Hoyle–Lyttleton Accretion onto Accretion Disks

We investigate Hoyle–Lyttleton accretion for the case where the central source is a luminous accretion disk. In classical Hoyle-Lyttleton accretion onto a “spherical” source, accretion takes place in an axially symmetric manner around a so-called accretion axis. In the spherical case the accretion rate Ṁ is given as ṀHL(1− Γ), where ṀHL is the accretion rate of the classical Hoyle–Lyttleton accretion onto a non-luminous object and Γ the luminosity of the central object normalized by the Eddington luminosity. If the central object is a compact star with a luminous accretion disk, the radiation field becomes “non-spherical”. In such a case the axial symmetry around the accretion axis breaks down; the accretion radius Racc generally depends on an inclination angle i between the accretion axis and the symmetry axis of the disk and the azimuthal angle φ around the accretion axis. Hence, the accretion rate Ṁ , which is obtained by integrating Racc around φ, depends on i. In the case of a pole-on accretion (i = 0), although the axisymmetry around φ is retained, the accretion radius becomes smaller than that of the spherical case. The accretion rate is therefore smaller than that of the spherical case. We found that the accretion rate is approximately expressed as Ṁ ∼ ṀHL(1 − Γ)(1 − 2Γ). In the case of an edge-on accretion (i = 90), the accretion radius depends strongly on φ and is somewhat larger than that of the spherical case. Depending on the central luminosity Γ, the shape of the accretion cross-section varies from a circle (Γ = 0), an ellipse, a hollow ellipse (Γ ∼ 0.5), and a twin lobe (Γ> ∼ 0.65). The accretion rate is larger than that of the spherical case and approximately expressed as Ṁ ∼ ṀHL(1− Γ) for Γ ≤ 0.65 and Ṁ ∼ ṀHL(2− Γ)/5 for Γ ≥ 0.65. Once the accretion disk forms and the anisotropic radiation fields are produced around the central object, the accretion plane will be maintained automatically (the direction of jets associated with the disk is also maintained). Thus, the anisotropic radiation field of accretion disks drastically changes the accretion nature, that gives a clue to the formation of accretion disks around an isolated black hole. Hoyle–Lyttleton type accretion onto the accretion disk may take place in various astrophysical situations, including the Galactic Center X-ray source 1E 1740.7−2942.