GCN Circular 16874

M. De Pasquale (INAF-IASFPA), N. Gehrels (NASA/GSFC),
H. A. Krimm (CRESST/GSFC/USRA), C. B. Markwardt (NASA/GSFC),
A. Maselli (INAF-IASFPA), K. L. Page (U Leicester),
D. M. Palmer (LANL), B. Sbarufatti (INAF-OAB/PSU) and
M. H. Siegel (PSU) report on behalf of the Swift Team:

At 04:44:43 UT, the Swift Burst Alert Telescope (BAT) triggered and
located a source consistent with the position of the Be star HD 305560
(trigger=614193).  Swift slewed immediately to the location. 
The BAT on-board calculated location is 
RA, Dec 161.614, -60.581 which is 
   RA(J2000) = 10h 46m 27s
   Dec(J2000) = -60d 34' 51"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT lightcurve shows variation due to
Vela X-1, which is also in the BAT FOV, but no other obvious variation
that can be attributed to the new source, as is usual for an image trigger. 

The trigger time corresponds to the start of the observation, so the
source may have been active before it came into the BAT field of view. 

The XRT began observing the field at 04:51:41.0 UT, 417.7 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
fading, uncatalogued X-ray source located at RA, Dec 161.61728,
-60.56433 which is equivalent to:
   RA(J2000)  = 10h 46m 28.15s
   Dec(J2000) = -60d 33' 51.6"
with an uncertainty of 4.1 arcseconds (radius, 90% containment). This
location is 60 arcseconds from the BAT onboard position, within the BAT
error circle. This position may be improved as more data are received;
the latest position is available at http://www.swift.ac.uk/sper. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density consistent with the Galactic value of 1.42
x 10^22 cm^-2 (Willingale et al. 2013). 

The initial flux in the 2.5 s image was 1.36e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the U filter
starting 873 seconds after the BAT trigger. The X-ray source is
spatially coincident with HD 305560, which is saturated in the UVOT
image, consistent with its listed U magnitude of 9.5.  Further
analysis will be needed to determine if there is any variation in
the optical flux. No new optical source is seen in the 2.7'x2.7'
sub-image. The typical 3-sigma upper limit has been about 19.2 mag. 

Although high energy emission has not been previously detected
from HD 305560, its status as a bright Be star in close
coincidence with the XRT location leads us to believe that this
star is the detected source.