Mouchet, M.; Bonnet-Bidaud, J.-M.; Van Box Som, L.; Falize, E.; Buckley, D. A. H.; Breytenbach, H.; Ashley, R. P.; Marsh, T. R.; Dhillon, V. S.
Bibliographical reference
Astronomy and Astrophysics, Volume 600, id.A53, 13 pp.
Description
Quasi-periodic oscillations (QPOs) of a few seconds have been detected
in some polars, the synchronised subclass of cataclysmic systems
containing a strongly magnetised white dwarf which accretes matter from
a red dwarf companion. The QPOs are thought to be related to
instabilities of a shock formed in the accretion column, close to the
white dwarf photosphere above the impact region. We present optical
observations of the polar V834 Centauri performed with the fast ULTRACAM
camera mounted on the ESO-VLT simultaneously in three filters (u', He ii
λ4686, r') to study these oscillations and characterise their
properties along the orbit when the column is seen at different viewing
angles. Fast Fourier transforms and wavelet analysis have been performed
and the mean frequency, rms amplitude, and coherence of the QPOs are
derived; a detailed inspection of individual pulses has also been
performed. The observations confirm the probable ubiquity of the QPOs
for this source at all epochs when the source is in a high state, with
observed mean amplitude of 2.1% (r'), 1.5% (He ii), and 0.6% (u'). The
QPOs are present in the r' filter at all phases of the orbital cycle,
with a higher relative amplitude around the maximum of the light curve.
They are also detected in the He ii and u' filters but at a lower level.
Trains of oscillations are clearly observed in the r' light curve and
can be mimicked by a superposition of damped sinusoids with various
parameters. The QPO energy distribution is comparable to that of the
cyclotron flux, consistent for the r' and He ii filters but requiring a
significant dilution in the u' filter. New 1D hydrodynamical simulations
of shock instabilities, adapted to the physical parameters of
V834 Cen, can account for the optical QPO amplitude
and X-ray upper limit assuming a cross section of the accretion column
in the range (4 - 5) × 1014 cm2. However,
the predicted frequency is larger than the observed one by an order of
magnitude. This shortcoming indicates that the QPO generation is more
complex than that produced in a homogeneous column and calls for a more
realistic 3D treatment of the accretion flow in future modelling.
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