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.