Radiation pressure instability as a variability mechanism in the microquasar GRS 1915+105

The physical mechanism responsible for the high viscosity in accretion disks is still under debate. The parameterization of the viscous stress as alphaP proved to be a successful representation of this mechanism in the outer parts of the disk, explaining the dwarf novae and X-ray novae outbursts as being due to ionization instability. We show that this parameterization can also be adopted in the innermost part of the disk where the adoption of the or-viscosity law implies the presence of the instability in the radiation pressure-dominated region. We study the time evolution of such disks. We show that the time-dependent behavior of GRS 1915+105 can be well reproduced if the or-viscosity disk model is calculated accurately (with proper numerical coefficients in vertically averaged equations and with advection included) and if the model is supplemented with (1) a moderate corona dissipating 50% of energy and (2) a jet carrying a luminosity-dependent fraction of energy. These necessary modifications in the form of the presence of a, corona and a jet are well justified observationally. The model predicts outbursts at a luminosity larger than 0.16(M) over dot(Edd), as required, and correct outburst timescales and amplitudes, including the effect of an increasing outburst timescale with mean luminosity. This result strongly suggests also that the or-viscosity law is a good description of the actual mechanism responsible for angular momentum transfer in the innermost, radiation pressure-dominated part of the disk around a black hole.