GCN Circular 20970

S. D. Barthelmy (GSFC), J. A. Kennea (PSU),
F. E. Marshall (NASA/GSFC), A. Melandri (INAF-OAB) and
B. Sbarufatti (INAF-OAB/PSU) report on behalf of the Swift Team:

At 19:00:53 UT, the Swift Burst Alert Telescope (BAT) triggered on the known
source SAX_J1806.5-221 (trigger=744773).  Swift slewed immediately to the source. 
The BAT on-board calculated location is 
RA, Dec 271.602, -22.242, which is 
   RA(J2000)  =  18h 06m 25s
   Dec(J2000) = -22d 14' 29"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  As is typical for image triggers, the real-time
light curve does not show anything significant. 

The XRT began observing the field at 19:02:54.6 UT, 121.5 seconds after
the BAT trigger. XRT found a bright X-ray source located at RA, Dec
271.6326, -22.2386 which is equivalent to:
   RA(J2000)  = +18h 06m 31.82s
   Dec(J2000) = -22d 14' 19.0"
with an uncertainty of 4.8 arcseconds (radius, 90% containment). This
position is 3.3 arcseconds from a known X-ray source: 1SXPS
J180632.0-221417. This source is in the Swift XRT 1SXPS catalogue with
a mean 0.3-10 keV count-rate of 3.300 +/- 0.019 ct/sec; see
http://www.swift.ac.uk/1SXPS/1SXPSJ180632.0-221417 for details of these
previous observations. No event data are yet available to determine the
column density using X-ray spectroscopy. 

The initial flux in the 2.5 s image was 6.04e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 129 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. Because of the density of catalogued stars, further
analysis is required to report an upper limit for any afterglow in the
sub-image. The 8'x8' region for the list of sources generated on-board covers
100% of the XRT error circle. Because of the density of catalogued stars,
further analysis is required to report an upper limit for any afterglow in the
region. No correction has been made for the large, but uncertain extinction
expected.