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GCN Circular 30845

Subject
Swift trigger 1073821: detection of IGR J16418-4532
Date
2021-09-18T08:23:26Z (3 years ago)
From
Scott Barthelmy at NASA/GSFC <scott@milkyway.gsfc.nasa.gov>
S. D. Barthelmy (GSFC), K. L. Page (U Leicester) and
B. Sbarufatti (PSU) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 07:58:20 UT, the Swift Burst Alert Telescope (BAT) triggered and
located IGR J16418-4532 (trigger=1073821).  Swift slewed immediately to the source. 
The BAT on-board calculated location is 
RA, Dec 250.455, -45.540 which is 
   RA(J2000)  =  16h 41m 49s
   Dec(J2000) = -45d 32' 22"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve is an image trigger
and as such does not show any peak. 

The XRT began observing the field at 08:00:28.1 UT, 128.0 seconds after
the BAT trigger. XRT found a bright X-ray source located at RA, Dec
250.4669, -45.5421 which is equivalent to:
   RA(J2000)  = 16h 41m 52.06s
   Dec(J2000) = -45d 32' 31.6"
with an uncertainty of 4.9 arcseconds (radius, 90% containment). This
location is 30 arcseconds from the BAT onboard position, within the BAT
error circle. This position is 15.1 arcseconds from a known X-ray
source: 2SXPS J164150.7-453225, also known as IGRJ16418-4532. This 
source is in the Swift XRT 2SXPS catalogue with a mean 0.3-10 keV 
count-rate of 0.2893 +/- 0.0028 ct/sec; see 
https://www.swift.ac.uk/2SXPS/2SXPS%20J164150.7-453225 for
details of these previous observations. No event data are yet available
to determine the column density using X-ray spectroscopy. 

UVOT took a finding chart exposure of 250 seconds with the U filter starting
133 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 19.2 mag. The
coverage of the XRT error circle by the 8'x8' region for the list of sources
generated on-board is uncertain because the large number of sources filled the
available telemetry. The list of sources is typically complete to about 18.0
mag. No correction has been made for the large, but uncertain, extinction
expected.