Detailed Diagnostics of an X-Ray Flare in the Single Giant HR 9024

We analyze a 96 ks Chandra HETGS observation of the single G-type giant HR 9024. The high flux allows us to examine spectral line and continuum diagnostics at high temporal resolution, to derive plasma parameters. A time-dependent one-dimensional hydrodynamic model of a loop with half-length L=5×1011 cm (~R*/2) and cross-sectional radius r=4.3×1010 cm, with a heat pulse of 15 ks and 2×1011 ergs cm-2 s-1 deposited at the loop footpoints, satisfactorily reproduces the observed evolution of temperature and emission measure, derived from the analysis of the strong continuum emission. For the first time we can compare predictions from the hydrodynamic model with single spectral features, other than with global spectral properties. We find that the model closely matches the observed line emission, especially for the hot (~108 K) plasma emission of the Fe XXV complex at ~1.85 Å. The model loop has L/R*~1/2 and aspect ratio r/L~0.1, as typically derived for flares observed in active stellar coronae, suggesting that the underlying physics is the same for these very dynamic and extreme phenomena in stellar coronae independently of stellar parameters and evolutionary stage.