A Multi-Phase Suzaku Study of X-rays from tau Sco

Ignace, Oskinova, Jardine, Cassinelli, Cohen, Donati, Townsend, ud-Doula

ETSU, U of Potsdam, U of St-Andrews, U of Wisconsin, Swarthmore, Observatoire Midi-Pyrenees, U of Wisconsin, Penn State Worthington Scranton

We obtained relatively high signal-to-noise X-ray spectral data of
the early massive star $tau$~Sco (B0.2V) with the {it Suzaku} XIS
instrument. This source displays several unusual features that motivated
our study: (a) redshifted absorption in UV P~Cygni lines to approximately
$+250$ km~s$^{-1}$ suggestive of infalling gas, (b) unusually hard X-ray
emission requiring hot plasma at temperatures in excess of 10~MK whereas
most massive stars show relatively soft X-rays at a few~MK, and (c)
a complex photospheric magnetic field of open and closed field lines.
In an attempt to understand the hard component better, X-ray data were
obtained at six roughly equally spaced phases within the same epoch
of $tau$~Sco's 41~day rotation period. The XIS instrument has three
operable detectors: XIS1 is back illuminated with sensitivity down to
0.2~keV; XIS0 and XIS2 are front illuminated with sensivitity only down
to 0.4~keV and have overall less effective area than XIS1. The XIS0
and XIS3 detectors show relatively little variability. In contrast,
there is a $approx 4sigma$ detection of a $approx 4%$ drop in the
count rate of the XIS1 detector at one rotational phase. In addition,
all three detectors show a $approx 3%$ increase in count rate at the
same phase. The most optimistic prediction of X-ray variability allows
for a 40% change in the count rate, particularly near phases where we
have pointings. Observed modulations in the X-ray light curve on the
rotation cycle is an order of magnitude smaller than this, which
places new stringent constraints on future modeling of this interesting
magnetic massive star.

Reference: to appear in ApJ
Status: Manuscript has been accepted

Weblink: astroph/1008.1552


Email: ignace@etsu.edu