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

Abstract

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.

Optical observations and cyclops post-shock region modelling of the polar V348 Pav

“…Such a feature is consistent with previous works and is a consequence of the sudden decrease of the plasma velocity while reaching the WD surface (e.g. Hayashi & Ishida 2014;Van Box Som et al 2018). Additionally, at the PSR top, it significantly differs from a power-law distribution, since in our modelling a dipole-like geometry is assumed as well as a magnetospheric boundary and nonnegligible cyclotron cooling.…”

“…Cropper, Wu, Ramsay & Kocabiyik 1999) and considered equipartition between ions and electrons, which is a reasonable assumption for most polars (e.g. Van Box Som et al 2018). We have adopted a quasi-dipolar geometry (i.e.…”

“…The plasma cools down from the shock until the WD surface due to effects of both bremsstrahlung and cyclotron radiative processes. Bremsstrahlung cooling and the relative importance between the two processes are described as power-law functions following Van Box Som et al (2018), and cyclotron cooling is described following Canalle, Saxton, Wu, Cropper & Ramsay (2005). We would like to stress that PSR profiles obtained with such an updated ver- sion of the cyclops code are consistent with previous works (e.g.…”

Fast quasi-periodic oscillations in the eclipsing polar VV Puppis from VLT and XMM-Newton observations

“…Coupling the shock energy distribution with the properties of the shock oscillations may theoretically provide a full determination of the major accretion parameters (white dwarf mass, column cross section, accretion rate). Due to well-controlled physical processes, MCV accretion columns also represent a unique astrophysical environment that can be validly tested by both numerical simulations (Van Box Som et al 2018a) and laser astrophysics experiments (Cross et al 2016).…”

Laser experiment for the study of accretion dynamics of Young Stellar Objects: Design and scaling

“…at the rear surface of the laser-irradiate target) exhibits a good scalability with accreting binary systems (the so-called cataclysmic variables) where a white dwarf accretes material coming from its companion. Accretion in cataclysmic variables exhibits more "extreme" parameters than the one of CTTSs: flow density of about 10 −7.5 g cm −3 , flow speed v f low ∼ 5000 km s −1 , magnetic field strength B ∼ 10−200 M G and a resulting post shock temperature of T ps ∼ 10 8 K [6,7]. We should add that filling the tube with a high Z gas (often used is Xe gas), offer the possibility to study radiative shocks, where radiations start to impact the hydrodynamic behavior [8,9].…”