ACCRETING ASTROPHYSICAL BLACK HOLES ARE NATURAL EXAMPLES OF CLASSICAL ANALOGUE MODEL

In the framework of acoustic geometry, we investigate the transonic accretion onto gravitating astrophysical objects to establish that such a configuration is a unique example of an classical analogue system realized naturally in the Universe. Only for an accreting astrophysical black hole, on which we mainly concentrate in our work, both kind of horizons, gravitational as well as acoustic (generated due to transonicity of accreting fluid) are simultaneously present in the same system, which indicates that accreting astrophysical black holes are the most ideal candidate to study theoretically and to compare the properties of these two different kind of horizons. Such a system is also unique in the aspect that accretion onto the black holes represents the only classical analogue system found in the nature so far, where the analogue Hawking temperature may exceed the actual Hawking temperature.