Tracking X-ray outflows with optical/infrared footprint lines

Abstract

We use cloudy photoionization models to predict the flux profiles for optical/infrared (IR) emission lines that trace the footprint of X-ray gas, such as [Fe X] 6375 Å and [Si X] 1.43 $\mu\mathrm{m}$. These are subset of coronal lines, from ions with ionization potential greater than or equal to that of O VII, i.e. 138 eV. The footprint lines are formed in gas over the same range in ionization state as the H- and He-like of O and Ne ions, which are also the source of X-ray emission lines. The footprint lines can be detected with optical and IR telescopes, such as the Hubble Space Telescope/STIS and James Webb Space Telescope/NIRSpec, and can potentially be used to measure the kinematics of the extended X-ray emission gas. As a test case, we use the footprints to quantify the properties of the X-ray outflow in the type 1 Seyfert galaxy NGC 4151. To confirm the accuracy of our method, we compare our model predictions to the measured flux from archival STIS spectra and previous ground-based studies, and the results are in good agreement. We also use our X-ray footprint method to predict the mass profile for the X-ray emission-line gas in NGC 4151 and derive a total spatially integrated X-ray mass of $7.8(\pm 2.1) \times 10^{5}\, {\rm M}_{\odot }$, in comparison to $5.4(\pm 1.1) \times 10^{5}\, {\rm M}_{\odot }$ measured from a Chandra X-ray analysis. Our results indicate that high-ionization footprint emission lines in the optical and near-IR can be used to accurately trace the kinematics and physical conditions of active galactic nucleus-ionized, X-ray emission-line gas.