Robotic reverberation mapping of the broad-line radio galaxy 3C 120
  We carried out photometric and spectroscopic observations of the well-studied
broad-line radio galaxy 3C 120 with the Las Cumbres Observatory (LCO) global
robotic telescope network from 2016 December to 2018 April as part of the LCO
AGN Key Project on Reverberation Mapping of Accretion Flows. Here, we present
both spectroscopic and photometric reverberation mapping results. We used the
interpolated cross-correlation function (ICCF) to perform multiple-line lag
measurements in 3C 120. We find the H$\gamma$, He II $\lambda 4686$, H$\beta$
and He I $\lambda 5876$ lags of $\tau_{\text{cen}} = 18.8_{-1.0}^{+1.3}$,
$2.7_{-0.8}^{+0.7}$, $21.2_{-1.0}^{+1.6}$, and $16.9_{-1.1}^{+0.9}$ days
respectively, relative to the V-band continuum. Using the measured lag and rms
velocity width of the H$\beta$ emission line, we determine the mass of the
black hole for 3C 120 to be
$M=\left(6.3^{+0.5}_{-0.3}\right)\times10^7\,(f/5.5)$ M$_\odot$. Our black hole
mass measurement is consistent with similar previous studies on 3C 120, but
with small uncertainties. In addition, velocity-resolved lags in 3C 120 show a
symmetric pattern across the H$\beta$ line, 25 days at line centre decreasing
to 17 days in the line wings at $\pm4000$ km s$^{-1}$. We also investigate the
inter-band continuum lags in 3C 120 and find that they are generally consistent
with $\tau\propto\lambda^{4/3}$ as predicted from a geometrically-thin,
optically-thick accretion disc. From the continuum lags, we measure the best
fit value $\tau_{\rm 0} = 3.5\pm 0.2$ days at $\lambda_{\rm 0} = 5477$A. It
implies a disc size a factor of $1.6$ times larger than prediction from the
standard disc model with $L/L_{\rm Edd} = 0.4$. This is consistent with
previous studies in which larger than expected disc sizes were measured.
