A Report on the X-ray Properties of the tau Sco Like Stars

Ignace^1, R, Oskinova^2, L, Massa^3, D

^1 Physics & Astronomy, East Tennessee State University
^2 Institute for Physics and Astronomy, University of Potsdam
^3 Space Telescope Science Institute

An increasing number of OB stars have been shown to possess magnetic
fields. Although the sample remains small, it is surprising that
the magnetic and X-ray properties of these stars appear to be far
less correlated than expected. This contradicts model predictions,
which generally indicate that the X-rays from magnetic stars are harder
and more luminous than their non-magnetic counterparts. Instead,
the X-ray properties of magnetic OB stars are quite diverse.

$tau$~Sco is one example where the expectations are better met.
This bright main sequence, early B star has been studied extensively
in a variety of wavebands. It has a surface magnetic field of
around 500~G, and Zeeman Doppler tomography has revealed an unusual
field configuration. Furthermore, $tau$~Sco displays an unusually
hard X-ray spectrum, much harder than similar, non-magnetic OB
stars. In addition, the profiles of its UV P~Cygni wind lines have
long been known to possess a peculiar morphology.

Recently, two stars, HD~66665 and HD~63425, whose spectral types
and UV wind line profiles are similar to those of $tau$~Sco, have
also been determined to be magnetic. In the hope of establishing
a magnetic field -- X-ray connection for at least a sub-set of the
magnetic stars, we obtained XMM-Newton EPIC spectra of these two
objects. Our results for HD~66665 are somewhat inconclusive. No
especially strong hard component is detected; however, the number
of source counts is insufficient to {em rule out} hard emission.
longer exposure is needed to assess the nature of the X-rays from
this star. On the other hand, we do find that HD~63425 has a
substantial hard X-ray component, thereby bolstering its close
similarity to $tau$~Sco.

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

Weblink: http://lanl.arxiv.org/abs/1211.0861


Email: ignace@etsu.edu