Long-term profile variability of double-peaked emission lines in active galactic nuclei

We present up to two decades of spectroscopic monitoring of the double-peaked broad H alpha emission lines of seven broad-line radio galaxies. These extremely broad, double-peaked lines are successfully modeled by emission from gas in the outer accretion disk, and their profiles vary on timescales of months to years. This variability is a valuable probe for understanding the structure and dynamics of the accretion disk. We characterize the long-term broad-line profile variability in detail and compare the evolution of the line profiles with predictions from a few simple physical models for dynamical processes in the accretion disk. We find no evidence for persistent, periodic variability that would be indicative of a precessing elliptical disk, or a circular disk with a long-lived, single-armed spiral or warp. We do find transient, periodic variability on the orbital timescale, possibly resulting from shocks induced by tidal perturbations, and variability in the wings of the profile, which suggests changes in the emissivity of the inner accretion flow. Dramatic but localized profile changes are observed during flares and high states of broad-line flux. In 3C 332, these changes can be explained by a slow, smooth, secular change in disk illumination. In Arp 102B and 3C 390.3, however, a simple disk model cannot explain the asymmetric profile changes, suggesting a need for more sophisticated models. We also observe sharp, transient features that appear in the blue peak of the objects, which require a well-organized velocity field.