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Low and intermediate frequency quasi-periodic oscillations (QPOs) are thoughtto be due to oscillations of Comptonizing regions or hot blobs embedded inKeplerian disks. Any movement of these perturbations is expected systematicallyto change the QPO frequency. Our goal is to find systems where such asystematic drifts have been observed. We also try to find the real cause ofsuch drifts and whether they shed some light on the accretion disk dynamics.Using archival data of the recent outburst of GRO J1655-40, we report thepresence of such systematic drifts not only during the rising phase from the25th of February 2005 to the 12th March 2005, when the QPO frequencymonotonically increased from 82mHz to 17.78Hz but also in the decline phasefrom the 15th September 2005 to the 5th of October 2005, when the QPO frequencydecreased from 13.14Hz to 34mHz. We fitted the frequency drifts with thepropagatory oscillating shock solution. In the shock-oscillation solution, thefrequency is inversely proportional to the infall time scale from the shocklocation. We obtained the shock location and strength through such a fit. Theastonishing smoothness of the variation of the QPO frequency over a period ofweeks directly supports the view that it may due to the drift of an oscillatingshock rather than the movements of a blob inside a differentially rotatingdisk.

Evolution of the quasi-periodic oscillation frequency in GRO J1655-40 -- Implications for accretion disk dynamics