Tracking X-ray Outflows with Optical/IR Footprint Lines

Abstract

Abstract We use Cloudy photoionisation models to predict the flux profiles for optical/IR emission lines that trace the footprint of X-ray gas, such as [Fe X] 6375Å and [Si X] 1.43μm. These are a subset of coronal lines, from ions with ionisation potential ≥ that of O VII, i.e., 138eV. The footprint lines are formed in gas over the same range in ionisation 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 Seyfert 1 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( ± 2.1) × 105 M⊙, in comparison to 5.4( ± 1.1) × 105 M⊙ measured from a Chandra X-ray analysis. Our results indicate that high-ionisation footprint emission lines in the optical and near-infrared can be used to accurately trace the kinematics and physical conditions of AGN ionised, X-ray emission line gas.