Steps toward determination of the size and structure of the broad-line region in active galactic nuclei. XII. Ground-based monitoring of 3C 390.3

Results of a ground-based optical monitoring campaign on 3C 390.3 in 1994-1995 are presented. The broadband fluxes (B, V, R, and I), the spectrophotometric optical continuum flux F-lambda(5177 Angstrom), and the integrated emission-line fluxes of H alpha, H beta, H gamma, He I lambda 5876, and He II lambda 4686 all show a nearly monotonic increase with episodes of milder short-term variations superposed. The amplitude of the continuum variations increases with decreasing wavelength (4400-9000 Angstrom). The optical continuum variations follow the variations in the ultraviolet and X-ray with time delays, measured from the centroids of the crosscorrelation functions, typically around 5 days, but with uncertainties also typically around 5 days; zero time delay between the high-energy and low-energy continuum variations cannot be ruled out. The strong optical emission lines H alpha, H beta, H gamma, and He I lambda 5876 respond to the high-energy continuum variations with time delays typically about 20 days, with uncertainties of about 8 days. There is some evidence that He II lambda 4686 responds somewhat more rapidly, with a time delay of around 10 days, but again, the uncertainties are quite large (similar to 8 days). The mean and rms spectra of the H alpha and H beta line profiles provide indications for the existence of at least three distinct components located at +/-4000 and 0 km s(-1) relative to the line peak. The emission-line profile variations are largest near line center.