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 1D hydrodynamic model of a loop with half-length $L = 5 \times 10^{11}$cm($\sim R_{\star}/2$),cross-sectionradius$r = 4.3 \times 10^{10}$cm,withaheatpulseof15ksand$2 \times 10^{11}$ergcm$^{-2}$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 ($\sim {10}^8$ K) plasma emission of the FeXXV complex at $\sim 1.85$\AA. The model loop has $L\mathrm{/}{R}_{\star }\sim 1\mathrm{/}2$ and aspect ratio $r\mathrm{/}L\sim 0.1$ 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 on stellar parameters and evolutionary stage.