2012ApJ...757...53D

We have undertaken a new ground-based monitoring campaign on the broad-line radio galaxy 3C 390.3 to improve the measurement of the size of the broad emission-line region and to estimate the black hole mass. Optical spectra and g-band images were observed in late 2005 for three months using the 2.4 m telescope at MDM Observatory. Integrated emission-line flux variations were measured for the hydrogen Balmer lines Hα, Hβ, Hγ, and for the helium line He IIλ4686, as well as g-band fluxes and the optical active galactic nucleus (AGN) continuum at λ = 5100 Å. The g-band fluxes and the optical AGN continuum vary simultaneously within the uncertainties, τ_cent= (0.2±1.1) days. We find that the emission-line variations are delayed with respect to the variable g-band continuum by τ(Hα) = 56.3^+2.4_- 6.6_days, τ(Hβ) = 44.3^+3.0_- 3.3_days, τ(Hγ) = 58.1^+4.3_- 6.1_days, and τ(He II 4686) = 22.3^+6.5_- 3.8_days. The blue and red peaks in the double-peaked line profiles, as well as the blue and red outer profile wings, vary simultaneously within ±3 days. This provides strong support for gravitationally bound orbital motion of the dominant part of the line-emitting gas. Combining the time delay of the strong Balmer emission lines of Hα and Hβ and the separation of the blue and red peaks in the broad double-peaked profiles in their rms spectra, we determine M ^vir_bh= 1.77^+0.29_- 0.31_x10⁸ M_☉and using σ_line of the rms spectra M ^vir_bh= 2.60^+0.23_- 0.31_x10⁸ M_☉ for the central black hole of 3C 390.3, respectively. Using the inclination angle of the line-emitting region which is measured from superluminal motion detected in the radio range, accretion disk models to fit the optical double-peaked emission-line profiles, and X-ray observations, the mass of the black hole amounts to M_bh= 0.86^+0.19_- 0.18_x10⁹ M_☉(peak separation) and M_bh= 1.26^+0.21_- 0.16_x10⁹ M_☉(σ_line), respectively. This result is consistent with the black hole masses indicated by simple accretion disk models to describe the observed double-peaked profiles, derived from the stellar dynamics of 3C 390.3, and with the AGN radius-luminosity relation. Thus, 3C 390.3 as a radio-loud AGN with a low Eddington ratio, L_edd/L_bol= 0.02, follows the same AGN radius-luminosity relation as radio-quiet AGNs.