The thermal-viscous disk instability model in the AGN context

TL;DR

This study systematically applies the disk instability model to AGN accretion disks, revealing that the thermal-viscous instability does not naturally produce large outbursts or long-term variability in AGNs due to the dynamics of heating and cooling fronts.

Contribution

The paper demonstrates that the thermal-viscous disk instability model, when applied to AGN disks, cannot account for observed long-term variability, challenging previous assumptions about AGN outbursts.

Findings

Heating and cooling fronts are too narrow to be resolved numerically.

Front propagation occurs on short timescales, causing only small accretion rate variations.

Outbursts are not naturally produced even with high irradiation efficiencies.

Abstract

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.