GCN Circular 27696

S. D. Barthelmy (GSFC), J.D. Gropp (PSU), J. A. Kennea (PSU),
N. P. M. Kuin (UCL-MSSL), S. Laha (GSFC/UMBC/CRESST),
F. E. Marshall (NASA/GSFC), M. J. Moss (GWU),
K. L. Page (U Leicester), D. M. Palmer (LANL), B. Sbarufatti (PSU),
M. H. Siegel (PSU) and A. Tohuvavohu (U Toronto) report on behalf of
the Neil Gehrels Swift Observatory Team:

At 17:36:50 UT, the Swift Burst Alert Telescope (BAT) triggered on
Swift J1818.0-1607 (trigger=969823).  Swift slewed immediately to the source. 
The BAT on-board calculated location is 
RA, Dec 274.498, -16.152, which is 
   RA(J2000)  =  18h 18m 00s
   Dec(J2000) = -16d 09' 06"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows a single spike
with a duration of about 0.3 sec.  The peak count rate
was ~1200 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 17:37:52.5 UT, 62.6 seconds after
the BAT trigger. Using promptly downlinked data we find an 
X-ray source with an enhanced position: RA, Dec 274.5008, -16.1312
which is equivalent to:
   RA(J2000)  = 18h 18m 00.18s
   Dec(J2000) = -16d 07' 52.3"
with an uncertainty of 3.6 arcseconds (radius, 90% containment). This
location is 75 arcseconds from the BAT onboard position, within the BAT
error circle, and 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 65 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 100% of
the XRT 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
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. 

This source was first reported on 2020 March 12 (GCN #27373; Evans et al.)
Since then it has been monitored every 3-7 days with Swift follow-up 
observations.