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

The physical mechanism responsible for the high viscosity in accretion disk s is still under debate. The parameterization of the viscous stress as alph aP 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 th e or-viscosity law implies the presence of the instability in the radiation pressure-dominated region. We study the time evolution of such disks. We s how that the time-dependent behavior of GRS 1915+105 can be well reproduced if the or-viscosity disk model is calculated accurately (with proper numer ical coefficients in vertically averaged equations and with advection inclu ded) and if the model is supplemented with (1) a moderate corona dissipatin g 50% of energy and (2) a jet carrying a luminosity-dependent fraction of e nergy. These necessary modifications in the form of the presence of a, coro na and a jet are well justified observationally. The model predicts outburs ts at a luminosity larger than 0.16(M) over dot(Edd), as required, and corr ect outburst timescales and amplitudes, including the effect of an increasi ng outburst timescale with mean luminosity. This result strongly suggests a lso that the or-viscosity law is a good description of the actual mechanism responsible for angular momentum transfer in the innermost, radiation pres sure-dominated part of the disk around a black hole.