TITLE: GCN CIRCULAR NUMBER: 30845 SUBJECT: Swift trigger 1073821: detection of IGR J16418-4532 DATE: 21/09/18 08:23:26 GMT 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.