GCN Circular 27746

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.