TITLE: GCN CIRCULAR NUMBER: 13907 SUBJECT: GRB 121027A: Swift detection of a burst DATE: 12/10/27 07:58:39 GMT FROM: Phil Evans at U of Leicester P. A. Evans (U Leicester), C. Gronwall (PSU), H. A. Krimm (CRESST/GSFC/USRA), C. B. Markwardt (NASA/GSFC), K. L. Page (U Leicester) and M. H. Siegel (PSU) report on behalf of the Swift Team: At 07:32:29 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 121027A (trigger=536831). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 63.610, -58.801 which is RA(J2000) = 04h 14m 26s Dec(J2000) = -58d 48' 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 80 sec. The peak count rate was ~2000 counts/sec (15-350 keV), at ~3 sec after the trigger. The XRT began observing the field at 07:33:37.0 UT, 67.4 seconds after the BAT trigger. Using promptly downlinked data we find a bright, uncatalogued X-ray source located at RA, Dec 63.59612, -58.82887 which is equivalent to: RA(J2000) = 04h 14m 23.07s Dec(J2000) = -58d 49' 43.9" with an uncertainty of 4.0 arcseconds (radius, 90% containment). This location is 103 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 consistent with the Galactic value of 1.50 x 10^20 cm^-2 (Kalberla et al. 2005). The initial flux in the 2.5 s image was 6.10e-09 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of 250 seconds with the U filter starting 288 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 the expected extinction corresponding to E(B-V) of 0.02. 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 P. A. Evans (pae9 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.)