TITLE: GCN CIRCULAR NUMBER: 20970 SUBJECT: Swift triggered on SAX J1806.5-221 DATE: 17/04/01 19:16:38 GMT FROM: Scott Barthelmy at NASA/GSFC 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.