GCN Circular 17764

S. D. Barthelmy (GSFC), A. P. Beardmore (U Leicester),
J. R. Cummings (NASA/UMBC), P. A. Evans (U Leicester),
J. A. Kennea (PSU), K. L. Page (U Leicester), D. M. Palmer (LANL),
P. Romano (INAF-IASFPA), B. Sbarufatti (INAF-OAB/PSU) and
M. H. Siegel (PSU) report on behalf of the Swift Team:

At 14:18:02 UT, the Swift Burst Alert Telescope (BAT) triggered and
located IGRJ16418-4532 (trigger=639199).  Swift slewed immediately to the source. 
The BAT on-board calculated location is 
RA, Dec 250.461, -45.533, which is 
   RA(J2000)  =  16h 41m 51s
   Dec(J2000) = -45d 31' 57"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  As is usual for image triggers (duration 320 sec),
the TDRSS real-time light curve does not show anything significant. 

The XRT began observing the field at 14:25:34.6 UT, 452.1 seconds after
the BAT trigger. Using promptly downlinked data we find an X-ray source
with an enhanced position: RA, Dec 250.4627, -45.5410 which is
equivalent to:
   RA(J2000)  = 16h 41m 51.05s
   Dec(J2000) = -45d 32' 27.6"
with an uncertainty of 2.2 arcseconds (radius, 90% containment). This
location is 29 arcseconds from the BAT onboard position, within the BAT
error circle; and 7.7 arcseconds from the location of the known X-ray
source IGRJ16418-4532 in the BAT catalogue. The XRT position may be
improved as more data are received; the latest position is available at
http://www.swift.ac.uk/sper. This position is 1.6 arcseconds from that
of a known X-ray source: 3XMM J164150.8-453225 in the XMM-NEWTON XMMSSC
catalogue. 

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 149 seconds with the U filter starting
877 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 18.0 mag. No correction has been made for the large, but uncertain
extinction expected.