A wind-based unification model for NGC 5548: spectral holidays, non-disk
  emission, and implications for changing-look quasars
  The 180-day Space Telescope and Optical Reverberation Mapping campaign on NGC
5548 discovered an anomalous period, the broad-line region (BLR) holiday, in
which the emission lines decorrelated from the continuum variations. This is
important since the correlation between the continuum-flux variations and the
emission-line response is the basic assumption for black hole (BH) mass
determinations through reverberation mapping. During the BLR holiday, the
high-ionization intrinsic absorption lines also decorrelated from the continuum
as a result of variable covering factor of the line of sight (LOS) obscurer.
The emission lines are not confined to the LOS, so this does not explain the
BLR holiday. If the LOS obscurer is a disk wind, its streamlines must extend
down to the plane of the disk and the base of the wind would lie between the BH
and the BLR, forming an equatorial obscurer. This obscurer can be transparent
to ionizing radiation, or can be translucent, blocking only parts of the SED,
depending on its density. An emission-line holiday is produced if the wind
density increases only slightly above its transparent state. Both obscurers are
parts of the same wind, so they can have associated behavior in a way that
explains both holidays. A very dense wind would block nearly all ionizing
radiation, producing a Seyfert 2 and possibly providing a contributor to the
changing-look AGN phenomenon. Disk winds are very common and we propose that
the equatorial obscurers are too, but mostly in a transparent state.
