Choked Accretion onto a Schwarzschild Black Hole: A Hydrodynamical Jet-launching Mechanism

Abstract

We present a novel, relativistic accretion model onto a Schwarzschild black hole. This consists of a purely hydrodynamical mechanism in which, by breaking spherical symmetry, a radially accreting flow transitions into an inflow-outflow configuration. The spherical symmetry is broken by considering that the accreted material is more concentrated on an equatorial belt, leaving the polar regions relatively under-dense. What we have found is a flux-limited accretion regime in which, for a sufficiently large accretion rate, the incoming material chokes at a gravitational bottleneck and the excess flux is redirected by the density gradient as a bipolar outflow. The threshold value at which the accreting material chokes is of the order of the mass accretion rate found in the spherically symmetric case studied by Bondi and Michel. We describe the choked accretion mechanism first in terms of a general relativistic, analytic toy model based on the assumption of an ultrarelativistic stiff fluid. We then relax this approximation and, by means of numerical simulations show that this mechanism can operate also for general polytropic fluids. Interestingly, the qualitative inflow-outflow morphology obtained appears as a generic result of the proposed symmetry break, across analytic and numeric results covering both the Newtonian and relativistic regimes. Finally, we discuss the applicability of this model as a jet-launching mechanism in different astrophysical settings.

Spherical accretion: Bondi, Michel and rotating black holes

“…On the other hand, a relationship between the fluid state at infinity and at the sonic point can be obtained by substituting equations (2.15a) and (2.15b) into equation (2.13b). Doing this results in the following cubic equation for hs (see Tejeda et al 2020, Appendix A) (2.16) as well as the corresponding equation for the sound speed…”

“…Recent works have also studied deviations away from spherical symmetry by introducing large-scale, small-amplitude density anisotropies, finding that even a slight equator-to-poles density contrast can drastically modify Bondi's solution, giving rise to an inflow-outflow configuration consisting of equatorial accretion and a bipolar outflow. The resulting steady-state configuration, dubbed choked accretion, was studied in Aguayo-Ortiz et al (2019) at the Newtonian level and, within a general relativistic framework, in Tejeda et al (2020) and Aguayo-Ortiz et al (2021) for Schwarzschild and Kerr black holes, respectively. In these series of works, it was found that the total mass flux that reaches the central accretor is of the order of magnitude of the corresponding Bondi mass accretion rate, while all the excess flux is redirected by the density gradient as outflow.…”

Choked accretion onto a Kerr black hole

“…Based on the general solution presented in [30], and following on the work of [31] and [32], Tejeda, Aguayo-Ortiz and Hernandez [33] presented a simple, hydrodynamical mechanism for launching astrophysical jets from a choked accretion flow onto a Schwarzschild black hole. This model starts from a spherically symmetric accretion flow onto a central massive object and introduces a deviation away from spherical symmetry by considering a small-amplitude, large-scale density gradient in such a way that the equatorial region of the accreting material is over dense as compared to the polar regions.…”

“…This anisotropic density field translates into a pressure-driven force that, provided a sufficiently large mass accretion rate, can deflect a fraction of the originally radial accretion flow onto a bipolar outflow. The threshold value for the accretion rate determining whether the inflow chokes and the launching mechanism is successful or not is found to be of the order of the mass accretion rate corresponding to the spherically symmetric cases discussed by Bondi and Michel. Even though the approximation of a stiff fluid has a rather limited applicability in astrophysics, the mechanism presented in [33] was shown to be valid for more general equations of state by means of full hydrodynamic numerical simulations. Moreover, as discussed in [32], this mechanism is also valid in the context of Newtonian gravity.…”

“…In this work, we present an extension of the choked accretion model introduced in [32,33] to the case of a rotating central black hole as described by the Kerr metric. We study this problem using both an analytic solution for an ultrarelativistic stiff fluid as well as full hydrodynamic numerical simulations for fluids described by more general equations of state.…”