Optical monitoring of the broad-line radio galaxy 3C390.3

We have undertaken a new ground-based monitoring campaign on the broad-line radio galaxy 3C390.3 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 2.4m 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_-6.6days, τ(Hβ) = 44.3^+3.0_-3.3days, τ(Hγ) = 58.1 ^+4.3_-6.1days, and τ(He II4686) = 22.3 ^+6.5_-3.8days. 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 × 10⁸ M_Ȯ and using σ_line of the rms spectra M ^vir _bh = 2.60^+0.23_-0.31 × 10⁸ M_Ȯ for the central black hole of 3C390.3, 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 × 10⁹ M_Ȯ (peak separation) and M_bh = 1.26^+0.21_-0.16 × 10⁹ M_Ȯ (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 3C390.3, and with the AGN radius-luminosity relation. Thus, 3C390.3 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.