TITLE: GCN CIRCULAR NUMBER: 10097 SUBJECT: GRB 091029: Swift detection of a burst with an optical counterpart DATE: 09/10/29 04:05:12 GMT FROM: David Palmer at LANL D. Grupe (PSU), J. R. Cummings (NASA/UMBC), F. E. Marshall (NASA/GSFC), C. Pagani (PSU), D. M. Palmer (LANL), M. H. Siegel (PSU), M. Stamatikos (OSU/NASA/GSFC), T. N. Ukwatta (GSFC/GWU) and L. Vetere (PSU) report on behalf of the Swift Team: At 03:53:22 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 091029 (trigger=374210). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 60.183, -55.994 which is RA(J2000) = 04h 00m 44s Dec(J2000) = -55d 59' 37" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a double-peaked structure with a duration of about 50 sec. The peak count rate was ~3000 counts/sec (15-350 keV), at ~18 sec after the trigger. The XRT began observing the field at 03:54:41.9 UT, 79.3 seconds after the BAT trigger. Using promptly downlinked data we find a bright, uncatalogued X-ray source located at RA, Dec 60.17774, -55.95574 which is equivalent to: RA(J2000) = 04h 00m 42.66s Dec(J2000) = -55d 57' 20.7" with an uncertainty of 4.4 arcseconds (radius, 90% containment). This location is 138 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 consistent with the Galactic value of 1.14e+20 cm^-2 (Kalberla et al. 2005). The initial flux in the 2.5 s image was 1.80e-09 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of 150 seconds with the White filter starting 90 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 04:00:42.58 = 60.17742 DEC(J2000) = -55:57:20.0 = -55.95555 with a 90%-confidence error radius of about 0.70 arc sec. This position is 2.3 arc sec. from the center of the XRT error circle. The estimated magnitude is 20.31 with a 1-sigma error of about 0.18. No correction has been made for the expected extinction corresponding to E(B-V) of 0.02. Burst Advocate for this burst is D. Grupe (grupe AT astro.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.)