TITLE: GCN CIRCULAR NUMBER: 27746 SUBJECT: Swift Detection of a burst from SGR Swift J1818.0-1607 DATE: 20/05/16 15:21:15 GMT FROM: David Palmer at LANL C. Gronwall (PSU), J.D. Gropp (PSU), J. A. Kennea (PSU), S. Laha (GSFC/UMBC/CRESST), A. Y. Lien (GSFC/UMBC), F. E. Marshall (NASA/GSFC), K. L. Page (U Leicester) and D. M. Palmer (LANL) report on behalf of the Neil Gehrels Swift Observatory Team: At 15:03:28 UT, the Swift Burst Alert Telescope (BAT) triggered and located a burst from Soft Gamma Repeater SGR Swift J1818.0-1607 (trigger=972614). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 274.550, -16.193 which is RA(J2000) = 18h 18m 12s Dec(J2000) = -16d 11' 32" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a single peak structure with a duration of about 100 ms. The peak count rate was ~1000 counts/sec (15-350 keV), at ~0 sec after the trigger. The XRT began observing the field at 15:04:27.5 UT, 59.0 seconds after the BAT trigger. Using promptly downlinked data we find an X-ray source with an enhanced position: RA, Dec 274.5010, -16.1314 which is equivalent to: RA(J2000) = 18h 18m 00.23s Dec(J2000) = -16d 07' 53.0" with an uncertainty of 3.0 arcseconds (radius, 90% containment). This location is 279 arcseconds from the BAT onboard position, consistent with the known position of Swift J1818.0-1607. This position may be improved as more data are received; the latest position is available at https://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 1.35 x 10^22 cm^-2 (Willingale et al. 2013). UVOT took a finding chart exposure of 150 seconds with the White filter starting 62 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 none of the XRT error circle. No correction has been made for the large, but uncertain, extinction expected. Swift is currently monitoring this source every 3 days under a ToO program.