X-ray emission mechanisms in accreting white dwarfs

TL;DR

This paper reviews the mechanisms of X-ray emission in various types of accreting white dwarf systems, highlighting differences based on magnetic properties and accretion processes, and discussing spectral and variability characteristics.

Contribution

It provides a comprehensive overview of X-ray emission processes in different subclasses of cataclysmic variables, integrating spectral and temporal variability insights.

Findings

X-ray emission arises from shocks and hydrogen burning in CVs.

Magnetic and non-magnetic CVs show distinct X-ray behaviors.

Spectral and variability patterns help classify CV types.

Abstract

In this chapter we consider the processes which can lead to X-ray emission from different types of cataclysmic variable stars (CVs). CVs are semi-detached, binary star systems where material is transferred from the donor star (also known as the companion or secondary star) onto the white dwarf primary. CVs are divided into several sub-classes based on the observed phenomenology in the optical and X-ray bands, which, in turn, is largely defined by the magnetic field strength of the accretor. In non-magnetic systems, a variety of observed behaviours are identified, depending on the accretion rate: novae, dwarf novae, nova-like variables, symbiotic binaries and supersoft sources are all examples of non-magnetic CVs. In magnetic systems (polars and intermediate polars, or AM Her and DQ Her systems, respectively), the accretion flow is channelled to polar regions, and the observational appearance is different. X-rays are typically produced through hot or energetic processes, and in CVs they are formed via shocks (within a boundary layer or accretion column, or through interactions either internal to the nova ejecta, or between the ejecta and a stellar wind) or from hydrogen burning (either steady fusion, or a thermonuclear runaway). All of these different types of accreting white dwarfs are discussed here, considering both spectral and temporal variability in the different populations.