The peculiar cataclysmic variable FS Aurigae with a magnetic and freely precessing white dwarf.

FS Aurigae represents one of the most unusual cataclysmic variable to have ever been observed. The system is famous for a variety of uncommon and puzzling periodic photometric and spectroscopic variabilities which do not fit well into any of the established sub-classes of cataclysmic variables.

It was previously proposed that the precession of a fast-rotating magnetically accreting white dwarf can successfully explain these phenomena. Due to the magnetic nature of the white dwarf, the accreting material in FS Aur should be controlled and channelled by the strong magnetic field of the white dwarf within its magnetospheric radius, as it is in an ordinary intermediate polar. However, as the white dwarf is involved in two periodic motions – rotational and precessional – one could expect to observe stable modulations in the optical and X-ray light curves with both the periods. Observational properties of these modulations should depend, among other things, on geometrical factors such as the angles between the magnetic, rotational and precessional axes, but they should both be similar in appearance to the spin modulation in intermediate polars.

In our new paper, we present a study of FS Aur based on two extensive sets of optical photometric observations and three X-ray data sets in which we intended to verify whether the observational properties of the long period modulations observed in FS Aur are similar in appearance to the spin modulation in ordinary intermediate polars. We compared most reliable observable properties of the Long Spectroscopic Period (LSP) pulsation such as energy dependence and phasing of the LSP pulses in different wavelengths, with those of the spin modulation of ironclad and confirmed IPs. We have found strong indications that the LSP signal detected in FS Aur’s X-rays and optical photometry and spectroscopy is similar in nature to the spin modulation of the IPs, confirming FS Aur as a member of the class. Thus, FS Aur is the first cataclysmic variable which received an observational confirmation to contain a magnetic and freely precessing white dwarf.

Figure: A schematic representation of the suggested geometry and components for FS Aur. The magnetic field of the white dwarf disrupts the inner accretion disc and forces the accreting material to flow along field lines onto magnetic poles, forming arc-shaped, azimuthally extended accretion curtains. The spin component of the emission lines observed in an ordinary intermediate polar originates in a radiating gas which flows along the curtains (yellow) and has the radial infall velocity. In FS Aur, however, the radial velocity variations with the short spin period are smeared out during the Long Spectroscopic Period (LSP). Instead, a spin-averaged X-ray beam from FS Aur’s WD illuminates different segments of the accretion disc when sweeping around with the precession (LSP) period. These segments (green) reprocess high-energy emission into the optical and have the Keplerian velocity. It crosses the line of sight in front of the whire dwarf at the optical LSP pulse maximum when the upper precessional pole points to the observer. The inner hemisphere of the secondary is the most probable candidate for being both the source of the orbital modulation and the reprocessing site of the sideband, LPP modulation.

For more details see the paper “Steps towards a solution of the FS Aurigae puzzle. II. Confirmation of the intermediate polar status”, 2013, MNRAS, Volume 432, Issue 3, p.2596-2614; by Vitaly Neustroev, Gagik Tovmassian, Sergey Zharikov and George Sjoberg. It can be found from the MNRAS website and astro-ph.