GCN Circular 16707

J. R. Cummings (NASA/UMBC), N. Gehrels (NASA/GSFC),
D. Malesani (DARK/NBI), F. E. Marshall (NASA/GSFC) and
B. Sbarufatti (INAF-OAB/PSU) report on behalf of the Swift Team:

At 17:10:03 UT, the Swift Burst Alert Telescope (BAT) triggered and
located a source consistent with the position of SAX J1712.6-3739
(trigger=609878).  Swift slewed immediately to the location. 
The BAT on-board calculated location is 
RA, Dec 258.161, -37.657 which is 
   RA(J2000) = 17h 12m 39s
   Dec(J2000) = -37d 39' 22"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows a ~10-second peak at ~T+130.  
This was a 332-second image trigger.  BAT retriggered on the same source 
with the next 320-second image trigger attempt (trigger=609879), with a 
higher flux.  The lightcurve shows rapid fluctuations.  The peak count 
rate was ~200 counts/sec (15-350 keV), at 130 sec after the trigger. 

The XRT began observing the field at 17:16:59.4 UT, 415.8 seconds after
the BAT trigger. XRT found a bright X-ray source located at RA, Dec
258.1547, -37.6444 which is equivalent to:
   RA(J2000)  = +17h 12m 37.13s
   Dec(J2000) = -37d 38' 39.8"
with an uncertainty of 4.7 arcseconds (radius, 90% containment). This
location is 48 arcseconds from the BAT onboard position, within the BAT
error circle. This is 4.5 arcseconds away from the position of SAX J1712.6-3739
(Wiersema et al. 2009, MNRAS 397, 6). This source is in the Swift XRT 1SXPS
catalogue as 1SXPS J171236.9-373842 with a mean 0.3-10 keV count-rate of 
3.830 +/- 0.033 ct/sec; see http://www.swift.ac.uk/1SXPS/1SXPSJ171236.9-373842 
for details of these previous observations. 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 1.69e-08 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White
filter  starting 425 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.  
We note that this is a crowded field with large expected  extinction. 
No correction has been made for the large, but uncertain extinction 
expected. 

This source has been detected by BAT many times, with the most recent
on-board detection in 2014 May, and it is variable but persistently
visible in BAT ground analysis.