Long-Term Profile Variability in Active Galactic Nuclei with
  Double-Peaked Balmer Emission Lines
  An increasing number of Active Galactic Nuclei (AGNs) exhibit broad,
double-peaked Balmer emission lines,which represent some of the best evidence
for the existence of relatively large-scale accretion disks in AGNs. A set of
20 double-peaked emitters have been monitored for nearly a decade in order to
observe long-term variations in the profiles of the double-peaked Balmer lines.
Variations generally occur on timescales of years, and are attributed to
physical changes in the accretion disk. Here we characterize the variability of
a subset of seven double-peaked emitters in a model independent way. We find
that variability is caused primarily by the presence of one or more discrete
"lumps" of excess emission; over a timescale of a year (and sometimes less)
these lumps change in amplitude and shape, but the projected velocity of these
lumps changes over much longer timescales (several years). We also find that
all of the objects exhibit red peaks that are stronger than the blue peak at
some epochs and/or blueshifts in the overall profile, contrary to the
expectations for a simple, circular accretion disk model, thus emphasizing the
need for asymmetries in the accretion disk. Comparisons with two simple models,
an elliptical accretion disk and a circular disk with a spiral arm, are unable
to reproduce all aspects of the observed variability, although both account for
some of the observed behaviors. Three of the seven objects have robust
estimates of the black hole masses. For these objects the observed variability
timescale is consistent with the expected precession timescale for a spiral
arm, but incompatible with that of an elliptical accretion disk. We suggest
that with the simple modification of allowing the spiral arm to be fragmented,
many of the observed variability patterns could be reproduced.
