Space Telescope and Optical Reverberation Mapping Project. VII.
  Understanding the UV anomaly in NGC 5548 with X-Ray Spectroscopy
  During the Space Telescope and Optical Reverberation Mapping Project (STORM)
observations of NGC 5548, the continuum and emission-line variability became
de-correlated during the second half of the 6-month long observing campaign.
Here we present Swift and Chandra X-ray spectra of NGC 5548 obtained as a part
of the campaign. The Swift spectra show that excess flux (relative to a
power-law continuum) in the soft X-ray band appears before the start of the
anomalous emission-line behavior, peaks during the period of the anomaly, and
then declines. This is a model-independent result suggesting that the soft
excess is related to the anomaly. We divide the Swift data into on- and
off-anomaly spectra to characterize the soft excess via spectral fitting. The
cause of the spectral differences is likely due to a change in the intrinsic
spectrum rather than being due to variable obscuration or partial covering. The
Chandra spectra have lower signal-to-noise ratios, but are consistent with
Swift data. Our preferred model of the soft excess is emission from an
optically thick, warm Comptonizing corona, the effective optical depth of which
increases during the anomaly. This model simultaneously explains all the three
observations: the UV emission line flux decrease, the soft-excess increase, and
the emission line anomaly.
