Optical Monitoring of the Broad-Line Radio Galaxy 3C390.3

We have undertaken a new ground-based monitoring campaign on the BLRG 3C390.3 to improve the measurement of the size of the BLR and to estimate the black hole mass. Optical spectra and g-band images were observed in 2005 using the 2.4m telescope at MDM Observatory. Integrated emission-line flux variations were measured for Ha, Hb, Hg, and for HeII4686, as well as g-band fluxes and the optical AGN continuum at 5100A. The g-band fluxes and the AGN continuum vary simultaneously within the uncertainties, tau=(0.2+-1.1)days. We find that the emission-line variations are delayed with respect to the variable g-band continuum by tau(Ha)=56.3(+2.4-6.6)days, tau(Hb)=44.3(+3.0_-3.3)days, tau(Hg)=58.1(+4.3-6.1)days, and tau(HeII4686)=22.3(+6.5-3.8)days. The blue and red peak 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 Ha and Hb and the separation of the blue and red peak in the broad double-peaked profiles in their rms spectra, we determine Mbh_vir=1.77(+0.29-0.31)x10^8Msol and using sigma_line of the rms spectra Mbh_vir=2.60(+0.23-0.31)x10^8Msol for the central black hole of 3C390.3, respectively. Using the inclination angle of the line emitting region the mass of the black hole amounts to Mbh=0.86(+0.19-0.18)x10^9 Msol (peak-separation) and Mbh=1.26(+0.21-0.16)x10^9 Msol (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 3C390.3, and with the AGN radius-luminosity relation. Thus, 3C390.3 as a radio-loud AGN with a low Eddington ratio, Ledd/Lbol=0.02, follows the same AGN radius-luminosity relation as radio-quiet AGN.