Acoustic black hole in Schwarzschild spacetime: Quasinormal modes, analogous Hawking radiation, and shadows

Various properties of acoustic black holes constructed in Minkowski spacetime have been widely studied in the past decades. Recently the acoustic black holes in general spacetime were proposed. In this paper, we first investigate the basic characteristics of ``curved'' acoustic black holes in Schwarzschild spacetime, including the quasinormal modes, grey-body factor, and analogous Hawking radiation. We find that the signal of the quasinormal mode is weaker than that of the Schwarzschild black hole. Moreover, as the tuning parameter increases, both the positive real part and negative imaginal part of the quasinormal frequency approach to the horizonal axis, but they will not change sign. This means that all the perturbations could die off and the system is stable under those perturbations. Since the larger tuning parameter suppresses the effective potential barrier, so it enhances the grey-body factor. The energy emission rate of Hawking radiation does not monotonically increase the tuning parameter because of the nonmonotonicity of the Hawking temperature. Finally, as a first attempt, we study the acoustic black hole shadow. The radius of the acoustic shadow becomes larger as the tuning parameter increases because both the related acoustic horizon and the acoustic sphere become larger. Our studies could help us to further understand the near horizon geometrical features of the black hole. We also expect that our observations could be detected experimentally in the near future.