GCN Circular 16632
M. G. Bernardini (INAF-OAB), P. D'Avanzo (INAF-OAB), C. B. Markwardt (NASA/GSFC), D. M. Palmer (LANL) and M. H. Siegel (PSU) report on behalf of the Swift Team: At 02:41:23 UT, the Swift Burst Alert Telescope (BAT) triggered and located a burst from the Soft Gamma Repeater SGR 1806-20 (trigger=607399). Swift slewed immediately to the burst location. The BAT on-board calculated location is RA, Dec 272.172, -20.437 which is RA(J2000) = 18h 08m 41s Dec(J2000) = -20d 26' 13" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty), which is 1.8 arcminutes from the location of the SGR. The BAT light curve showed a single-peaked structure with a duration of about 0.25 sec. The peak count rate was ~15000 counts/sec (15-350 keV), at ~0 sec after the trigger. The XRT began observing the field at 02:42:52.6 UT, 88.9 seconds after the BAT trigger. No source was detected in 303 s of promptly downlinked data. We are waiting for the full dataset to detect and localise the XRT counterpart. UVOT took a finding chart exposure of 150 seconds with the White filter starting 93 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 25% of the BAT error circle. The typical 3-sigma upper limit has been about 19.6 mag. The 8'x8' region for the list of sources generated on-board covers 100% of the BAT 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.