A report on the X-ray properties of the τ Sco-like stars the Small Magellanic Cloud
R. Ignace, L.M. Oskinova, D. Massa
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 to be harder and
more luminous than their non-magnetic counterparts. Instead, the X-ray
properties of magnetic OB stars are quite diverse.
τ 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 τ 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
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 τ Sco.
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