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We present results of a study of the fast timing variability of the magneticcataclysmic variable (mCV) EX Hya. It was previously shown that one may expectthe rapid flux variability of mCVs to be smeared out at timescales shorter thanthe cooling time of hot plasma in the post shock region of the accretioncurtain near the WD surface. Estimates of the cooling time and the massaccretion rate, thus provide us with a tool to measure the density of thepost-shock plasma and the cross-sectional area of the accretion funnel at theWD surface. We have probed the high frequencies in the aperiodic noise of oneof the brightest mCV EX Hya with the help of optical telescopes, namely SALTand the SAAO 1.9m telescope. We place upper limits on the plasma coolingtimescale $\tau<$0.3 sec, on the fractional area of the accretion curtainfootprint $f<1.6\times10^{-4}$, and a lower limit on the specific massaccretion rate $\dot{M}/A \gtrsim $3 g/sec/cm$^{-2}$. We show that measurementsof accretion column footprints via eclipse mapping highly overestimate theirareas. We deduce a value of $\Delta r/r \lesssim 10^{-3}$ as an upper limit tothe penetration depth of the accretion disc plasma at the boundary of themagnetosphere.

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