Multitransonic Black Hole Accretion Disks with Isothermal Standing Shocks

In this work we address the issue of shock formation in black hole accretion disks. We provide a generalized two-parameter solution scheme for multitransonic accretion and wind around Schwarzschild black holes, mainly by concentrating on accretion solutions that may contain steady, standing isothermal shocks. Such shocks conserve flow temperature by dissipating energy at the shock location. We use a vertically integrated 1.5-dimensional model to describe the disk structure, where the equations of motion apply to the equatorial plane of the central accretor, assuming the flow to be in hydrostatic equilibrium in the transverse direction. Unlike previous works in this field, our calculation is not restricted to any particular kind of post-Newtonian gravitational potentials; rather, we use all available pseudo-Schwarzschild potentials to formulate and solve the equations governing the accretion and wind only in terms of the flow temperature T and specific angular momentum λ of the flow. The accretion flow is assumed to be nondissipative everywhere, except possibly at the shock location, if any. We observe that a significant region of parameter space spanned by {λ, T} allows shock formation. Our generalized formalism ensures that the shock formation is not just an artifact of a particular type of gravitational potential; rather, the 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 components of rotating, advective accretion flows of isothermal fluid around a nonspinning astrophysical black hole. We identify all possible shock solutions that may be present in isothermal disk accretion and thoroughly study the dependence of various shock parameters on fundamental dynamical variables governing the accretion flow for all possible initial boundary conditions. Types of shocks discussed in this paper may appear to be "bright" because of the huge amount of energy dissipation at the shock, and the quick removal of such energy to maintain isothermality may power the strong X-ray flairs recently observed to be emerging from our Galactic center. The results are discussed in connection with other astrophysical phenomena of related interest, such as the quasi-periodic oscillation behavior of galactic black hole candidates.