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X-Ray Variability as a Probe of Advection-dominated Accretion in Low-Luminosity Active Galactic Nuclei

As a class, LINERs and low-luminosity active galactic nuclei (LLAGNs) tend to show little or no significant short-term variability (i.e., with timescales less than a day). This is a marked break from the trend of increased variability in Seyfert 1 galaxies with decreased luminosity. We propose that this difference is due to the lower accretion rate in LINERs and LLAGNs, which is probably causing the accretion flow to be advection-dominated. This results in a larger characteristic size for the X-ray-producing region than is the case in "normal" AGNs. Short-term variability may be caused by a localized instability or occultation events, but we note that such events would likely be accompanied by broadband spectral changes. Since the advection-dominated accretion flow (ADAF) is more compact in a Kerr metric, it is possible that the X-ray emission from ADAFs around rotating black holes would be more variable than X-ray emission from ADAFs in a Schwarzschild metric. Similar variability arguments also apply to other wavelengths, and accordingly multiwavelength monitoring of LLAGNs could serve to "map" the ADAF regions.