Long-Term Profile Variability in Active Galactic Nuclei with Double-Peaked Balmer Emission Lines

TL;DR

This study monitors long-term variability in double-peaked Balmer emission lines of AGNs, revealing that asymmetries and discrete emission lumps in accretion disks cause profile changes over years, challenging simple disk models.

Contribution

It provides a model-independent analysis of long-term profile variability in AGN double-peaked emitters, highlighting the role of disk asymmetries and proposing modifications to existing models.

Findings

Variability caused by discrete emission lumps changing over months to years.

Red peaks often stronger than blue, indicating asymmetries.

Spiral arm precession timescales match observed variability for some objects.

Abstract

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.