Radiation Pressure Instability as a Variability Mechanism in the Microquasar GRS 1915+105

Physical mechanism responsible for high viscosity in accretion disks is stillunder debate. Parameterization of the viscous stress as $\alpha P$ proved to bea successful representation of this mechanism in the outer parts of the disk,explaining the dwarf novae and X-ray novae outbursts as due to ionizationinstability. We show that this parameterization can be also adopted in theinnermost part of the disk where the adoption of the $\alpha$-viscosity lawimplies the presence of the instability in the radiation pressure dominatedregion. We study the time evolution of such disks. We show that thetime-dependent behavior of GRS 1915+105 can be well reproduced if$\alpha$-viscosity disk model is calculated accurately (with proper numericalcoefficients in vertically averaged equations and with advection included), andif the model is supplemented with (i) moderate corona dissipating 50% of energy(ii) jet carrying luminosity-dependent fraction of energy. These necessarymodifications in the form of the presence of a corona and a jet are welljustified observationally. The model predicts outbursts at luminosity largerthan 0.16$\dot M_{Edd}$, as required, correct outburst timescales andamplitudes, including the effect of increasing outburst timescale with meanluminosity. This result strongly suggests that the $\alpha$-viscosity law is agood description of the actual mechanism responsible for angular momentumtransfer also in the innermost, radiation pressure dominated part of the diskaround a black hole.Comment: 6 pages, 2 figures; accepted for publication in ApJ Letter