A High-Quality velocity-delay map of the broad-line region in NGC 5548
  NGC 5548 has been well spectroscopically monitored for reverberation mapping
of the central kinematics by 19 campaigns. Using the maximum entropy method in
this Letter, we build up a high-quality velocity-delay map of the H$\beta$
emission line in the light curves of the continuum and the line variations
observed between 2015-2016. The map shows the response strength and lags of the
velocity fields of the H$\beta$ emitting regions. The velocity-delay structure
of the map is generally symmetric, with strong red and blue wings at time lag
$\tau \leq 15$ days, a narrower velocity distribution at $\tau \geq 15$ days,
and a deficit of response in the core. This is suggestive of a disk geometry of
the broad-line region (BLR). The relatively weaker H$\beta$ response at the
longer lags in the red side indicates anisotropic emission from the outer part
of the BLR. We also recover the velocity-delay maps of NGC 5548 from the
historical data of 13 years to investigate the long-term variability of its
BLR. In general, the BLR of NGC 5548 was switching between the inflow and
virialized phases in the past years. The resultant maps of seven years reveal
inflow signatures and show decreasing lags, indicating that the changes in the
BLR size are related to the infalling BLR gas. The other four maps show
potential disk signatures which are similar to our map.
