Multi-transonic Black Hole Accretion Discs with Isothermal Standing Shocks

Abstract

In this work we would like to address the issue of shock formation in black hole accretion discs. We provide a {\it generalized two parameter solution scheme} for multi-transonic, accretion and wind around Schwarzschild black holes, by mainly concentrating on accretion solutions which may contain steady, standing isothermal shocks. Such shocks conserve flow temperature by the dissipation of energy at the shock location.Unlike previous works in this field, our calculation is {\it not} restricted to any particular kind of post-Newtonian gravitational potentials, rather we use {\it all} available pseudo-Schwarzschild potentials to formulate and solve the equations governing the accretion and wind only in terms of flow temperature $T$ and specific angular momentum $λ$ of the flow. Our generalized formalism assures that the shock formation is not just an artifact of a particular type of gravitational potential, rather inclusion of all available black-hole potentials demonstrates a substantially extended zone of parameter space allowing for the possibility of shock formation. We thus arrive at the conclusion that the standing shocks are essential ingredients in rotating, advective accretion flows of isothermal fluid around a non-spinning astrophysical black hole. Types of shocks discussed in this paper may appear to be `bright' because of the huge amount of energy dissipation at shock, and the quick removal of such energy to maintain the isothermality may power the strong X-ray flares recently observed to be emerged from our Galactic centre. The results are discussed in connection to other astrophysical phenomena of related interest, such as the QPO behaviour of galactic black hole candidates. (Abridged).