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GCN Circular 21369

Subject
Swift triggers on a new galactic source: Swift J181723.1-164300
Date
2017-07-28T17:33:02Z (7 years ago)
From
Scott Barthelmy at NASA/GSFC <scott@milkyway.gsfc.nasa.gov>
S. D. Barthelmy (GSFC), A. P. Beardmore (U Leicester),
D. N. Burrows (PSU), A. Cholden-Brown (PSU), N. P. M. Kuin (UCL-MSSL),
A. Y. Lien (GSFC/UMBC), F. E. Marshall (NASA/GSFC),
K. L. Page (U Leicester), D. M. Palmer (LANL), P. Romano (INAF-OAB),
B. Sbarufatti (INAF-OAB/PSU), M. H. Siegel (PSU) and
E. Troja (NASA/GSFC/UMCP) report on behalf of the Swift Team:

At 16:57:57 UT on 28 July 2017, the Swift Burst Alert Telescope (BAT) triggered and
located a new galactic source Swift J181723.1-164300 (trigger=765081). 
Swift slewed immediately to the source.  The BAT on-board calculated location is 
RA, Dec 274.368, -16.722 which is 
   RA(J2000)  =  18h 17m 28s
   Dec(J2000) = -16d 43' 17"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows several peaks
with a total duration of about 20 sec.  The peak count rate
was ~600 counts/sec (15-350 keV), at ~1 sec after the trigger. 

The XRT began observing the field at 16:59:22.4 UT, 84.5 seconds after
the BAT trigger. Using promptly downlinked data we find a fading,
uncatalogued X-ray source located at RA, Dec 274.34646, -16.71671 which
is equivalent to:
   RA(J2000)  = 18h 17m 23.15s
   Dec(J2000) = -16d 43' 00.2"
with an uncertainty of 3.6 arcseconds (radius, 90% containment). This
location is 76 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. 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 90 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. 

Due to the proximity of this source to the Galactic plane and center
(lon = 14.23 deg, lat = -0.28 deg) and the very soft and absorbed
spectrum seen in initial XRT analysis, we believe that this is
a Galactic source and not a GRB. 

GCN OPS NOTE(30july17):  Per author's request, the date of the event
was added for clarity.]
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