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GCN Circular 17679

Subject
Swift triggered on a source in Terzan 5 (trigger=637212)
Date
2015-04-05T18:45:15Z (9 years ago)
From
Scott Barthelmy at NASA/GSFC <scott@milkyway.gsfc.nasa.gov>
S. D. Barthelmy (GSFC), W. H. Baumgartner (GSFC/UMBC),
V. D'Elia (ASDC), L. Izzo (URoma/ICRA), J. A. Kennea (PSU) and
K. L. Page (U Leicester) report on behalf of the Swift Team:

At 18:15:06 UT, the Swift Burst Alert Telescope (BAT) triggered and
located a source within Terzan 5 (trigger=637212).  Swift slewed immediately
to the source.  The BAT on-board calculated location is 
RA, Dec 267.000, -24.772 which is 
   RA(J2000)  =  17h 48m 00s
   Dec(J2000) = -24d 46' 18"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  As is typical for image trigger, there is nothing
in the real-time TDRSS BAT light curve. 

The XRT began observing the field at 18:24:47.4 UT, 580.8 seconds after
the BAT trigger. XRT found a bright X-ray source located at RA, Dec
267.0200, -24.7801 which is equivalent to:
   RA(J2000)  = +17h 48m 4.80s
   Dec(J2000) = -24d 46' 48.4"
with an uncertainty of 5.4 arcseconds (radius, 90% containment). This
location is 71 arcseconds from the BAT onboard position, within the BAT
error circle. This position is 1.44 arcseconds from a known X-ray
source EXO 1745-248. No event data are yet available to
determine the column density using X-ray spectroscopy. 

The initial flux in the 2.5 s image was 1.34e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 589 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. Because of the density of catalogued stars, further analysis
is required to report an upper limit for any afterglow in the region. No
correction has been made for the large, but uncertain extinction expected. 

We note that the source EXO 1745-248 within Terzan 5 is currently in
outburst, so we suggest this may be the cause of the trigger.
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