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K. T. Lewis (Penn State/NASA GSFC)
A few percent of Active Galactic Nuclei (AGN) exhibit broad, double-peaked Balmer emission lines. There is considerable evidence to support the hypothesis that these lines originate in the outer portions of the AGN accretion disk. The profiles vary on timescales of years (much longer than the light crossing time) as a result of physical changes in the accretion disk. Consequently, the profile variability provides a way to test models for physical phenomena in the outer accretion disks in AGN. To this end, the H\alpha emission lines of 20 double-peaked emitters have been observed 1--3 times per year over the past decade.
The most obvious variability occurs in the relative strengths of the red and blue peaks and is caused by one or more distinct lumps of emission whose projected velocities vary slowly with time. Some frequently monitored objects also show variability on timescales of a few months, due to changes in the strength of a lump relative to the underlying profile. In order to interpret the variability time scales, I determined the mass of the black hole in three of these objects. The black hole masses were ~4\times107 M\sun, giving a dynamical timescale of a 2--3 months, suggesting that gross variability in the profile occurs on timescales that are an order of magnitude longer than the dynamical time, but the more rapid variability could be due to a dynamical effect.
I also compare the observations with the variability predicted by two simple models---an accretion disk with a spiral arm and a disk with eccentric orbits---which have been proposed to explain the variability in these objects. I find that neither of these models can adequately explain the full range of behavior observed in these objects, however modifications to the spiral arm model may produce better agreement with the data.
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Bulletin of the American Astronomical Society, 37 #4
© 2005. The American Astronomical Soceity.