Space Telescope and Optical Reverberation Mapping Project. X.
  Understanding the Absorption-Line Holiday in NGC 5548
  The flux variations in the emission lines in active galactic nuclei (AGNs)
are driven by variations in the ionizing continuum flux --which are usually
reflected in the observable UV-optical continuum. The "Reverberation mapping"
technique measures the delay between line and continuum variations to determine
the size of the line emitting region, this is the basis for measurements of the
central black hole mass in AGNs. The Space Telescope and Optical Reverberation
Mapping Project (AGN STORM) on NGC 5548 in 2014 is the most intensive
multi-wavelength AGN monitoring campaign ever. For most of the campaign, the
emission-line variations followed changes in the continuum with a time lag, as
expected. However, the lines varied independently of the observed UV-optical
continuum during a 60 -- 70 day "holiday." To understand this remarkable
phenomenon, we study the intrinsic absorption lines present in NGC 5548. We
identify a novel cycle that reproduces the absorption line variability and thus
identify the physics that allows the holiday to occur. In our model, variations
in this obscurer's line-of-sight covering factor modify the soft X-ray
continuum. This leads to changes in the ionization of helium gas in the
broad-line region. Ionizing radiation produced by recombining helium then
affects the ionization of other species as observed during the AGN STORM
holiday. It is likely that any other model which selectively changes the soft
X-ray part of the continuum during the holiday can also explain the anomalous
emission line behavior observed.
