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Register a new userAperiodic optical variability of intermediate polars - cataclysmic variables with truncated accretion disks
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Authors
M. Revnivtsev
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We study the power spectra of the variability of seven intermediate polars containing magnetized asynchronous accreting white dwarfs, XSS J00564+4548,IGR J00234+6141, DO Dra, V1223 Sgr, IGR J15094-6649, IGR J16500-3307 and IGR J17195-4100, in the optical band and demonstrate that their variability can be well described by a model based on fluctuations propagating in a truncated accretion disk. The power spectra have breaks at Fourier frequencies, which we associate with the Keplerian frequency of the disk at the boundary of the white dwarfs magnetospheres. We propose that the properties of the optical power spectra can be used to deduce the geometry of the inner parts of the accretion disk, in particular: 1) truncation radii of the magnetically disrupted accretion disks in intermediate polars, 2) the truncation radii of the accretion disk in quiescent states of dwarf novae
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Accretion disks around supermassive black holes in active galactic nuclei produce continuum radiation at ultraviolet and optical wavelengths. Physical processes in the accretion flow lead to stochastic variability of this emission on a wide range of timescales. We measure the optical continuum variability observed in 67 active galactic nuclei and the characteristic timescale at which the variability power spectrum flattens. We find a correlation between this timescale and the black hole mass, extending over the entire mass range of supermassive black holes. This timescale is consistent with the expected thermal timescale at the ultraviolet-emitting radius in standard accretion disk theory. Accreting white dwarfs lie close to this correlation, suggesting a common process for all accretion disks.
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We model the post-shock accretion column (PSAC) for intermediate polars (IPs), with parameterizing specific accretion rate between 0.0001 and 100 g cm-2 s-1 and metal abundance between 0.1 and 2 times of solar abundance, and taking into account the gravitational potential and non-equipartition between ions, electrons and ionization degree. We assume the cylinder and dipole as geometry of the PSAC. The PSAC becomes higher against the white dwarf (WD) radius for lower specific accretion rate and more massive WD, and may be comparable to the WD radius. The consideration of the dipolar geometry significantly reduces the density and temperature over the whole PSAC comparing with the cylindrical case when the specific accretion rate is lower than a threshold which the PSAC height reachs 0.2 RWD with and is decreased by the more massive white dwarf. We calculate the spectra of the cylindrical and dipolar PSACs with the wide range of the specific accretion rate. Although the spectra soften as the specific accretion rate decreases for the both geometrical assumptions under the specific accretion rate threshold, the softening is more speedy for the dipolar PSAC. The fact means that the both geometrical assumptions lead the different WD masses for each other when their spectra are applied to the IPs hosting the low accretion or a massive WD. Although the ionization non-equilibrium are also involved for the spectral calculation, the effects are trivial because the radiation from ionization non-equilibrium plasma is a few percent of the whole at most.
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