Low frequency oscillations from accretion disks in x‐ray binaries

The global nonlinear time dependent evolution of thermal and viscous instabilities in Keplerian accretion disks in x‐ray binary sources has been investigated to understand the low frequency (∼0.04 Hz) quasi‐periodic oscillations (QPOs) seen recently in some black hole candidates (Cyg X–1 and GRO J0422+32) and the Rapid Burster MXB 1730–335. Within the framework of the α‐viscosity model, we assume the viscous stress scales with gas pressure only but that the α parameter is formulated as a function of the local scale height, h. Specifically, α=min[α0(h/r)n, αmax], where r is the distance from the compact object, and n, α0, and αmax are constants. It is found that nonsteady behavior may arise for sufficiently large n (∼1.2). We show that, the low frequency QPOs may be explicable in terms of a thermal‐viscous instability in accretion disks, which results in periodic or quasi‐periodic oscillations in the mass accretion rate. These oscillations are globally coherent in the unstable regions of the disk and the d...