The power of intermediate polars

Abstract

We construct a simplified multicomponent model for the line and continuum emission from accretion funnels and shocks in intermediate polars, which encapsulates the essential physics of the various processes involved. The model is used to calculate the optical and X-ray power spectra for conditions appropriate to disced and discless intermediate polars. We show that, for stream-fed accretion, significant power is expected in the optical at the frequency 2(ω - Ω) if the field distribution has an up (above orbital plane)-down (below orbital plane) symmetry, or at (ω - Ω) if this symmetry is broken. The frequency ω can also appear in the power spectra, but its relative importance depends on the optical thickness of the funnel, which is linked to the mass accretion rate, extent of the coupling region and viewing geometry. In contrast, for disc accretion, the dominant power in the continuum and line fluxes is always at the spin frequency ω. However, 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 radial velocity variations. Disc-fed accretion is characterized by low radial velocity amplitudes (∼ 50-100 km s-1), while stream-fed accretion is characterized by high radial velocity amplitudes (∼ 500-1000 km s-1). Magnetic fields also play an important role in determining the characteristics of the power spectra. As the field increases, the polarized cyclotron emission from the shocks becomes comparable to the optical radiation from the 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 with a stream-fed accretion model. On the other hand, the observations of systems 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.