First Application of the FexviiI(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 A)/I(17.05 A) line ratio observed in theChandra HETG spectrum of the intermediate polar EX Hydrae is significantlysmaller than that observed in the Sun or other late-type stars. Using theLivermore X-ray Spectral Synthesizer, which calculates spectral models ofhighly charged ions based on HULLAC atomic data, we find that the observedI(17.10 A)/I(17.05 A) line ratio can be explained if the plasma density n_e >3x10^{14} cm^{-3}. However, if photoexcitation is included in the levelpopulation kinetics, the line ratio can be explained for any density if thephotoexcitation temperature T_bb > 55 kK. For photoexcitation to dominate theFe XVII level population kinetics, the relative size of the hotspot on thewhite dwarf surface must be f < 2%. This constraint and the observed X-ray fluxrequires a density n > 2x10^{14} cm^{-3} for the post-shock flow. Either way,then, the Chandra HETG spectrum of EX Hya requires a plasma density which isorders of magnitude greater than that observed in the Sun or other late-typestars.Comment: 13 pages including 1 table and 4 encapsulated postscript figures; LaTeX format, uses aastex.cls; accepted on 2001 June 27 for publication in The Astrophysical Journa