Transonic Black Hole Accretion as Analogue System

Classical black hole analogues (alternatively, the analogue systems) are fluid dynamical analogue of general relativistic black holes. Such analogue effects may be observed when acoustic perturbations (sound waves) propagate through a classical dissipation-less tran-sonic fluid. The acoustic horizon, which resembles the actual black hole event horizon in many ways, may be generated at the transonic point in the fluid flow. Acoustic horizon emits quasi thermal phonon spectra, which is analogous to the actual Hawking radiation, and possesses the temperature referred as the analogue Hawking temperature, or simply, the analogue temperature. Transonic accretion onto astrophysical black holes is a very interesting example of classical analogue system found naturally in the Universe. An accreting black holes system as a classical analogue is unique in the sense that only for such a system, both kind of horizons, the electromagnetic and the acoustic (generated due to transonicity of accreting fluid) are simultaneously present in the same system. Hence an accreting astrophysical black hole is the ideal-most candidate to theoretically study and to compare the properties of these two different kind of horizons. Also such system is unique in the aspect that general relativistic spherical accretion onto the Schwarzschild black hole represents the only classical analogue system found in the nature so far, where the analogue Hawking temperature may be higher than the actual Hawking temperature. 1 Black Holes Black holes are vacuum solutions of Einstein’s field equations in general relativity. Classically, these objects are conceived as singularities in space time, censored from the rest of the Universe by mathematically defined one way surfaces, the event horizons. The space time metric defining the vacuum exterior of a classical black hole, and the black hole itself, is characterized by only three parameters, the mass of the black hole MBH , the rotation (spin) J and charge q. For J = q = 0, one obtains a Schwarzschild black hole, and for q = 0 one obtains a Kerr black hole. These two kind of black holes are important in astrophysics. In astrophysics, black holes are the end point of gravitational collapse of massive celestial objects. Astrophysical black holes may be broadly classified into two categories, the stellar mass (MBH∼ a few M⊙, where M⊙ is the mass of the Sun), and super massive (MBH≥10M⊙) black holes (SMBH). While the birth history of the stellar mass black holes is theoretically known with almost absolute certainty (they are the endpoint of the gravitational collapse of massive stars), the formation scenario of the supermassive black hole is not unanimously understood. A SMBHmay form through the monolithic collapse of early proto-spheroid gaseous mass originated at the time of galaxy formation. Or a number of stellar/intermediate mass black holes may merge to form it. Also the runaway growth of a seed black hole by accretion in a specially favoured highdensity environment may lead to the formation of SMBH. However, it is yet to be well understood exactly which of the above processes routes towards the SMBH formation, see, e.g. Rees 2002 for a comprehensive review on the formation and evolution of SMBH. Both kind of astrophysical black holes, the stellar mass and SMBH, however, accrete matter from the surrounding. Depending on the intrinsic angular momentum content of accreting material, either spherically symmetric (for zero angular momentum flow), or axisymmetric (for flow with non-zero finite angular momentum) flow geometry is invoked to study an accreting black hole system (Frank, King & Raine 1992).