  We present a study of the centroid frequencies and phase lags of the
quasi-periodic oscillations (QPOs) as functions of photon energy for GRS
1915+105. It is found that the centroid frequencies of the 0.5-10 Hz QPOs and
their phase lags are both energy dependent, and there exists an
anti-correlation between the QPO frequency and phase lag. These new results
challenge the popular QPO models, because none of them can fully explain the
observed properties. We suggest that the observed QPO phase lags are partially
due to the variation of the QPO frequency with energy, especially for those
with frequency higher than 3.5 Hz.

High Energy Astrophysical Phenomena;Astrophysics of Galaxies