  Through studying the hard lags between the soft (3.3 - 5.8 keV) and hard
(13.0 - 41.0 keV) photons of the 0.5 - 10 Hz QPOs in GRS 1915+105, we have
classified them into three types: 0.5 - 2.0 Hz QPOs, 2.0 - 4.5 Hz QPOs, and 4.5
- 10 Hz QPOs. They are closely related to different temporal and spectral
states. The first type of QPOs (0.5 - 2 Hz) have positive hard lags at both the
QPO fundamental and first harmonic frequencies. These QPOs were observed in the
quiescent soft state. The second type of QPOs (2 - 4.5 Hz), which were also
detected in the quiescent soft state, have opposite signs of hard lags at the
QPO fundamental and first harmonic frequencies. The third type (4.5 - 10 Hz),
which showed up in medium soft quiescent/out-burst states, do not have
significant higher harmonic peaks. There is a smooth transition between these
three types of QPO behaviors. We did not detect 0.5 - 10 Hz QPOs in the very
soft state. We discuss some of the implications of these results.
