A UNIFIED MODEL FOR THE SPECTRAL VARIABILITY IN GRS-1915+105

We have analyzed the spectral variations of the superluminal black hol e X-ray binary GRS 1915+105 by using data obtained with the PCA on the Rossi XTE. We find that, despite the marked differences in the struct ure and the timescale of variability, all spectral changes can be attr ibuted to the rapid disappearing of the inner region of an accretion d isk, followed by a slower refilling of the emptied region. The timesca le for each event is determined by the extent of the missing part of t he disk. The observed relation between the duration of an event and th e radius of the disappearing region matches remarkably well the expect ed radius dependence of the viscous timescale for the radiation-pressu re-dominated region of an accretion disk.