Stabilization of radiation pressure dominated accretion disks through viscous fluctuations

Abstract

Aims. The standard thin accretion disk model has successfully explained the soft X-ray spectra of Galactic black hole systems and perhaps the UV emission of active galactic nuclei. However, radiation pressure dominated disks are known to be viscously unstable and should produce high-amplitude oscillations that are typically not observed. Instead, these sources exhibit a stochastic variability that may naturally arise through viscous fluctuations in a turbulent disk. Here we investigate whether these aperiodic viscous fluctuations can stabilize the inner radiation pressure dominated disks and hence maybe the answer to a forty-year old problem in accretion disk theory. Methods. The structure and evolution of a time-dependent accretion disk around a black hole is solved numerically. We incorporated fluctuations in the disk by considering stochastic variations in the viscous parameter α on the local viscous time-scale. We studied both locally stable disks where the viscous stress scales with the gas pressure and locally unstable disks where the stress scales as the total pressure. We considered steady state disk parameters pertaining to both stellar and supermassive black holes. Results. For locally stable disks, the power spectra of the luminosity variations are found to inversely scale with frequency, i.e., P(f) ∞ 1/f as expected from analytical studies of linear viscous fluctuations. For unstable disks, where the viscous stress scales with the total pressure, the standard oscillatory solutions are seen when the viscous fluctuation amplitude is low. Increasing the fluctuation amplitude decreases the amplitude of these oscillations until for a sufficiently high (near unity) fluctuation amplitude, the oscillatory behavior disappears and the luminosity variation of the disk becomes stochastic. This study may explain why many accreting black holes, although they show sufficiently high accretion rates to develop the radiation pressure instability, do not exhibit high-amplitude regular outbursts, or, like the micro-quasar GRS 1915+105, produce diminished versions of these outburst only in some specific spectral states. © 2012 ESO.