Anatomy of the AGN in NGC 5548: VII. Swift study of obscuration and
  broadband continuum variability
  We present our investigation into the long-term variability of the X-ray
obscuration and optical-UV-X-ray continuum in the Seyfert 1 galaxy NGC 5548. In
2013 and 2014, the Swift observatory monitored NGC 5548 on average every day or
two, with archival observations reaching back to 2005, totalling about 670 ks
of observing time. Both broadband spectral modelling and temporal rms
variability analysis are applied to the Swift data. We disentangle the
variability caused by absorption, due to an obscuring weakly-ionised outflow
near the disk, from variability of the intrinsic continuum components (the soft
X-ray excess and the power-law) originating from the disk and its associated
coronae. The spectral model that we apply to this extensive Swift data is the
global model that we derived for NGC 5548 from analysis of the stacked spectra
from our multi-satellite campaign of summer 2013 (including XMM-Newton, NuSTAR
and HST). The results of our Swift study show that changes in the covering
fraction of the obscurer is the primary and dominant cause of variability in
the soft X-ray band on timescales of 10 days to ~ 5 months. The obscuring
covering fraction of the X-ray source is found to range between 0.7 and nearly
1.0. The contribution of the soft excess component to the X-ray variability is
often much less than that of the obscurer, but it becomes comparable when the
optical-UV continuum flares up. We find that the soft excess is consistent with
being the high-energy tail of the optical-UV continuum and can be explained by
warm Comptonisation: up-scattering of the disk seed photons in a warm,
optically thick corona as part of the inner disk. To this date, the Swift
monitoring of NGC 5548 shows that the obscurer has been continuously present in
our line of sight for at least 4 years (since at least February 2012).
