2003ApJ...588L.101M

Using the Livermore X-ray Spectral Synthesizer, which calculates spectral models of highly charged ions based primarily on HULLAC atomic data, we investigate the temperature, density, and photoexcitation dependence of the I(11.92 Å)/I(11.77 Å) line ratio of Fe XXII. We find that this line ratio has a critical density n_c~5x10¹³ cm^–3, is approximately 0.3 at low densities and 1.5 at high densities, and is very insensitive to temperature and photoexcitation; therefore, it is a useful density diagnostic for sources like magnetic cataclysmic variables, in which the plasma densities are high and in which the efficacy of the He-like ion density diagnostic is compromised by the presence of a bright ultraviolet continuum. Applying this diagnostic to the Chandra High Energy Transmission Grating spectrum of the intermediate polar EX Hya, we find that the electron density of its T_e~12 MK plasma is n_e=1.0^+2.0_–0.5x10¹⁴ cm^–3, which is orders of magnitude greater than that typically observed in the Sun or other late-type stars.