TITLE: GCN CIRCULAR NUMBER: 16677 SUBJECT: Swift trigger 609139 is probably 4U 1850-03 DATE: 14/08/10 14:50:34 GMT FROM: Scott Barthelmy at NASA/GSFC S. D. Barthelmy (GSFC), S. B. Cenko (GSFC), H. A. Krimm (CRESST/GSFC/USRA), N. P. M. Kuin (UCL-MSSL), D. Malesani (DARK/NBI), C. B. Markwardt (NASA/GSFC), F. E. Marshall (NASA/GSFC), K. L. Page (U Leicester) and M. H. Siegel (PSU) report on behalf of the Swift Team: At 13:42:11 UT, the Swift Burst Alert Telescope (BAT) triggered and probably located 4U 1850-03 (trigger=609139). Swift slewed immediately to the location. The BAT on-board calculated location is RA, Dec 282.080, -2.444, which is RA(J2000) = 18h 48m 19s Dec(J2000) = -02d 26' 37" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). Given that this is a 21-min image trigger (8.11 sigma), the information in the BAT ring buffer has been wrapped, so there is no real-time TDRSS light curve available. The XRT began observing the field at 14:08:41.9 UT, 1590.3 seconds after the BAT trigger. Using promptly downlinked data we find an X-ray source located at RA, Dec 282.07027, -2.42364 which is equivalent to: RA(J2000) = 18h 48m 16.86s Dec(J2000) = -02d 25' 25.1" with an uncertainty of 3.6 arcseconds (radius, 90% containment). This location is 81 arcseconds from the BAT onboard position, within the BAT error circle. This position may be improved as more data are received; the latest position is available at http://www.swift.ac.uk/sper. This position is 5.5 arcseconds from a known X-ray source: 1SXPS J184816.5-022522. This source is in the Swift XRT 1SXPS catalogue with a mean 0.3-10 keV count-rate of 2.97 x 10^-3 ct/sec; see http://www.swift.ac.uk/1SXPS/1SXPSJ184816.5-022522 for details of these previous observations. 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 150 seconds with the White filter starting 1593 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 19.6 mag. 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 mag. No correction has been made for the large, but uncertain extinction expected. The above positions appear to be consistent with the known high mass X-ray binary 4U 1850-03, and we believe the BAT trigger indicates activity from this known source. The identifier 1SXPSJ184816.5-022522, derived from the Swift XRT point source survey, is most consistent with the current Swift XRT localization, but it is possible this source is the same as the 4U 1850-03. However, as this is a location 0.4 deg from the Galactic Plane, we cannot rule out the possibility of a distinct Galactic source, nearby to 4U 1850-03.