The Quiescence of Dwarf Novae and X-ray Transients

Our understanding of the structure and properties of accretion disks in quiescent Dwarf Novae and X-ray Transients is very limited. Observations during quiescence challenge some of the standard Disk Instability Model (DIM) predictions. Our ignorance of the nature of viscosity may be the main source of uncertainties in quiescent disk models. A “layered accretion” alternative to the DIM, in which accretion proceeds via X-ray ionized surface layers, illustrates the magnitude of these uncertainties. A detection of molecular hydrogen in quiescent disks may provide direct constraints on the nature of viscosity.