Column accretion on to white dwarfs

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Abstract
We analyse the global behaviour of all accretion column solutions in which the accreting matter reaches equipartition (Te = Ti) before landing on the white dwarf. We show that all such columns remain very optically thin, and divide them into two classes corresponding to the thermal conductive flux into the star becoming large (type 1) or always remaining small (type 2). We show that the unique equipartition solution which can be matched to a perturbed white dwarf atmosphere is one in which the conductive flux into the star vanishes, thus establishing that equipartition flows cannot account for the very large soft X-ray excesses of the AM Herculis stars. We argue that instead the accretion process in these objects involves transport by energetic (E ≲ kTs, Ts = shock temperature) ‘non-local’ electrons which are not in equipartition with the ions (and is thus analogous to the accretion process for neutron stars, involving direct ion transport). In consequence the hot, optically thin post-shock region of the accretion flow must be limited in size by the equipartition length. This in turn limits the hard X-ray luminosity to a small fraction of the total accretion luminosity, in full agreement with observations of AM Herculis.

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