[vsnet-grb-info 15540] Swift detection of outburst from 4U 1700+24

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  16829
SUBJECT: Swift detection of outburst from 4U 1700+24
DATE:    14/09/17 14:11:04 GMT
FROM:    David Palmer at LANL  <palmer at lanl.gov>

D. N. Burrows (PSU), S. B. Cenko (GSFC), V. D'Elia (ASDC),
N. Gehrels (NASA/GSFC), C. Gronwall (PSU), J. A. Kennea (PSU),
H. A. Krimm (CRESST/GSFC/USRA), V. Mangano (PSU),
F. E. Marshall (NASA/GSFC), P. T. O'Brien (U Leicester),
C. Pagani (U Leicester), D. M. Palmer (LANL), T. Sakamoto (AGU),
B. Sbarufatti (INAF-OAB/PSU), M. H. Siegel (PSU), C. A. Swenson (PSU)
and E. Troja (NASA/GSFC/UMCP) report on behalf of the Swift Team:

At 13:30:35 UT, the Swift Burst Alert Telescope (BAT) triggered and
located the symbiotic star 4U 1700+24 (trigger=612974). 
Swift slewed immediately to the source location. 
The BAT on-board calculated location is 
RA, Dec 256.633, +23.950 which is 
   RA(J2000) = 17h 06m 32s
   Dec(J2000) = +23d 57' 01"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  Because this is a 15 minute image trigger,
the BAT lightcurve is not immediately available. 

The XRT began observing the field at 13:50:12.7 UT, 1177.2 seconds
after the BAT trigger. XRT found a bright X-ray source located at RA,
Dec 256.6459, 23.9711 which is equivalent to:
   RA(J2000)  = +17h 06m 35.02s
   Dec(J2000) = +23d 58' 16.0"
with an uncertainty of 5.2 arcseconds (radius, 90% containment). This
location is 87 arcseconds from the BAT onboard position, within the BAT
error circle. This position is 6.4 arcseconds from a known X-ray
source: 1SXPS J170634.5+235816, and 7.3 arc-seconds from the optical 
position of V934 Her, thought to be the optical counterpart of 4U 1700+24. 
This source is in the Swift XRT 1SXPS catalogue with a mean 0.3-10 keV 
count-rate of 0.3810 +/- 0.0067 ct/sec; see
http://www.swift.ac.uk/1SXPS/1SXPSJ170634.5%2B235816 for
details of these previous observations. 

The initial flux in the 2.5 s image was 1.79e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 64 seconds with the White filter
starting 1185 seconds after the BAT trigger. The only source within
the XRT error circle is the known bright (V=7.6) optical source V934
Her. 

The source 4U 1700+24 has been rising in brightness in the Swift/BAT
hard X-ray transient monitor since 12 September 2014.  It is currently
seen in the one-day average for 17 September at 0.011 +/- 0.004
ct/s/cm^2 (~50 mCrab; 15-50 keV).