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In recent work, we analyzed the dynamics of spherical and nonsphericalacoustic perturbations of the Michel flow, describing the steady radialaccretion of a relativistic perfect fluid into a nonrotating black hole. Weshowed that such perturbations undergo quasi-normal oscillations and computedthe corresponding complex frequencies as a function of the black hole mass Mand the radius r_c of the sonic horizon. It was found that when r_c is muchlarger than the Schwarzschild radius r_H = 2GM/c^2 of the black hole, thesefrequencies scale like the surface gravity of the analogue black holeassociated with the acoustic metric.  In this work, we analyze the Newtonian limit of the Michel solution and itsacoustic perturbations. In this limit, the flow outside the sonic horizonreduces to the transonic Bondi flow, and the acoustic metric reduces to the oneintroduced by Unruh in the context of experimental black hole evaporation. Weshow that for the transonic Bondi flow, Unruh's acoustic metric describes ananalogue black hole and compute the associated quasi-normal frequencies. Weprove that they do indeed scale like the surface gravity of the acoustic blackhole, thus providing an explanation for our previous results in therelativistic setting.

Quasi-normal acoustic oscillations in the transonic Bondi flow