Spectral optical monitoring of 3C 390.3 in 1995-2007 I. Light curves and flux variation in the continuum and broad lines

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 H alpha and H beta line fluxes and the continuum flux. To detect variations in the BLR, we analyze the H alpha and H beta 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 H beta 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 H beta and H alpha lines, we discuss the structure of the BLR. We also cross-correlate the continuum flux with H beta and Ha lines to determine the dimensions of the BLR. Results. During the monitoring period, we found that the broad emission component of the H alpha and H beta lines, and the continuum flux varied by a factor of approximate to 4-5. We also detected different structure in the line profiles of H alpha and H beta. An additional central component appears to be present and superimposed on the disk emission. In the period of high activity (after 2002), H beta became broader than H alpha and red wing of H beta was higher than that of Ha. We detected time lags of similar to 95 days between the continuum and H beta flux, and about 120 days between the continuum and H alpha 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.