[vsnet-grb-info 2520] Swift detection of an RS CVn flare star

[GCN Circulars]

TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  4357
SUBJECT: Swift detection of an RS CVn flare star
DATE:    05/12/16 12:27:05 GMT
FROM:    Scott Barthelmy at NASA/GSFC  <Scott at lheamail.gsfc.nasa.gov>

S.T. Holland (GSFC/USRA), S. Barthelmy (GSFC), A. Beardmore (U Leicester),
D. Burrows (PSU), M. Capalbi (ASDC), G. Chincarini (OAB),
S. Covino (OAB), N. Gehrels (GSFC), O. Godet (U. Leicester),
C. Gronwall (PSU), S. Hunsberger (PSU), J. Kennea (PSU),
H. Krimm (GSFC/USRA), A. Moretti (OAB), K. Page (U Leicester),
D. Palmer (LANL), M. Perri (ASDC), G. Tagliaferri (OAB)
on behalf of the Swift team:

At 11:21:52 UT, Swift-BAT triggered on hard x-ray emission from probably
a non-GRB source (trigger=172969).  The spacecraft slewed immediately.
The BAT on-board calculated location is RA,Dec 358.776d,+28.607d
{23h 55m 06s,+28d 36' 26"} (J2000), with an uncertainty of 3 arcmin
(radius, 90% containment, stat+sys).  This is an image trigger (64-sec duration)
and the TDRSS lightcurve shows no obvious variation as is typical for
image triggers.  We also note that the RS CVn star II Peg is close,  and
is probably the cause of the BAT and XRT detections.

XRT began observing the field at 11:24:08 UT, 136 seconds after the BAT
trigger.  On-board centroiding found a bright X-ray
source in the field of view at the following coordinates:
RA(J2000) = 23 55 04.6
Dec(J2000)= 28 38 02
We estimate the uncertainty of this position to be 6 arcseconds (radius,
90% containment).  This position lies 99 arcseconds from the BAT position
above.  The approximate initial flux is 6.4e-09 erg/cm2/sec (0.2-10 keV).
We note that the XRT count rate is not decreasing and that this position is 
7 arcseconds from the RS CVn star II Peg.  It is likely that the XRT 
centroided on this star.  There is no other X-ray source in the 2 x 2 arcmin
postage stamp image.  A check for other sources in the BAT error circle
will require ground analysis following the next Malindi contact.

UVOT began observing the field at UT 11:24:12, 140 seconds after the BAT
trigger.  II Peg is seen as a bright source.  No new source with respect 
to the DSS is seen in the 200 sec finding chart image to a limiting 
magnitude of V=18.6.  The finding chart covers 12% of the BAT error circle.
There is also no new source seen in the parameterized source list.