Quasi-periodic oscillations in accreting magnetic white dwarfs
Abstract
Context. Magnetic cataclysmic variables are close binary systems
containing a strongly magnetized white dwarf that accretes matter coming from an M-dwarf
companion. The high magnetic field strength leads to the formation of an accretion column
instead of an accretion disk. High-energy radiation coming from those objects is emitted
from the column close to the white dwarf photosphere at the impact region. Its properties
depend on the characteristics of the white dwarf and an accurate accretion column model
allows the properties of the binary system to be inferred, such as the white dwarf mass,
its magnetic field, and the accretion rate.Aims. We study the temporal and spectral behaviour of the accretion
region and use the tools we developed to accurately connect the simulation results to the
X-ray and optical astronomical observations.Methods. The radiation hydrodynamics code Hades was adapted to
simulate this specific accretion phenomena. Classical approaches were used to model the
radiative losses of the two main radiative processes: bremsstrahlung and cyclotron.
Synthetic light curves and X-ray spectra were extracted from numerical simulations. A fast
Fourier analysis was performed on the simulated light curves. The oscillation frequencies
and amplitudes in the X-ray and optical domains are studied to compare those numerical
results to observational ones. Different dimensional formulae were developed to complete
the numerical evaluations. Results. The complete characterization of the emitting region is
described for the two main radiative regimes: when only the bremsstrahlung losses and when
both cyclotron and bremsstrahlung losses are considered. The effect of the non-linear
cooling instability regime on the accretion column behaviour is analysed. Variation in
luminosity on short timescales (~1 s quasi-periodic oscillations) is an expected consequence of
this specific dynamic. The importance of secondary shock instability on the quasi-periodic
oscillation phenomenon is discussed. The stabilization effect of the cyclotron process is
confirmed by our numerical simulations, as well as the power distribution in the various
modes of oscillation.
Origin : Publication funded by an institution
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