  We have analyzed the X-ray spectra of the microquasar GRS 1915+105, as
observed with the PCA on the Rossi XTE, during periods of stable weak emission,
outbursts and rapid flaring. We find that the complicated X-ray intensity curve
of this source can be described by the rapid removal and replenishment of
matter forming the inner part of an optically thick accretion disk, probably
caused by a thermal-viscous instability analogous to that operating in dwarf
novae, but here driven by the Lightman-Eardley instability. We find that the
mass accretion rate in quiescence is 10^-6 solar masses per year. Only a small
fraction of the energy liberated by accretion is emitted as radiation. We
suggest that most of this energy is advected into the black hole in the
high-viscosity state of the outburst cycle.
