Reverberation Mapping of the Broad-line Region in NGC 5548: Evidence for
  Radiation Pressure?
  NGC 5548 is the best-observed reverberation-mapped active galactic nucleus
with long-term, intensive monitoring. Here we report results from a new
observational campaign between January and July, 2015. We measure the centroid
time lag of the broad H$\beta$ emission line with respect to the 5100 \AA
continuum and obtain $\tau_{\rm cent} = 7.20^{+1.33}_{-0.35}$ days in the rest
frame. This yields a black hole mass of $M_{\bullet}=8.71^{+3.21}_{-2.61} $x$
10^{7}M_{\odot}$ using a broad H$\beta$ line dispersion of $3124\pm302$ km
s$^{-1}$ and a virial factor of $f_{_{\rm BLR}}=6.3\pm1.5$ for the broad-line
region (BLR), consistent with the mass measurements from previous H$\beta$
campaigns. The high-quality data allow us to construct a velocity-binned delay
map for the broad H$\beta$ line, which shows a symmetric response pattern
around the line center, a plausible kinematic signature of virialized motion of
the BLR. Combining all the available measurements of H$\beta$ time lags and the
associated mean 5100 {\AA} luminosities over 18 campaigns between 1989 and
2015, we find that the H$\beta$ BLR size varies with the mean optical
luminosity, but, interestingly, with a possible delay of $2.35_{-1.25}^{+3.47}$
yrs. This delay coincides with the typical BLR dynamical timescale of NGC 5548,
indicating that the BLR undergoes dynamical changes, possibly driven by
radiation pressure.
