Intermediate Polars
Cataclysmic Variables are close interacting binary stars which consist of a white dwarf accreting material from a Roche-lobe-filling companion, usually a late main-sequence star. Intermediate polars are an important subset of CVs in which the magnetic field of the WD is strong enough to disrupt the inner accretion disc or even prevent disc formation completely and to force the accreting material to flow along field lines onto one or both magnetic poles.
The spin period modulation is unambiguously detected in all the confirmed IPs as this is the defining characteristic of the class. This modulation is a direct consequence of the magnetically confined accretion flow onto the magnetic poles of the WD whose magnetic field is of sufficient strength to disrupt the accretion disc and to control the flow before it reaches the surface of the star. At some distance from the WD surface, the infalling material undergoes a strong shock, releasing X-rays as it cools by thermal bremsstrahlung and Compton cooling processes. Thus, it is widely supposed that the X-ray modulation reflects this physical process most clearly whereas the optical range is heavily affected by X-ray reprocessing. This, however, is not always the case. For example, DQ Her – the prototype of the subclass of IPs – is characterized by a very low X-ray flux and the strong spin modulation in the optical band.
IPs show a large variety of photometric and spectroscopic behaviour, both in X-rays and the optical range. In X-rays, for example, some IPs show a single-peaked pulsation whereas others show a double-peaked pulsation. There is also observed a strong dependence on the photon energy (usually, increasing modulation depth with decreasing energy).
The optical spin modulation, if observed, is usually correlated with the X-ray one. However, the optical variability is caused by the reprocessing of variable X-ray irradiation that often resulted in significant sideband modulations. These sidebands and spin modulations can also have a strong wavelength-dependence (see examples below). Furthermore, these modulations are observed not only in continuum but also in the hydrogen and He ii emission lines. In many IPs the spin component, appearing in the far wings of the emission lines, has maximum blueshift near optical and/or X-ray pulse maximum (Hellier 1999).
Despite such complex behaviour, our understanding of IPs is now quite good. Most observable properties can be understood within the context of the widely accepted accretion curtain model (Rosen, Mason, & Cordova 1988; Hellier, Cropper, & Mason 1991). In this model, the material flows towards the magnetic poles of the WD in an arc-shaped curtain, and the largest X-ray and optical flux is seen when the upper pole is on the far side of the WD.
It is not our intention to assess the strong and weak aspects of the model. Instead, we aim to compare the key, most reliable observable properties of FS Aur with those of the “ironclad” IPs. First of all, we pay attention to the optical brightness and colour variability with a spin period, and a mutual phasing between them and the emission line spin variability.