Anatomy of the AGN in NGC 5548 VIII. XMM-Newton's EPIC detailed view of
  an unexpected variable multilayer absorber
  In 2013 we conducted a large multi-wavelength campaign on the archetypical
Seyfert 1 galaxy NGC 5548. Unexpectedly, this usually unobscured source
appeared strongly absorbed in the soft X-rays during the entire campaign, and
signatures of new and strong outflows were present in the almost simultaneous
UV HST/COS data. Here we carry out a comprehensive spectral analysis of all
available XMM-Newton observations of NGC 5548 (for a total of ~763 ks) in
combination with 3 simultaneous NuSTAR obs. We obtain a best-fit composed by i)
a weakly varying flat (Gamma~1.5-1.7) power-law component; ii) a constant, cold
reflection (FeK + continuum) component; iii) a soft excess, possibly due to
thermal Comptonization; and iv) a constant, ionized scattered emission-line
dominated component. Our main findings are that, during the 2013 campaign, the
first 3 of these components appear to be partially covered by a heavy and
variable obscurer located along the l.o.s. that is consistent with a multilayer
of cold and mildly ionized gas. We characterize in detail the short timescale
(~ks-to-days) spectral variability of this new obscurer, and find it is mostly
due to a combination of Nh and Cf variations, on top of intrinsic power-law
variations. In addition, our best-fit spectrum is left with several (but
marginal) absorption features at rest-frame energies ~6.7-6.9 keV and ~8 keV,
as well as a weak broad emission line feature redwards of the 6.4 keV emission
line. These could indicate a more complex underlying model, e.g. a P-Cygni-type
profile if allowing for a large velocity and wide-angle outflow. These findings
are consistent with a picture where the obscurer represents the manifestation
along the l.o.s. of a multilayer of gas that is likely outflowing at high
speed, and simultaneously producing heavy obscuration and scattering in the
X-rays and broad absorption features in the UV.
