ABSORPTION OF WAVES IN ACCRETION DISKS .2. RESONANCE AT THE HEIGHT WHERE WAVE FREQUENCY MATCHES THE EPICYCLIC FREQUENCY

We consider axisymmetric, adiabatic, and inviscid oscillations of a di sk rotating around a compact object. Assuming the wavelength in the ra dial direction to be short compared with the typical length scale in t he radial direction, we find that low frequency waves propagating vert ically are resonantly absorbed at the height where the wave frequency omega matches the epicyclic frequency kappa. As an example, we conside r a weakly isentropic disk having coronae. In the equatorial cool regi on of the disk, low-frequency waves with omega < kappa are transmissiv e. Above and below the equatorial cool disk, there are supposed to be hot coronae which are nearly non-rotating (kappa < 0). In the coronae, low-frequency waves are no longer transmissive, because their frequen cy omega exceeds kappa. In the transition layer located between the eq uatorial cool disk and either of the coronae, there is a surface where omega matches kappa. We give a mathematical proof that waves are abso rbed at this surface. Some implications of this result with respect to the time variabilities observed from X-ray stars are briefly discusse d.