Space Telescope and Optical Reverberation Mapping Project. XII.
  Broad-Line Region Modeling of NGC 5548
  We present geometric and dynamical modeling of the broad line region for the
multi-wavelength reverberation mapping campaign focused on NGC 5548 in 2014.
The dataset includes photometric and spectroscopic monitoring in the optical
and ultraviolet, covering the H$\beta$, C IV, and Ly$\alpha$ broad emission
lines. We find an extended disk-like H$\beta$ BLR with a mixture of
near-circular and outflowing gas trajectories, while the C IV and Ly$\alpha$
BLRs are much less extended and resemble shell-like structures. There is clear
radial structure in the BLR, with C IV and Ly$\alpha$ emission arising at
smaller radii than the H$\beta$ emission. Using the three lines, we make three
independent black hole mass measurements, all of which are consistent.
Combining these results gives a joint inference of $\log_{10}(M_{\rm
BH}/M_\odot) = 7.64^{+0.21}_{-0.18}$. We examine the effect of using the $V$
band instead of the UV continuum light curve on the results and find a size
difference that is consistent with the measured UV-optical time lag, but the
other structural and kinematic parameters remain unchanged, suggesting that the
$V$ band is a suitable proxy for the ionizing continuum when exploring the BLR
structure and kinematics. Finally, we compare the H$\beta$ results to similar
models of data obtained in 2008 when the AGN was at a lower luminosity state.
We find that the size of the emitting region increased during this time period,
but the geometry and black hole mass remain unchanged, which confirms that the
BLR kinematics suitably gauge the gravitational field of the central black
hole.
