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Session 78 - X-ray Binaries.
Oral session, Thursday, June 11
Sierra,
We show that the epicyclic frequency in the comoving\/ frame of orbits in the innermost parts of relativistic accretion disks is imaginary for r_ms < r < r_fr, r_ms being the radius of the last stable orbit. As a result, although this region of the disk is dynamically stable, it is unstable towards fragmentation, i.e., formation of clumps, if it is gas pressure dominated. We calculate the dependence of the fragmentation radius r_fr on the mass and angular momentum of the central compact object. We estimate the linear growth rate of this instability in a locally-Newtonian analysis, and compare it with that of the Lightman-Eardley instability of radiation pressure dominated disks at r > r_fr. We suggest that an observational signature of this instability is the limited range, \sim 500 Hz to \sim 1200 Hz, displayed by the upper peak of kilohertz quasiperiodic oscillations in low mass X-ray binaries, corresponding to the requirement that the disk's inner edge lie in the above ``clumpy'' region, within the framework of that class of models which depends critically on the accretion of clumps or blobs of matter.
