The power of intermediate polars

We construct a simplified multicomponent model for the line and continuum e mission from accretion funnels and shocks in intermediate polars, which enc apsulates the essential physics of the various processes involved. The mode l is used to calculate the optical and X-ray power spectra for conditions a ppropriate to disced and discless intermediate polars. We show that, for st ream-fed accretion, significant power is expected in the optical at the fre quency 2(omega - Omega) if the field distribution has an up (above orbital plane)-down (below orbital plane) symmetry, or at (omega - Omega) if this s ymmetry is broken. The frequency w can also appear in the power spectra, bu t its relative importance depends on the optical thickness of the funnel, w hich is linked to the mass accretion rate, extent of the coupling region an d viewing geometry. In contrast, for disc accretion, the dominant power in the continuum and Line fluxes is always at the spin frequency omega. Howeve r, the single most important feature that allows a clear distinction to be made between disc-fed and stream-fed accretion is the amplitude of the radi al velocity variations. Disc-fed accretion is characterized by low radial v elocity amplitudes (similar to 50-100 km s(-1)), while stream-fed accretion is characterized by high radial velocity amplitudes (similar to 500-1000 k m s(-1)). Magnetic fields also play an important role in determining the characterist ics of the power spectra. As the field increases, the polarized cyclotron e mission from the shocks becomes comparable to the optical radiation from th e funnels, and the dominant power shifts away from the side-band frequency, to the spin frequency of the white dwarf. The observed characteristics of the polarized intermediate polar RX J1712-2424 are shown to be consistent w ith a stream-fed accretion model. On the other hand, the observations of sy stems such as RX J1238-38 and TX Col are inconsistent with what is expected for stream-fed accretion, or for direct accretion via disc overflow.