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 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.

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

“…3. The secondary shock Mach number is around 1.2 − 1.7 which is in a good agreement with the published values by Busschaert et al (2015).…”

“…Firstly, for the case of constant specific accretion rate ( m = 1 g cm −2 s −1 ) and for the three white dwarf masses (blue region and models (a), (b) and (c)), the increase of the magnetic field leads to larger oscillation frequencies of both X-ray and optical luminosities. This is due to the link between the cooling characteristic time and the magnetic field as f ∼ 1/t cool ∼ B 2.85 as displayed by Busschaert et al (2015). At the same time, we observe a drastic decrease of their relative amplitudes, as expected with the increase of cyclotron cooling.…”

“…where Busschaert et al (2015) have shown that the dynamics of the post-shock is in good agreement despite the difference of boundary conditions. The simulations are performed using a 1D uniform Cartesian grid without the available Adaptive Mesh Refinement (AMR) present in RAMSES.…”

“…Numerical simulations were performed by Busschaert et al (2015) to study the dynamics of the accretion column and the evolution of QPOs. Oscillations coupled in the X-ray and optical luminosities were highlighted.…”

Quasi-Periodic Oscillation of a Magnetic Cataclysmic Variable, DO Draconis

“…Recently, on the characteristics of QPOs in the X-Ray region and the optical region for polar-type magnetic white dwarfs, Bonnet-Bidaud et al (2015) have suggested that QPOs with short periods of 1 sec ~ 3 sec are formed by shock waves. Also, Busschaert et al (2015) have provided an explanation that QPOs show different behavior in the region of bremsstrahlung only compared to the region where both the cyclotron radiation and the bremsstrahlung radiation occur at the same time through numerical modeling to explain the observation data. QPOs were also observed at TT Ari (Tremko et al 1996;Andronov et al 1999;Smak 2014), and while QPOs in IP-type stars are believed to be generated by the bright bulb around the white dwarf with magnetic fields, detailed numerical calculation results have not been reported yet.…”