Spectral variability of the 3C 390.3 nucleus for more than 20 years -- II. Variability of the broad emission-line profiles and He II $\lambda4686$A emission-line fluxes

Abstract

Results of the analysis of the variability of the H$\beta$ and H$\alpha$ broad emission-line profiles and the He II $\lambda4686$A 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 $26\pm 8$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 scalefactor $c(t)$: $F_{line}\propto c(t)\,F_{cont}^a$, 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$\beta$ and H$\alpha$ lines, respectively. It means that the variability amplitude is the largest in the He II, less in H$\beta$, and more less in H$\alpha$. 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) $R_{max}\approx 3$ that is much greater than the variability amplitudes of both the narrow Balmer lines and the narrow [O III] $\lambda$5007A line.