LONG-TERM PROFILE VARIABILITY IN ACTIVE GALACTIC NUCLEUS WITH DOUBLE-PEAKED BALMER EMISSION LINES

Abstract

An increasing number of Active Galactic Nuclei (AGNs) exhibit broad,\ndouble-peaked Balmer emission lines,which represent some of the best evidence\nfor the existence of relatively large-scale accretion disks in AGNs. A set of\n20 double-peaked emitters have been monitored for nearly a decade in order to\nobserve long-term variations in the profiles of the double-peaked Balmer lines.\nVariations generally occur on timescales of years, and are attributed to\nphysical changes in the accretion disk. Here we characterize the variability of\na subset of seven double-peaked emitters in a model independent way. We find\nthat variability is caused primarily by the presence of one or more discrete\n"lumps" of excess emission; over a timescale of a year (and sometimes less)\nthese lumps change in amplitude and shape, but the projected velocity of these\nlumps changes over much longer timescales (several years). We also find that\nall of the objects exhibit red peaks that are stronger than the blue peak at\nsome epochs and/or blueshifts in the overall profile, contrary to the\nexpectations for a simple, circular accretion disk model, thus emphasizing the\nneed for asymmetries in the accretion disk. Comparisons with two simple models,\nan elliptical accretion disk and a circular disk with a spiral arm, are unable\nto reproduce all aspects of the observed variability, although both account for\nsome of the observed behaviors. Three of the seven objects have robust\nestimates of the black hole masses. For these objects the observed variability\ntimescale is consistent with the expected precession timescale for a spiral\narm, but incompatible with that of an elliptical accretion disk. We suggest\nthat with the simple modification of allowing the spiral arm to be fragmented,\nmany of the observed variability patterns could be reproduced.\n