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The properties of sub-second time variability of the X-ray emission of theblack-hole binary GRS 1915+105 are very complex and strictly connected to itspatterns of variability observed on long time scales. A key aspect fordetermining the geometry of the accretion flow is the study of time lagsbetween emission at different energies, as they are associated to key timescales of the system. In particular, it is important to examine the lagsassociated to the strong low-frequency Quasi-periodic Oscillations (QPOs), asthe QPOs provide unambiguous special frequencies to sample the variability. Wehave analyzed data from an observation with the AstroSat satellite, in whichthe frequency of the low-frequency QPO varies smoothly between 2.5 and 6.6 Hzon a time scale of ~10 hours. The derived phase lags show the same propertiesand evolution of those observed on time scales of a few hundred days,indicating that changes in the system geometry can take place on times belowone day. We fit selected energy spectra of the source and rms and phase-lagspectra of the QPO with a time-variable Comptonization model, as donepreviously to RossiXTE data of the same source, and find that indeed thederived parameters match those obtained for variations on much longer timescales.

Fast-varying time lags in the Quasi-periodic Oscillation in GRS 1915+105