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GCN Circular 19499

Subject
Trigger 689353: Swift detection of a transient near Sgr A*
Date
2016-06-06T13:25:59Z (8 years ago)
From
Scott Barthelmy at NASA/GSFC <scott@milkyway.gsfc.nasa.gov>
S. D. Barthelmy (GSFC), P.A. Evans (U Leicester), J. A. Kennea (PSU),
H. A. Krimm (CRESST/GSFC/USRA), A. Y. Lien (GSFC/UMBC),
B. Mingo (U Leicester), K. L. Page (U Leicester), D. M. Palmer (LANL)
and T. N. Ukwatta (LANL) report on behalf of the Swift Team:

At 12:37:43 UT, the Swift Burst Alert Telescope (BAT) triggered and
located a source near Sgr A* (trigger=689353).  Swift slewed immediately
to the source.  The BAT on-board calculated location is 
RA, Dec 266.411, -29.007, which is 
   RA(J2000)  =  17h 45m 39s
   Dec(J2000) = -29d 00' 25"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  As is typical for image triggers, the real-time
TDRSS light curve does not show anything significant. 

The XRT began observing the field at 12:44:50.6 UT, 427.1 seconds after
the BAT trigger. Using promptly downlinked data we find two X-rays sources
very close to each other. The approximate positions of the sources are:

Source 1:
   RA(J2000)  = 17h 45m 40.29s  = 266.41787
   Dec(J2000) = -29d 00' 41.18" = -29.01144

Source 2:
   RA(J2000)  = 17h 45m 40.65s  = 266.41938
   Dec(J2000) = -29d 00' 13.58" = -29.00377

Typical uncertainties are around 5 arcsec (radius, 90% containment
however as these sources are partially blended the uncertainties
may be slightly higher. These source both correspond to known
transients. 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 429 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 BAT Transient is consistent with the location of several   
transient sources in the Galactic Center, as well as the location of  
Sgr A*. Most recently a new transient has been reported in the   
Galactic Center, Swift J174540.2-290037 (ATEL #9109), which lies 24   
arcseconds from the BAT position. The sources found in the XRT SPER
correspond to this source, and also AX J1745.6-2901 (ATEL #5222, #5226). 
As the Galactic Center is a dense region, we cannot rule out that
the BAT trigger is from another transient, however, Swift J174540.2-290037
is the brightest source seen in prompt XRT data taken after the BAT trigger,
and has been rising in brightness since its initial detection by
Swift.
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