A unified model for the spectral variability in GRS 1915+105

We have analyzed the spectral variations of the superluminal black-hole 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 structure and the time scale of variability, all spectral changes can be attributed to the rapid disappearing of the inner region of an accretion disk, followed by a slower re-filling of the emptied region. The time scale for each event is determined by the extent of the missing part of the disk. The observed relation between the duration of an event and the radius of the disappearing region matches remarkably well the expected radius dependence of the viscous time scale for the radiation-pressure dominated region of an accretion disk. Subject headings: accretion, accretion disks — binaries: close — black hole physics – instabilities – X-rays: stars — stars: individual GRS 1915+105