Stream-fed and disc-fed accretion in TX Columbae

We report on two recent X-ray observations of the intermediate polar TX Col made with ASCA and ROSAT. From a consideration of the power spectra of their light curves, we show that the system was accreting predominantly via a disc in 1994 October, but had reverted to a discoverflow mode of accretion by 1995 October, at which time a substantial amount of the accretion was directly via a stream. We discuss possible reasons for this change. By considering the relative phases of the spin and beat pulsations seen in the ROSAT data, as well as their orbital harmonics, we demonstrate that the stream-fed accretion regions on the white dwarf surface remain fixed at about orbital phase 0.7, and thus migrate around the white dwarf. We suggest that the observed orbital modulation results from a combination of two effects: first, attenuation of emission sites by material located at the stream-disc boundary at orbital phase 0.8, and, secondly, the changing visibility of the stream-fed accretion sites which are always on the side of the white dwarf facing the stream.