Quasi-periodic oscillations from post-shock accretion column of polars

TL;DR

This study investigates the structure and variability of post-shock accretion columns in polars, revealing how radiation processes and gravity influence quasi-periodic oscillations observed in optical emissions.

Contribution

It provides a detailed analysis of the effects of bremsstrahlung, cyclotron radiation, and gravity on the variability of post-shock accretion columns in polars, linking theoretical models to observed QPOs.

Findings

Cyclotron emission reduces overall variability.

Stratification due to gravity decreases thermal instability.

Dense plasma near the surface may enhance variability.

Abstract

A set of strongly magnetized accreting white dwarfs (polars) shows quasi-periodic oscillations (QPOs) with frequency about a Hz in their optical luminosity. These Hz-frequency QPOs are thought to be generated by intensity variations of the emitted radiation originating at the post-shock accretion column. Thermal instability in the post-shock region, triggered by efficient cooling process at the base, is believed to be the primary reason behind the temporal variability. Here we study the structure and the dynamical properties of the post-shock accretion column including the effects of bremsstrahlung and cyclotron radiation. We find that the presence of significant cyclotron emission in optical band reduces the overall variability of the post-shock region. In the case of a larger post-shock region above the stellar surface, the effects of stratification due to stellar gravity becomes important. An accretion column, influenced by the strong gravity, has a smaller variability as the strength of the thermal instability at the base of the column is reduced. On the other hand, the cool, dense plasma, accumulated just above the stellar surface, may enhance the post-shock variability due to the propagation of magnetic perturbations. These characteristics of the post-shock region are consistent with the observed properties of V834~Cen and in general with Cataclysmic Variable sources that exhibit QPO frequency of about a Hz.