[vsnet-grb-info 15388] Swift trigger 609139 is probably 4U 1850-03

[GCN Circulars]

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  <scott at milkyway.gsfc.nasa.gov>

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.