TITLE: GCN CIRCULAR NUMBER: 19781 SUBJECT: Swift Trigger 707545 is likely 4U 1850-03/GS1843-02 DATE: 16/08/06 11:32:39 GMT FROM: Judith Racusin at GSFC P. D'Avanzo (INAF-OAB), J. A. Kennea (PSU), H. A. Krimm (CRESST/GSFC/USRA), A. Melandri (INAF-OAB), K. L. Page (U Leicester) and J. L. Racusin (NASA/GSFC) report on behalf of the Swift Team: At 10:58:23 UT, the Swift Burst Alert Telescope (BAT) triggered and located an outburst from 4U 1850-03 (= GS1843-02; trigger=707545). Swift slewed immediately to the location. The BAT on-board calculated location is RA, Dec 282.077, -2.446 which is RA(J2000) = 18h 48m 18s Dec(J2000) = -02d 26' 44" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). As is typical for an image trigger, the BAT light curve shows no significant features. The XRT began observing the field at 11:06:22.2 UT, 478.9 seconds after the BAT trigger. Using promptly downlinked data we find an X-ray source with an enhanced position: RA, Dec 282.0707, -2.4236 which is equivalent to: RA(J2000) = 18h 48m 16.97s Dec(J2000) = -02d 25' 24.9" with an uncertainty of 2.3 arcseconds (radius, 90% containment). This location is 83 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 6.9 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 [+1.01, -0.85]) 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 482 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. Note that BAT has previously triggered on this source (GCNs 18675, 16677, ATel #4350). 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.