Intermediate polars as low-field magnetic cataclysmic variables

Detailed calculations are presented of the polarization properties of extended accretion shocks on the surface of a magnetic white dwarf, where allowance is made both for field spread and for the change in shock height as a function of specific accretion rate. These results are used to show conclusively that the null detection of circular polarization in most IPs imply fields of less than 5 MG. The X-ray properties of MCVs depends critically on the fractional area of the white-dwarf surface over which accretion occurs, and on the type of accretion (smooth or clumpy). In the known IPs, accretion occurs via a disk. The accretion flow is smooth and a strong shock forms making them a powerful source of hard X-rays. There is a new class of MCV distinct from the IPs, where the white dwarf is asynchronous and accretes without a disk in which the accretion is clumpy and the radiation is mainly in the EUV region. A significant fraction of the IPs above the period gap evolve into such systems below the gap and not into AM Hers, as some workers have previously proposed. Any IP-AM Her transitions occur mainly above the period gap at P(O) greater than 4 hr.