Assessment the area of accretion curtains from fast aperiodic time variability of Intermediate Polars

Abstract

Results of a study of the fast timing variability of the magnetic cataclysmic variable (mCV) EX Hya are presented. It was shown that one may expect the rapid flux variability of mCVs to be smeared out at timescales shorter than the cooling time of hot plasma in the post shock region of the accretion curtain near the WD surface. Estimates of the cooling time and the mass accretion rate, thus provide us with a tool to measure the density of the post-shock plasma and the crosssectional area of the accretion funnel at the WD surface. We have probed the high frequencies in the aperiodic noise of one of the brightest mCV EX Hya with the help of optical telescopes, namely SALT and the SAAO 1.9m telescope. We place upper limits on the plasma cooling timescale τ < 0.3 sec, on the fractional area of the accretion curtain footprint f < 1.6 × 10 −4 , and a lower limit on the specific mass accretion rateṀ/A > 3 g sec −1 cm −2 . We show that measurements of accretion column footprints via eclipse mapping highly overestimate their areas. From the measurements we estimate an upper limit to the penetration depth of the accretion disk plasma at the magnetosphere boundary δ r/r < 10 3