GCN Circular 23849

S. D. Barthelmy (GSFC), M. G. Bernardini (INAF-OAB),
P. D'Avanzo (INAF-OAB), J.D. Gropp (PSU), N. J. Klingler (PSU),
N. P. M. Kuin (UCL-MSSL), D. M. Palmer (LANL) and A. Tohuvavohu (PSU)
report on behalf of the Neil Gehrels Swift Observatory Team:

At 21:18:17 UT, the Swift Burst Alert Telescope (BAT) triggered and
located SWIFT J1728.9-3613 (trigger=887541).  Swift slewed immediately to the source. 
The BAT on-board calculated location is 
RA, Dec 262.301, -36.267 which is 
   RA(J2000) = 17h 29m 12s
   Dec(J2000) = -36d 16' 01"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty). As is usual with an image trigger, the available
BAT light curve shows no significant structure. 

The XRT began observing the field at 21:28:10.1 UT, 592.5 seconds after
the BAT trigger. XRT found a bright X-ray source located at RA, Dec
262.2442, -36.2421 which is equivalent to:
   RA(J2000)  = 17h 28m 58.61s
   Dec(J2000) = -36d 14' 31.6"
with an uncertainty of 4.7 arcseconds (radius, 90% containment). This
location is 187 arcseconds from the BAT onboard position, outside the
BAT error circle. This position is consistent with the position of Swift J1728.9-3613 (J. Kennea ATel #12445). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 602 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.