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

TL;DR

This study uses comprehensive numerical simulations to investigate shock oscillations and QPOs in accreting magnetic white dwarfs, revealing complex dynamics and limitations of current models in matching observations.

Contribution

It demonstrates the importance of secondary shocks and complex shock dynamics in QPO formation, highlighting the need for multi-dimensional simulations.

Findings

Shock oscillations result from interplay of radiative instabilities.

Secondary shocks form at the acoustic horizon, supporting steady-state models.

Current one-dimensional simulations cannot fully reproduce observed QPOs.

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.