An Unstable Central Disk in the Superluminal Black Hole X-Ray Binary GRS 1915+105

We have analyzed the X-ray spectra of the microquasar GRS 1915+105, as observed with the Proportional Counter Array (PCA) on the Rossi X-Ray Timing Explorer, 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 about 10-6 Msolar yr-1. 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.