Density diagnostics of the hot plasma in AE aquarii with XMM-Newton

We report on XMM-Newton observations of the magnetic CV AE Aqr. High-resolution spectroscopy of the He-like triplet of N and O with the RGS has enabled us to measure the electron number density of the plasma as similar to 10(11) cm(-3). Incorporating this with the emission measure, we estimate the geometrical scale of the plasma emitting these lines to be l(p) similar or equal to (2-3) x 10(10) cm. Since the density and the scale are both incompatible with the standard postshock accretion column of an mCV, the plasma cannot be a product of mass accretion onto the white dwarf. The widths of the H-like K alpha emission lines of N and O (=1250 - 1600 km s(-1)) are of similar magnitude to the thermal velocity dispersion of the hottest part of the plasma, where kT(max) is 4.6 keV. It is known that Balmer series and UV emission lines also show a similar velocity dispersion. In addition, like these lines, the X-ray emission lines are found to show dramatic flaring activity. These facts strongly suggest that all these broad emission lines from X-ray to optical wave bands are produced in the course of adiabatic cooling of the plasma once heated up to T-max in the deep gravitational potential of the white dwarf. This interpretation can resolve the problem of the absence of the high-velocity component in the H alpha emission line spectrum by a scenario where the plasma that is expelled due to the propeller action is still too hot to emit the H alpha line within a region of r < l(p) (roughly equal to the Roche lobe size) from the white dwarf where the high-velocity component is expected to originate.