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The general problem of perturbing a (non-self-gravitating) perfect fluid potential flow in an arbitrary background gravitational field is formulated. The special case of perturbing stationary, spherical accretion onto a Schwarzschild black hole is then considered, and the following stability results are derived: (1) no unstable normal modes exist which extend outside the sound horizon of the background flow; and (2) there are no unstable modes which represent a standing shock at the sound horizon. The high-frequency (JWKB) approximation to traveling wave perturbations is also derived and it is shown that these approximate solutions are regular across the black hole's event horizon.
Vincent Moncrief (Fri,) studied this question.