Intermediate Polar Type Cataclysmic Variables

A cataclysmic variable (CV) is a close interacting binary system in which a white dwarf (WD) accretes material from its late-type low mass main sequence companion. CVs can be studied in two main classes: non-magnetic CVs (that are characterized by their eruptive behavior) and magnetic CVs (MCVs). MCVs are divided into two sub-classes according to the degree of synchronization of the binary. Synchronization is caused by the interaction of the magnetic fields of the WD and the companion star. If a system is highly synchronized, then it is called a Polar type MCV or an AM Her star; otherwise it is called an Intermediate Polar (IP) or a DQ Her star. A review of CVs can be found in Warner (Cataclysmic Variable Stars, Cambridge University Press, Cambridge 1995).

In the evolutionary sequence, the secondary star fills its Roche lobe and consequently the WD starts to accrete matter from its companion.
Due to the angular momentum of the accreting matter, the material starts circulating around the WD, forming an accretion disk. However, the infalling plasma is channeled on to the magnetic field lines of the WD at around the Alfven radius. In the case of IPs, a disk remains outside the Alfven radius, since the magnetic field
strength of these stars are medium range. On the other hand, Polars have no accretion disks due to the large value of the Alfven radius which exceeds the L1 (inner Lagrangian) point of the binary. In both types of binaries the final impact of the plasma occurs at the magnetic poles of the WD. The radial infall velocity of the plasma is calculated
to be in the range of 3000-10000 km/s which is greater than the sonic speed
\(\approx\)100 km/s (Patterson PASP,106, 209 1994). Therefore, the infalling plasma releases its energy in a shock as it is coming to a stop above the magnetic poles of the WD. The resulting shock has T\(\approx{10^8}\)K and emits hard X-rays. In addition, if the poles are inclined to the spin axis, intensity modulations of the X-ray, UV and optical light will be observed at the spin period, one of its harmonics (eg., \({\bf two~pole}\) accretion) or sidebands (i.e., due to reprocessed light from structures fixed in the reference frame of the binary or material over-flowing the disk).

Power spectrum of YY Dra using data from the GIS3 detector on board ASCA. The periodic signals at 526.3 and 264.9 seconds are clearly visible. Figure is taken from E. Yazgan (2002).

E. Yazgan (2002), “Timing Characteristics of Intermediate Polar Type Cataclysmic Variables and Detailed Data Analysis on YY Draconis” , MS Thesis.

E. Yazgan, and S. Balman (2002) “Analysis of the ASCA data of the intermediate polar YY Dra” , in The Physics of Cataclysmic Variables and Related Objects, ASP Conference Proceedings, Vol. 261. Edited by B. T. Gänsicke, K. Beuermann, and K. Reinsch. ISBN: 1-58381-101-X. San Francisco: Astronomical Society of the Pacific, 2002, p. 177.