Warm absorbers in X-rays (WAX), a comprehensive high resolution grating
  spectral study of a sample of Seyfert galaxies: I. A global view and
  frequency of occurrence of warm absorbers
  We present results from a homogeneous analysis of the broadband 0.3-10 keV
CCD resolution as well as of soft X-ray high-resolution grating spectra of a
hard X-ray flux-limited sample of 26 Seyfert galaxies observed with XMM-Newton.
Our goal is to characterise the warm absorber (WA) properties along the
line-of-sight to the active nucleus. We significantly detect WAs in $65\%$ of
the sample sources. Our results are consistent with WAs being present in at
least half of the Seyfert galaxies in the nearby Universe, in agreement with
previous estimates . We find a gap in the distribution of the ionisation
parameter in the range $0.5<\log\xi<1.5$ which we interpret as a thermally
unstable region for WA clouds. This may indicate that the warm absorber flow is
probably constituted by a clumpy distribution of discrete clouds rather than a
continuous medium. The distribution of the WA column densities for the sources
with broad Fe K$\alpha$ lines are similar to those sources which do not have
broadened emission lines. Therefore the detected broad Fe K$\alpha$ emission
lines are bonafide and not artifacts of ionised absorption in the soft X-rays.
The WA parameters show no correlation among themselves, with the exception of
the ionisation parameter versus column density. The shallow slope of the
$\log\xi$ versus $\log v_{\rm out}$ linear regression ($0.12\pm 0.03$) is
inconsistent with the scaling laws predicted by radiation or
magneto-hydrodynamic-driven winds. Our results suggest also that WA and Ultra
Fast Outflows (UFOs) do not represent extreme manifestation of the same
astrophysical system.
