QUASI-PERIODIC OSCILLATIONS IN LUMINOUS ACCRETION DISKS

We examine the time-dependent evolution of radial pulsational instabilities in luminous accretion discs around neutron stars, using a one-dimensional hydrodynamic code. In geometrically thin and optically thick accretion discs, radial oscillations are overstable to axisymmetric perturbations, as was first found by Kato by linear analysis. It is due to a mechanism similar to the epsilon mechanism in stellar pulsation. We find that this instability induces large amplitude oscillations (more than 100 per cent) in accretion rates and small modulations (a few per cent) in temperature and surface density in the inner part of the disc, both with local Keplerian frequencies. In particular, oscillation amplitudes are greatly increased by the presence of thermal relaxation oscillations, when the disc is radiation-pressure dominated and the viscous stress is prescribed as proportional to the total pressure. As a result, the disc luminosity also fluctuates by a few to 10 per cent with frequencies 3-60 Hz. Observational implications of our model are discussed in the context of quasi-periodic oscillations observed in low-mass X-ray binaries.