First Application of the Fe xvii I(17.10 A)/I(17.05 A) Line Ratio to Constrain the Plasma Density of a Cosmic X‐Ray Source

We show that the Fe XVII I (17.10 Å)/ I (17.05 Å) line ratio observed in the Chandra High-Energy Transmission Grating (HETG) spectrum of the intermediate polar EX Hya is significantly smaller than that observed in the Sun or other late-type stars. Using the Livermore X-Ray Spectral Synthesizer, which calculates spectral models of highly charged ions based on HULLAC atomic data, we find that the observed I (17.10 Å)/ I (17.05 Å) line ratio can be explained if the plasma density n e ≳ 3 × 10 14 cm -3 . However, if photoexcitation is included in the level-population kinetics, the line ratio can be explained for any density if the photoexcitation temperature T bb ≳ 55 kK. For photoexcitation to dominate the Fe XVII level-population kinetics, the relative size of the hot spot on the white dwarf surface must be f ≲ 2%. This constraint, and the observed X-ray flux, requires a density n ≳ 2 × 10 14 cm -3 for the postshock flow. Either way, then, the Chandra HETG spectrum of EX Hya requires a plasma density that is orders of magnitude greater than that observed in the Sun or other late-type stars.