OPTICAL MONITORING OF THE BROAD-LINE RADIO GALAXY 3C 390.3*

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 H?, H?, H?, and for the helium line He II?4686, as well as g-band fluxes and the optical active galactic nucleus (AGN) continuum at ? =?5100 ?. The g-band fluxes and the optical AGN continuum vary simultaneously within the uncertainties, ?cent = (0.2 ? 1.1)?days. We find that the emission-line variations are delayed with respect to the variable g-band continuum by ?(H?) = 56.3+2.4 ? 6.6?days, ?(H?) = 44.3+3.0 ? 3.3?days, ?(H?) = 58.1+4.3 ? 6.1?days, and ?(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 H? and H? 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 ? and using ?line of the rms spectra M vir bh = 2.60+0.23 ? 0.31 ? 108 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 ? 109 M ? (peak separation) and M bh = 1.26+0.21 ? 0.16 ? 109 M ? (?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.