TITLE: GCN CIRCULAR NUMBER: 13844 SUBJECT: Trigger 535747: Swift detection of an outburst from SGR1806-20 DATE: 12/10/11 03:13:02 GMT FROM: Scott Barthelmy at NASA/GSFC S. D. Barthelmy (GSFC), S. T. Holland (STScI) and J. A. Kennea (PSU) report on behalf of the Swift Team: At 02:54:21 UT, the Swift Burst Alert Telescope (BAT) triggered and located an outburst from SGR 1806-20 (trigger=535747). Swift slewed immediately to the source. The BAT on-board calculated location is RA, Dec 272.130, -20.410 which is RA(J2000) = 18h 08m 31s Dec(J2000) = -20d 24' 35" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve shows a single spike with a duration of about 0.064 sec. The peak count rate was ~1000 counts/sec (15-350 keV), at ~0.0 sec after the trigger. The XRT began observing the field at 02:55:19.9 UT, 58.6 seconds after the BAT trigger. Using promptly downlinked data we find an X-ray source with an enhanced position: RA, Dec 272.1635, -20.4112 which is equivalent to: RA(J2000) = 18h 08m 39.24s Dec(J2000) = -20d 24' 40.3" with an uncertainty of 4.1 arcseconds (radius, 90% containment). This location is 113 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. This position is 1.4 arcseconds from that of SGR 1806-20. A power-law fit to a spectrum formed from promptly downlinked event data does not constrain the column density. UVOT took a finding chart exposure of 150 seconds with the White filter starting 63 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 none of the XRT error circle. 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 mag. No correction has been made for the large, but uncertain extinction expected.