Grating X-ray Spectroscopy of High-Velocity Outflows from Active Galaxies

Abstract

X‐ray absorption and emission lines now serve as powerful diagnostics of the outflows from active galaxies. Detailed X‐ray line studies of outflows have recently been enabled for a significant number of active galaxies via the grating spectrometers on Chandra and XMM‐Newton. We will review some of the recent X‐ray findings on active galaxy outflows from an observational perspective. We also describe some future prospects. X‐ray absorption lines from H‐like and He‐like ions of C, N, O, Ne, Mg, Al, Si, and S are often seen. A wide range of ionization parameter appears to be present in the absorbing material, and inner‐shell absorption lines from lower ionization ions, Fe L‐shell lines, and Fe M‐shell lines have also been seen. The X‐ray absorption lines are typically blueshifted relative to the systemic velocity by a few hundred km s−1, and they often appear kinematically consistent with UV absorption lines of C iv, N v, and H i. The X‐ray absorption lines can have complex profiles with multiple kinematic components present as well as filling of the absorption lines by emission‐line photons. A key remaining uncertainty is the characteristic radial location of the outflowing gas; only after this quantity is determined will it be possible to calculate reliably the amount of outflowing gas and the kinetic luminosity of the outflow.