Accretion Column Structure of Magnetic Cataclysmic Variables from X-Ray Spectroscopy

Using Chandra HETG data we present light curves for individual spectral linesof Mg XI and Mg XII for EX Hydrae, an intermediate-polar type cataclysmicvariable. The Mg XI light curve, folded on the white dwarf spin period, showstwo spikes that are not seen in the Mg XII or broad-band light curves.Occultation of the accretion column by the body of the white dwarf wouldproduce such spikes for an angle between the rotation axis and the accretioncolumns of alpha = 18 degrees and a height of the Mg XI emission above thewhite dwarf surface of < 0.0004 white dwarf radii or < 4 km. The absence ofspikes in the Mg XII and broad-band light curves could then be explained if thebulk of its emission forms at much larger height, > 0.004 white dwarf radii or> 40 km, above the white dwarf surface, although this is not consistent withthe predictions of the standard Aizu model of the accretion column.Comment: 12 pages, 3 figures, accepted for publication in the ApJ Letter