Transonic Black Hole Accretion as Analogue System

TL;DR

This paper explores transonic accretion onto astrophysical black holes as a natural classical analogue system, highlighting the simultaneous presence of electromagnetic and acoustic horizons and their potential to exhibit analogue Hawking radiation.

Contribution

It identifies astrophysical black hole accretion as a unique natural classical analogue system with both electromagnetic and acoustic horizons present simultaneously.

Findings

Accretion systems can generate acoustic horizons similar to black hole event horizons.

Such systems may exhibit analogue Hawking radiation with higher temperature than actual Hawking radiation.

Astrophysical black hole accretion uniquely combines electromagnetic and acoustic horizons in nature.

Abstract

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.