TITLE: GCN CIRCULAR NUMBER: 17764 SUBJECT: Trigger 639199: Swift detection of IGRJ16418-4532 DATE: 15/04/27 14:39:29 GMT FROM: Scott Barthelmy at NASA/GSFC 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.