Low-frequency quasi-periodic oscillations in low-mass x-ray binaries and galactic black hole candidates

We consider the inner regions of accretion disks surrounding black holes and neutron stars and investigate the nonlinear time-dependent evolution of thermal-viscous instabilities. The viscous stress is assumed to be proportional to the gas pressure with the viscosity parameter formulated as α = min[α₀(h/r)ⁿ, α_max], where h is the local scale height, r is the distance from the central compact object, and n, α₀, and α_max are constants. It is found that the disk is unstable for α sufficiently sensitive to h (n ≳ 1.2). The instabilities are globally coherent in the entire unstable region of the disk, and, depeding on the viscosity parameters, the time variability of the mass accretion rates are manifested as periodic or quasi-periodic oscillations. We show that, the low-frequency (∼004 Hz) quasi-periodic oscillations (QPOs) discovered recently in some of the black hole candidates (Cyg X-1 and GRO J0422 + 32) and a low-mass X-ray binary (Rapid Burster MXB 1730-335) may be explicable by the thermal-viscous instabilities in accretion disks. The observations of QPOs place constraints on the viscosity parameters and suggest that (n, α₀) ∼ (1.6, 30) for the Rapid Burster with a 1.4 M_⊙ neutron star. In the case of black hole candidates, the dependence of α on h/r is less steep corresponding to n ∼ 1.2-1.3 for black holes less than 10 M_⊙.