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

International audienceWe 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 alpha, H beta, H gamma, and for the helium line He II lambda 4686, as well as g-band fluxes and the optical active galactic nucleus (AGN) continuum at lambda = 5100 angstrom. The g-band fluxes and the optical AGN continuum vary simultaneously within the uncertainties, tau(cent) = (0.2 +/- 1.1) days. We find that the emission-line variations are delayed with respect to the variable g-band continuum by tau(H alpha) = 56.3(-6.6)(+2.4) days, tau(H beta) = 44.3(-3.3)(+3.0) days, tau(H gamma) = 58.1(-6.1)(+4.3) days, and tau(He II 4686) = 22.3(-3.8)(+6.5) 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 alpha and H beta and the separation of the blue and red peaks in the broad double-peaked profiles in their rms spectra, we determine M-bh(vir) = 1.77(-0.31)(+0.29) x 10(8) M-circle dot and using sigma(line) of the rms spectra M-bh(vir) = 2.60(-0.31)(+0.23) x 10(8) M-circle dot 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(-0.18)(+0.19) x 10(9) M-circle dot (peak separation) and M-bh = 1.26(-0.16)(+0.21) x 10(9) M-circle dot (sigma(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