OSCILLATORY MODES IN NEAR-CRITICAL ACCRETION FLOWS

Bright accretion-powered systems exhibit many varieties of time variability, from eclipses and ''dipping'' to flaring and quasi-periodic oscillations (QPOs). The former are extrinsic in the sense that they are caused by the obscuration of a steady source of photons, while the latter may reflect variations in the source itself intrinsic to rapid accretion. In this paper we examine intrinsic time variability by studying the time-dependent response of radiation-dominated spherical accretion flows to radial and nonradial perturbations. Such flows may serve as useful first approximations to the actual accretion flows in low-mass X-ray binaries and other systems. Feedback between the production of radiation and the flow of gas to the radiation-producing regions promotes the existence of global modes, each of which is associated with a characteristic oscillation about steady flow. The feedback mechanism responsible for the modes is very general implying that radiation hydrodynamic oscillations may be a universal feature of rapidly accreting systems. These modes are accompanied by characteristic variations in the radiation output of the accretion flow. Because heavily damped modes are unlikely to be observable, we examine how the frequencies and the damping rates of global modes depend on parameters such as the mass accretion rate. Under conditions thought to be typical of low-mass X-ray binaries, the radial modes are weakly damped and some nonradial modes are even linearly unstable. Our results for the frequencies and structure of radial modes confirm the earlier findings of Fortner, Lamb, and Miller, which were based on one-dimensional time-dependent simulations. Radial mode frequencies depend on both the spatial extent of the accretion flow and the luminosity of the accreting system: larger or higher luminosity flows support slower oscillations than smaller or less luminous ones. When nonradial modes are active, the flow develops regions through which material preferentially accretes, and regions through which radiation preferentially escapes. Our linear stability analysis suggests that these modes dominate the accretion flow behavior when the system luminosity approaches the Eddington limit, the critical point beyond which steady spherical accretion becomes impossible. Nonradial mode frequencies behave very differently from those of the radial modes, as they appear to depend only weakly on the system luminosity. A connection may exist between nonradial radiation hydrodynamic modes and the normal branch/flaring branch quasi-periodic intensity oscillations found in low-mass X-ray binaries, which also display striking frequency stability when the source is on the normal spectral branch. Moreover, at luminosities very close to the Eddington limit, higher frequency nonradial modes become less damped than either the radial modes or the lower frequency nonradial modes. This may explain the sudden increase in QPO frequency observed in Sco X-l as it moves from its normal to its flaring branch (Dieters 1994): the increase reflects a shift in dominance from a lower frequency to a higher frequency mode.