Spectral optical monitoring of 3C 390.3 in 1995–2007

Context. We present the results of the long-term (1995–2007) spectral monitoring of the broad-line radio galaxy 3C 390.3, a well known AGN with double-peaked broad emission lines, usually assumed to be emitted from an accretion disk. Aims. To explore dimensions and structure of the BLR, we analyze the light curves of the broad Hα and Hβ line fluxes and the continuum flux. To detect variations in the BLR, we analyze the Hα and Hβ line profiles, as well as the change in the line profiles during the monitoring period. Methods. We attempt first to find a periodicity in the continuum and Hβ light curves, finding that there is a high probability of measuring quasi-periodical oscillations. Using the line shapes and their characteristics (such as e.g., peak separation and their intensity ratio, or FWHM) of broad Hβ and Hα lines, we discuss the structure of the BLR. We also cross-correlate the continuum flux with Hβ and Hα lines to determine the dimensions of the BLR. Results. During the monitoring period, we found that the broad emission component of the Hα and Hβ lines, and the continuum flux varied by a factor of ≈4-5. We also detected different structure in the line profiles of Hα and Hβ. An additional central component appears to be present and superimposed on the disk emission. In the period of high activity (after 2002), Hβ became broader than Hα and red wing of Hβ was higher than that of Hα. We detected time lags of ~95 days between the continuum and Hβ flux, and about 120 days between the continuum and Hα flux. Conclusions. Variations in the line profiles, as well as correlation between the line and continuum flux during the monitoring period, are consistent with a disk origin of the broad lines and the possible contribution of some additional region and/or some kind of perturbation in the disk