2020MNRAS.495..971S

Results of the analysis of the variability of the H β and H α broad emission-line profiles and the He II λ4686 Å emission-line fluxes in the 3C 390.3 nucleus during 1992-2014 are present. The observed velocity-dependent lag for the Balmer lines is similar to that expected from the Keplerian disc configuration, although there are some differences. Probably, a radial infall motion can be present in the broad-line region of 3C 390.3 in addition to the Keplerian rotation. The lag of the broad He II line is 26 ± 8 d, significantly less than that of the Balmer lines, so the He II emission region is much smaller in size. In terms of the power-law relationship between line and optical continuum fluxes with slowly varying scale factor c(t): F_ line_∝c(t) F_ cont_^a, the power a is 1.03 for the broad He II line, while according to Paper I, the power is equal to 0.77 and 0.54 for the broad H β and H α lines, respectively. It means that the variability amplitude is the largest in the He II, less in H β, and more less in H α. However, the Balmer lines contain a long-term trend that is not seen in the helium line. The narrow He II line is variable with the amplitude (max-to-min ratio) R_max ≃ 3, which is much greater than the variability amplitudes of both the narrow Balmer lines and the narrow [O III] λ5007 Å line.