Geometry Diagnostics of a Stellar Flare from Fluorescent X-rays

TL;DR

This study uses X-ray fluorescence and 3D modeling to measure the height of a stellar flare's coronal plasma, providing insights into stellar flare geometry and comparing it with solar phenomena.

Contribution

It introduces a method combining fluorescence observations and Monte Carlo simulations to directly measure stellar flare coronal heights.

Findings

Fe fluorescent emission observed during a stellar flare.

Coronal plasma scale height estimated to be less than 0.3 stellar radii.

Flare properties consistent with X-ray photoionization mechanisms.

Abstract

We present evidence of Fe fluorescent emission in the Chandra HETGS spectrum of the single G-type giant HR 9024 during a large flare. In analogy to solar X-ray observations, we interpret the observed Fe K$\alpha$ line as being produced by illumination of the photosphere by ionizing coronal X-rays, in which case, for a given Fe photospheric abundance, its intensity depends on the height of the X-ray source. The HETGS observations, together with 3D Monte Carlo calculations to model the fluorescence emission, are used to obtain a direct geometric constraint on the scale height of the flaring coronal plasma. We compute the Fe fluorescent emission induced by the emission of a single flaring coronal loop which well reproduces the observed X-ray temporal and spectral properties according to a detailed hydrodynamic modeling. The predicted Fe fluorescent emission is in good agreement with the observed value within observational uncertainties, pointing to a scale height $\lesssim 0.3$\rstar. Comparison of the HR 9024 flare with that recently observed on II Peg by Swift indicates the latter is consistent with excitation by X-ray photoionization.