = 13.07433 DEC(J2000) = +49:56:23.1 = 49.93974 with a 90%-confidence error radius of about 0.61 arc sec. This position is 1.4 arc sec. from the center of the XRT error circle. The estimated magnitude is 15.26 with a 1-sigma error of about 0.14. No correction has been made for the expected extinction corresponding to E(B-V) of 0.16. 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.)