On the area of accretion curtains from fast aperiodic time variability of the intermediate polar EX Hya

Abstract

We present results of a study of the fast timing variability of the magnetic cataclysmic variable (mCV) EXHya. Itwas previously shown that one may expect the rapid flux variability of mCVs to be smeared out at time-scales shorter than the cooling time of hot plasma in the post-shock region of the accretion curtain near the white dwarf (WD) surface. Estimates of the cooling time and the mass accretion rate, thus provide us with a tool to measure the density of the postshock plasma and the cross-sectional area of the accretion funnel at the WD surface. We have probed the high frequencies in the aperiodic noise of one of the brightestmCVEX Hya with the help of optical telescopes, namely Southern African Large Telescope and the South African Astronomical Observatory 1.9 m telescope. We place upper limits on the plasma cooling time-scale τ < 0.3 s, on the fractional area of the accretion curtain footprint f < 1.6 × 10-4, and a lower limit on the specific mass accretion rate Ṁ/A > 3 gs-1 cm-2. We show that measurements of accretion column footprints via eclipse mapping highly overestimate their areas. We deduce a value of Λr/r ≲ 10-3 as an upper limit to the penetration depth of the accretion disc plasma at the boundary of the magnetosphere. © 2014 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.