[vsnet-grb-info 18066] Trigger 689956: Swift detection of a source near the Galactic center

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  19514
SUBJECT: Trigger 689956: Swift detection of a source near the Galactic center
DATE:    16/06/09 03:15:50 GMT
FROM:    Scott Barthelmy at NASA/GSFC  <scott at milkyway.gsfc.nasa.gov>

S. D. Barthelmy (GSFC), D. M. Palmer (LANL), T. G. R. Roegiers (PSU)
and M. H. Siegel (PSU) report on behalf of the Swift Team:

At 02:21:03 UT, the Swift Burst Alert Telescope (BAT) triggered and
located a possible new source (trigger=689956).  Swift did not immediately
slew to the source due to its low merit value. 
The BAT on-board calculated location is 
RA, Dec 266.422, -29.008, which is 
   RA(J2000)  =  17h 45m 41s
   Dec(J2000) = -29d 00' 29"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  As is typical for image triggers (especially
long triggers such as this 15 min one), the real-time TDRSS
data does not show anything significant. 

The XRT began observing the field at 02:56:14.5 UT, 2110.9 seconds
after the BAT trigger. Using promptly downlinked data we find an X-ray
source located at RA, Dec 266.41879, -29.01122 which is equivalent to:
   RA(J2000)  = 17h 45m 40.51s
   Dec(J2000) = -29d 00' 40.4"
with an uncertainty of 3.7 arcseconds (radius, 90% containment). 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 2.2
arcseconds from that of a known X-ray source: 1BMC 174540.6-290039 in
the CHANDRA BMWCHANCAT catalogue

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 2113 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. 

This source is presumably the same as the source that triggered 
2.5 days earlier (Barthelmy et al. GCN 19499).  This may
be the recently-detected source SWIFT J174540.2-290037 which
has continued to brighten in the XRT during routine monitoring
(Degenaar et al. Atel #9109), the previously detected and currently-
active SWIFT J174540.7-290015 (Reynolds et al; ATel #8649) or some
other source such as 1BMC 174540.6-290039, located by XRT in
the initial exposure. Due to the crowded field this close to the
Galactic center (~4 arcminutes) we cannot definitively determine 
which source produced either trigger with the currently-available data.