Observational evidence for matter propagation in accretion flows

Abstract

We study simultaneous X-ray and optical observations of three intermediate polars, EX Hya, V1223 Sgr and TV Col, with the aim of understanding the propagation of matter in their accretion flows. We show that in all cases the power spectra of the flux variability of binary systems in X-ray and optical bands are similar to each other, and the majority of X-ray and optical fluxes are correlated with time lag <1 s. These findings support the idea that the optical emission of accretion discs in these binary systems largely originates as the reprocessing of the X-ray luminosity of their white dwarfs. In the best obtained data set of EX Hya we see that the optical light curve unambiguously contains some component that leads the X-ray emission by ∼7 s. We interpret this in the framework of the model of propagating fluctuations and thus deduce the time of travel of matter from the innermost part of the truncated accretion disc to the white dwarf surface. This value agrees very well with the time expected for matter threaded on to the magnetosphere of the white dwarf to fall to its surface. The data sets of V1223 Sgr and TV Col in general confirm these findings, but have poorer quality.