GCN Circular 27373

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/)