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

TL;DR

This study investigates the rapid variability of the intermediate polar EX Hya to estimate properties of its accretion curtain, providing new constraints on plasma cooling times, accretion area, and penetration depth.

Contribution

It introduces a method to constrain accretion curtain properties in mCVs using high-frequency variability analysis, improving upon eclipse mapping estimates.

Findings

Upper limit on plasma cooling time: <0.3 sec

Upper limit on accretion footprint area: <1.6×10⁻⁴

Lower limit on mass accretion rate per area: >3 g/sec/cm²

Abstract

We present results of a study of the fast timing variability of the magnetic cataclysmic variable (mCV) EX Hya. It was previously 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 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 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 $\tau<$0.3 sec, on the fractional area of the accretion curtain footprint $f<1.6\times10^{-4}$, and a lower limit on the specific mass accretion rate $\dot{M}/A \gtrsim $3 g/sec/cm$^{-2}$. We show that measurements of accretion column footprints via eclipse mapping highly overestimate their areas. We deduce a value of $\Delta r/r \lesssim 10^{-3}$ as an upper limit to the penetration depth of the accretion disc plasma at the boundary of the magnetosphere.