[vsnet-grb-info 18333] Swift Trigger 707545 is likely 4U 1850-03/GS1843-02

[GCN Circulars]

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  <judith.racusin at nasa.gov>

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.