Thermal instability in accretion flows onto degenerate stars

Abstract

The discovery of a new kind of thermal instability in accretion flows onto stellar atmospheres is reported. The instability results in periodic variations in the height of the standoff shock which forms above the stellar surface. The hard X-ray temperature and luminosity vary with this period, as does any other radiation produced in the hot gas between the shock and the surface. Conditions for the appearance of this instability are most favorable in accreting white dwarf stars, such as cataclysmic variables and AM Her stars. It is suggested that a search for such variability be carried out in AM Her stars in both the hard X-ray and UV-optical bands. The resulting period, which is of order 1-100 s for AM Her stars, permits the determination of the accretion rate per unit area at the stellar surface.