The thermal-viscous disk instability model in the AGN context
  Accretion disks in AGN should be subject to the same type of instability as
in cataclysmic variables (CVs) or in low-mass X-ray binaries (LMXBs), which
leads to dwarf nova and soft X-ray transient outbursts. It has been suggested
that this thermal/viscous instability can account for the long term variability
of AGNs. We test this assertion by presenting a systematic study of the
application of the disk instability model (DIM) to AGNs. We are using the
adaptative grid numerical code we have developed in the context of CVs,
enabling us to fully resolve the radial structure of the disk. We show that,
because in AGN disks the Mach numbers are very large, the heating and cooling
fronts are so narrow that they cannot be resolved by the numerical codes that
have been used until now. In addition, these fronts propagate on time scales
much shorter than the viscous time. As a result, a sequence of heating and
cooling fronts propagate back and forth in the disk, leading only to small
variations of the accretion rate onto the black hole, with short quiescent
states occurring for very low mass transfer rates only. Truncation of the inner
part of the disk by e.g. an ADAF does not alter this result, but enables longer
quiescent states. Finally we discuss the effects of irradiation by the central
X-ray source, and show that, even for extremely high irradiation efficiencies,
outbursts are not a natural outcome of the model.
