Hlabathe, Michael S.Starkey, D.A.Horne, KeithRomero-Colmenero, ECrawford, Steven M.Valenti, StefanoWinkler, HartmutBarth, Aaron JOnken, ChristopherSand, David J.Treu, Tommaso2023-03-232023-03-230035-8711http://hdl.handle.net/1885/287319We 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 to perform multiple-line lag measurements in 3C 120. We find the H γ , He II λ4686, H β, and He I λ5876 lags of τcen = 18.8+1.3 −1.0, 2.7+0.7 −0.8, 21.2+1.6 −1.0, and 16.9+0.9 −1.1 d, respectively, relative to the V-band continuum. Using the measured lag and rms velocity width of the H β emission line, we determine the mass of the black hole for 3C 120 to be M = (6.3+0.5 −0.3) × 107 (f /5.5) M. 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 β line, 25 d at line centre decreasing to 17 d in the line wings at ±4000 km s−1. We also investigate the inter-band continuum lags in 3C 120 and find that they are generally consistent with τ ∝ λ4/3 as predicted from a geometrically thin, optically thick accretion disc. From the continuum lags, we measure the best-fitting value τ 0 = 3.5 ± 0.2 d at λ0 = 5477 Å. It implies a disc size a factor of 1.6 times larger than prediction from the standard disc model with L/LEdd = 0.4. This is consistent with previous studies in which larger than expected disc sizes were measured.MH and ERC acknowledge support from the South African National Research Foundation (NRF). KH acknowledges support from STFC grant ST/R000824/1. MH and SMC were supported in part by the Space Telescope Science Institute (STScI) Director’s Discretionary Research Fund (DDRF). Research by AJB was supported by NSF grants AST-1412693 and AST-1907290. Research by DJS is supported by NSF grants AST-1821967, 1821987, 1813708, 1813466, and 1908972. TT acknowledges support by the NSF through grants AST-1412315 and AST-1907208 and by the Packard Foundation through a Packard Fellowship.application/pdfen-AU© 2020 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Societygalaxies-Seyfertquasars-galaxiesnuclei-galaxiesindividual (3C 120)galaxiesemission linessupermassive black holes202010.1093/mnras/staa21712022-01-16