Oscillatory instability of radiative shocks with multiple cooling processes

Saxton, C. J.; Wu, Kinwah; Pongracic, H.; Shaviv, G.

doi:10.1046/j.1365-8711.1998.01839.x

Cited by 32 publications

(59 citation statements)

References 21 publications

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“…Considering the perturbative analysis, the system is expected to stabilize as cyclotron becomes a strong contribution in the radiative losses (Saxton et al 1998;Saxton 1999;Saxton & Wu 1999). In the results presented in this work, for a white dwarf Fig.…”

Section: Psac Efficiently Cooled By the Cyclotron Processmentioning

confidence: 70%

“…At such high temperatures, matter radiates strongly through optically thin bremsstrahlung, which dominates the hard X-ray emission (Frank et al 2002). Moreover, for the highest magnetized polars, the energy can also be transported efficiently by the cyclotron process (Saxton et al 1998), which emits mainly in the optical domain for the range of polar magnetic fields. As infalling material cools and slows down, it also gets denser with a sharp gradient near the surface.…”

Section: Brief Review Of the Accretion Column Standard Modelmentioning

confidence: 99%

“…A few studies have also described the effect of the coupling between several cooling processes (Wu et al 1992;Saxton et al 1998). In particular, recent astronomical observations (see Paper I) show that several polars are in regimes where either the bremsstrahlung dominates the losses or both bremsstrahlung and cyclotron have to be taken into account.…”

Section: Introductionmentioning

confidence: 99%

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Quasi-periodic oscillations in accreting magnetic white dwarfs

Busschaert

Falize

Michaut

et al. 2015

A&A

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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 H 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.

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Section: Psac Efficiently Cooled By the Cyclotron Processmentioning

confidence: 70%

Section: Brief Review Of the Accretion Column Standard Modelmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Quasi-periodic oscillations in accreting magnetic white dwarfs

Busschaert

Falize

Michaut

et al. 2015

A&A

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“…The cooling time is given by t cool ∼ P/Λ, with P the thermal pressure and Λ the cooling function. The parameter s can then be expressed as a function of the polar parameters (Saxton et al 1998) where m is the specific accretion rate. Bremsstrahlung emission is dominant when s < 1, whereas cyclotron emission dominates for s > 1.…”

Section: The Model Equationsmentioning

confidence: 99%

Numerical simulations of high-energy flows in accreting magnetic white dwarfs

Som

Falize

Bonnet‐Bidaud

et al. 2017

Monthly Notices of the Royal Astronomical Society

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Some polars show quasi-periodic oscillations (QPO) in their optical light curves which have been interpreted as the result of shock oscillations driven by the cooling instability. Although numerical simulations can recover this physics, they wrongly predict QPOs in the X-ray luminosity and have also failed to reproduce the observed frequencies, at least for the limited range of parameters explored so far. Given the uncertainties on the observed polar parameters, it is still unclear whether simulations can reproduce the observations. The aim of this work is to study QPOs covering all relevant polars showing QPOs. We perform numerical simulations including gravity, cyclotron and bremsstrahlung radiative losses, for a wide range of polar parameters, and compare our results with the astronomical data using synthetic X-ray and optical luminosities. We show that shock oscillations are the result of complex shock dynamics triggered by the interplay of two radiative instabilities. The secondary shock forms at the acoustic horizon in the post-shock region in agreement with our estimates from steady-state solutions. We also demonstrate that the secondary shock is essential to sustain the accretion shock oscillations at the average height predicted by our steady-state accretion model. Finally, in spite of the large explored parameter space, matching the observed QPO parameters requires a combination of parameters inconsistent with the observed ones. This difficulty highlights the limits of one-dimensional simulations, suggesting that multi-dimensional effects are needed to understand the non-linear dynamics of accretion columns in polars and the origins of QPOs.

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“…Chevalier & Imamura 1982;Imamura et al 1996;Saxton et al 1997Saxton et al , 1998Saxton & Wu 1999, 2001. When bremsstrahlung emission is the only cooling process the fundamental mode is stable, and the overtones are unstable, with instability δ R tending to increase with harmonic number.…”

Section: Perfect Stationary Wallmentioning

confidence: 99%

Effects of Lower Boundary Conditions on the Stability of Radiative Shocks

Saxton

2002

Publ. – Astron. Soc. Aust

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Thermal instabilities can cause a radiative shock to oscillate, thereby modulating the emission from the post-shock region. The mode frequencies are approximately quantised in analogy to those of a vibrating pipe. The stability properties depend on the cooling processes, the electron–ion energy exchange, and the boundary conditions. This paper considers the effects of the lower boundary condition on the post-shock flow, both ideally and for some specific physical models. Specific cases include constant perturbed velocity, pressure, density, flow rate, or temperature at the lower boundary, and the situation with nonzero stationary flow velocity at the lower boundary. It is found that for cases with zero terminal stationary velocity, the stability properties are insensitive to the perturbed hydrodynamic variables at the lower boundary. The luminosity responses are generally dependent on the lower boundary condition.

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Oscillatory instability of radiative shocks with multiple cooling processes

Cited by 32 publications

References 21 publications

Quasi-periodic oscillations in accreting magnetic white dwarfs

Quasi-periodic oscillations in accreting magnetic white dwarfs

Numerical simulations of high-energy flows in accreting magnetic white dwarfs

Effects of Lower Boundary Conditions on the Stability of Radiative Shocks

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