TITLE: GCN CIRCULAR NUMBER: 13913 SUBJECT: GRB 121028A: Swift detection of a burst DATE: 12/10/28 05:31:03 GMT FROM: David Palmer at LANL R. L. C. Starling (U Leicester), C. Gronwall (PSU), H. A. Krimm (CRESST/GSFC/USRA), K. L. Page (U Leicester), D. M. Palmer (LANL), M. H. Siegel (PSU) and T. N. Ukwatta (MSU) report on behalf of the Swift Team: At 05:04:31 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 121028A (trigger=536897). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 271.923, -2.285 which is RA(J2000) = 18h 07m 42s Dec(J2000) = -02d 17' 04" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a multi-peaked structure with a duration of about 30 sec. The peak count rate was ~2500 counts/sec (15-350 keV), at ~1 sec after the trigger. The XRT began observing the field at 05:06:41.7 UT, 129.8 seconds after the BAT trigger. Using promptly downlinked data we find an uncatalogued X-ray source with an enhanced position: RA, Dec 271.8995, -2.2936 which is equivalent to: RA(J2000) = 18h 07m 35.89s Dec(J2000) = -02d 17' 36.8" with an uncertainty of 2.3 arcseconds (radius, 90% containment). This location is 90 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 (2.48 x 10^21 cm^-2, Kalberla et al. 2005), with an excess column of 4.4 (+4.35/-3.23) x 10^21 cm^-2 (90% confidence). UVOT took a finding chart exposure of 150 seconds with the White filter starting 134 seconds after the BAT trigger. No credible afterglow candidate has been found in the initial data products. The 2.7'x2.7' sub-image covers 100% of the XRT error circle. The typical 3-sigma upper limit has been about 19.2 mag. The 8'x8' region for the list of sources generated on-board covers 100% of the XRT error circle. The list of sources is typically complete to about 18.0 mag. No correction has been made for extinction. Due to the maintenance work at Goddard this weekend (GCN 13905) there may be a delay in receiving the full dataset, and thus refined analyses may not be available until late Sunday/early Monday. Burst Advocate for this burst is R. L. C. Starling (rlcs1 AT star.le.ac.uk). 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.)