Variability of the Broad Balmer Emission Lines in 3C 390.3 from 1992 to 2000

Abstract

We describe the results of an optical spectral monitoring campaign on the Seyfert galaxy 3C 390.3 carried out with a CCD spectrograph at the 2.6 m Shajn Telescope of the Crimean Astrophysical Observatory in 1992-2000. Light curves for the Hα and Hβ emission lines and for the continuum in both the Hα and Hβ spectral regions are given. The lag between the continuum and Hβ emission-line variations (specifically the cross-correlation centroid τcent) is found to be τcent = 89 days, more than twice the lag found in an earlier investigation by Dietrich et al. We attribute this discrepancy to the difference between continuum autocorrelation functions for these two time series, which have very different durations. We show that a single emission-line transfer function is able to reproduce both cross-correlation results. The recovered transfer function for the Hβ line has a peak at a lag of 20 days, a tail that extends up to 300 days, and little or no correlation near zero lag. We find that the Hα line, which varies with a markedly lower amplitude than Hβ, has a lag of τcent = 162 days. We find no significant difference in lag between the blue and red wings of either Hα or Hβ, effectively ruling out kinematics dominated by simple radial motion. However, emission-line profile variations were clearly detected over the duration of the monitoring campaign. The mean and rms profiles computed for three selected time intervals are significantly different and show no correlation between profile width and continuum flux. Over the duration of the campaign, the red central part of the line profiles strongly decreased with respect to the total line flux. Moreover, the red bump nearly completely disappeared, while the blue bump has become more prominent. We find that some discrete profile features appear to propagate across the line profile, and we see such features moving in both directions, blue to red and vice versa. Variations in two profile segments at line-of-sight velocity ±(7500-9500) km s-1 are found to be poorly correlated with variations in the continuum or in other profile segments. Our conjecture is that the observed evolution of the Balmer line profiles in 3C 390.3 may be the result of the rotational redistribution of matter in a Keplerian disk and changes in the relative strength of an additional line component that dominates in the red central part of the profile. We conclude that the evolution of the broad-line profiles in 3C 390.3 in the context of the current models of the broad-line region remains unclear.