Nonaxisymmetric Neutral Modes in Relativistic Disks
  We perform a linear stability analysis of the axisymmetric, relativistic, self-similar isothermal disk against non-axisymmetric perturbations. Two sets of neutral modes are discovered. The first set corresponds to marginally unstable perturbations driven by gravitational radiation, and the other signals the onset of bifurcation to non-axisymmetric equilibrium solutions to the Einstein equations. 
Mike J. Cai & Frank H. Shu