The 1-12 Hz QPOs and dips in GRS 1915+105: tracers of Keplerian and viscous time scales?

We analyzed 9 RXTE/PCA observations of GRS 1915+105 in the flaring state, w hen the hardness of the source spectrum was changing on the time scales fro m few seconds up to 1000 seconds. The quasi periodic oscillations (QPOs) wi th the frequency varying between similar to I and 12 Hz are associated with the episodes of harder source spectrum. In each observation we found tight correlation between the duration of the hard episode and the characteristi c QPO frequency. For a half of the observations this correlation matches th e relation between the viscous time scale and the Keplerian frequency, when both quantities are evaluated for various radii in the radiation pressure dominated accretion disk. Assuming that the QPO frequency is proportional t o the Keplerian frequency at the boundary between an optically thick accret ion disk and a hot comptonization region the changes of the QPO frequency c an then be understood as due to variations of this boundary position on the viscous time scales.