TITLE: GCN CIRCULAR NUMBER: 27373 SUBJECT: Swift-BAT trigger 960986: Swift detection of a new SGR Swift J1818.0-1607 DATE: 20/03/12 21:47:38 GMT FROM: Phil Evans at U of Leicester P. A. Evans (U Leicester), J.D. Gropp (PSU), J. A. Kennea (PSU), N. J. Klingler (PSU), S. Laha (GSFC/UMBC/CRESST), A. Y. Lien (GSFC/UMBC), K. L. Page (U Leicester), T. Sakamoto (AGU) and A. Tohuvavohu (U Toronto) report on behalf of the Neil Gehrels Swift Observatory Team: At 21:16:47 UT, the Swift Burst Alert Telescope (BAT) triggered and located a new SGR (trigger=960986). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 274.471, -16.102 which is RA(J2000) = 18h 17m 53s Dec(J2000) = -16d 06' 05" 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 0.1 sec. The peak count rate was ~1500 counts/sec (15-350 keV), at ~0 sec after the trigger. The XRT began observing the field at 21:17:51.2 UT, 64.2 seconds after the BAT trigger. Using promptly downlinked data we find an uncatalogued X-ray source with an enhanced position: RA, Dec 274.5009, -16.1312 which is equivalent to: RA(J2000) = 18h 18m 00.22s Dec(J2000) = -16d 07' 52.3" with an uncertainty of 2.4 arcseconds (radius, 90% containment). This location is 147 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 https://www.swift.ac.uk/sper. 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 68 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. The extremely short and relatively soft nature of the BAT detection, combined with the source's closeness to the Galactic Plane (b=-0.14 deg), is suggestive that this trigger is from a previously undiscovered SGR. If this is a new SGR and not a GRB, we name the source Swift J1818.0-1607. 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/)