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

Abstract

Physical mechanism responsible for high viscosity in accretion disks is still under debate. Parameterization of the viscous stress as $\alpha P$ 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 due to ionization instability. We show that this parameterization can be also adopted in the innermost part of the disk where the adoption of the $\alpha$-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 $\alpha$-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 (i) moderate corona dissipating 50% of energy (ii) jet carrying 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 luminosity larger than 0.16$\dot M_{Edd}$, as required, correct outburst timescales and amplitudes, including the effect of increasing outburst timescale with mean luminosity. This result strongly suggests that the $\alpha$-viscosity law is a good description of the actual mechanism responsible for angular momentum transfer also in the innermost, radiation pressure dominated part of the disk around a black hole.