THE VISCOUS DISK OF GRS 1915+105

GRS 1915+105, one of the two known galactic microquasars, shows an ext remely complex variability in the X-ray band, comparable to no other X -ray source in the sky. Making use of RXTE/PCA data, we have analyzed the X-ray spectral distribution throughout the variability. We find th at all variations can be attributed to the rapid appearance and disapp earance of the inner region of an optically-thick accretion disk. Sinc e the time scale for each event is related to the maximum radius of th e disappearing region, the difference in time structure is due to the time distribution of such radii. The observed relation between the ext ent of the missing inner region of the disk and the duration of an eve nt is in remarkable agreement with the expected radius dependence of t he viscous time scale of the radiation-dominated region of an accretio n disk.