Spectral variability of the 3C 390.3 nucleus for more than 20 years – II. Variability of the broad emission-line profiles and He ii λ4686 Å emission-line fluxes

ABSTRACT 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): $F_{\rm line}\propto c(t)\, F_{\rm cont}^a$, 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) Rmax ≈ 3, which is much greater than the variability amplitudes of both the narrow Balmer lines and the narrow [O iii] λ5007 Å line.