Time Dependent Photoionization Modeling of Warm Absorbers in Active
  Galactic Nuclei
  Warm absorber spectra contain bound-bound and bound-free absorption features
seen in the X-ray and UV spectra from many active galactic nuclei (AGN). The
widths and centroid energies of these features indicate they occur in
outflowing gas, and the outflow can affect the gas within the host galaxy. Thus
the warm absorber mass and energy budgets are of great interest. Estimates for
these properties depend on models which connect the observed strengths of the
absorption features with the density, composition, and ionization state of the
absorbing gas. Such models assume that the ionization and heating of the gas
come primarily from the strong continuum near the central black hole. They also
assume that the various heating, cooling, ionization, and recombination
processes are in a time-steady balance. This assumption may not be valid, owing
to the intrinsic time-variability of the illuminating continuum, or other
factors which change the cloud environment. This paper presents models for warm
absorbers which follow the time dependence of the ionization, temperature, and
radiation field in warm absorber gas clouds in response to a changing continuum
illumination. We show that the effects of time variability are important over a
range of parameter values, that time dependent models differ from equilibrium
models in important ways, and that these effects should be included in models
which derive properties of warm absorber outflows.
