Absorption of Waves in Accretion Disks. II. Resonance at the Height Where Wave Frequency Matches the Epicyclic Frequency
We consider axisymmetric, adiabatic, and inviscid oscillations of a disk rotating around a compact object. Assuming the wavelength in the radial direction to be short compared with the typical length scale in the radial direction, we find that low-frequency waves propagating vertically are resonantly absorbed at the height where the wave frequency omega matches the epicyclic frequency kappa . As an example, we consider a weakly isentropic disk having coronae. In the equatorial cool region of the disk, low-frequency waves with omega < kappa are transmissive. 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 frequency omega exceeds kappa . In the transition layer located between the equatorial cool disk and either of the coronae, there is a surface where omega matches kappa . We give a mathematical proof that waves are absorbed at this surface. Some implications of this result with respect to the time variabilities observed from X-ray stars are briefly discussed.
Publications of the Astronomical Society of Japan
- Pub Date:
- October 1995
- ACCRETION DISKS;
- X-RAYS: BINARIES