Synthetic absorption lines for a clumpy medium: a spectral signature for
  cloud acceleration in AGN?
  There is increasing evidence that the highly ionized multiphase components of
AGN disk winds may be due to thermal instability. The ions responsible for
forming the observed X-ray absorption lines may only exist in relatively cold
clumps that can be identified with the so-called 'warm absorbers'. Here we
calculate synthetic absorption lines for such warm absorbers from first
principles by combining 2D hydrodynamic solutions of a two-phase medium with a
dense grid of photoionization models to determine the detailed ionization
structure of the gas. Our calculations reveal that cloud disruption, which
leads to a highly complicated velocity field (i.e. a clumpy flow), will only
mildly affect line shapes and strengths when the cold gas becomes highly mixed
but not depleted. Prior to complete disruption, clouds which are optically thin
to the driving UV resonance lines will cause absorption at an increasingly
blueshifted line of sight velocity as they are accelerated. This behavior will
imprint an identifiable signature on the line profile if warm absorbers are
enshrouded in an even broader absorption line produced by a high column of
intercloud gas. Interestingly, we show that it is possible to develop a
spectral diagnostic for cloud acceleration by differencing the absorption
components of a doublet line, a result which can be qualitatively understood
using a simple partial covering model. Our calculations also permit us to
comment on the spectral differences between cloud disruption and ionization
changes driven by flux variability. Notably, cloud disruption offers another
possibility for explaining absorption line variability.
