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 ) \times 10 ^ { 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 \pm 4000 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 _ { 0 } = 3.5 \pm 0.2 days at \lambda _ { 0 } = 5477 \text { \AA } .
It implies a disc size a factor of 1.6 times larger than prediction from the standard disc model with L / L _ { Edd } = 0.4 .
This is consistent with previous studies in which larger than expected disc sizes were measured .