Three‐dimensional Magnetohydrodynamic Simulations of Spherical Accretion

Abstract

We present three-dimensional numerical magnetohydrodynamic simulations of radiatively inefficient spherical accretion onto a black hole. The simulations are initialized with a Bondi Ñow and with a weak, dynamically unimportant, large-scale magnetic Ðeld. As the gas Ñows in, the magnetic Ðeld is ampliÐed. When the magnetic pressure approaches equipartition with the gas pressure, the Ðeld begins to reconnect and the gas is heated up. The heated gas is buoyant and moves outward, causing line stretching of the frozen-in magnetic Ðeld. This leads to further reconnection and more heating and buoyancy-induced motions, so that the Ñow makes a transition to a state of self-sustained convection. The radial structure of the Ñow changes dramatically from its initial Bondi proÐle, and the mass accretion rate onto the black hole decreases signiÐcantly. Motivated by the numerical results, we develop a simpliÐed analytical model of a radiatively inefficient spherical Ñow in which convective transport of energy to large radii plays an important role. In this "" convection-dominated Bondi Ñow,ÏÏ the accretion velocity is highly subsonic, and the density varies with radius as o P R~1@2 rather than the standard Bondi scaling o P R~3@2. We estimate that the mass accretion rate onto the black hole correspondingly scales as M 0 D where is a small multiple of the Schwarzschild radius of the black hole and is theaccretion radius ÏÏ at which the ambient gas in the surrounding medium is gravitationally captured by the black hole. Since the factor is typically very small, is signiÐcantly less than the Bondi R in /R a M 0 accretion rate. Convection-dominated Bondi Ñows may be relevant for understanding many astrophysical phenomena, e.g., post-supernova fallback and radiatively inefficient accretion onto supermassive black holes, stellar-mass black holes, and neutron stars.

The Effects of Thermal Conduction on Radiatively Inefficient Accretion Flows

“…Note that even for spherical accretion (i.e., in the absence of differential rotation and the MRI ), a significant fraction of the gravitational potential energy of the inflowing gas may be converted into heat. In particular, even initially weak magnetic fields are compressed by flux freezing until they become strong, leading to reconnection and significant heating (Igumenshchev & Narayan 2002). Because v e 3 v i unless T e P10 À3 T i , electron heat conduction tends to be more important than ion heat conduction.…”

Numerical simulations of axisymmetric hydrodynamical Bondi–Hoyle accretion on to a compact object

“…The stability of the b-h flow was put into question with numerical results glimpsing various instabilities without being able to agree whether their origin was physical or not (see Foglizzo et al 2005, and references therein). Meanwhile, analytical progresses were also made as physical effects were added or coupled together : small or large scale magnetic fields (Igumenshchev and Narayan 2002;Igumenshchev 2006;Pang et al 2011), net vorticity (Krumholz et al 2005), radiative feedback (Park and Ricotti 2013), finite size accretors (Ruffert 1994a), turbulence (Krumholz et al 2006), etc. Simulations of accretion onto not compact objects like stars orbiting an agb companion in a symbiotic binary (Theuns et al 1996;de Val-Borro et al 2009) have deeply improved our understanding of the barium enriched stars (Bidelman and Keenan 1951) ; but the wide dynamics introduced by the small size of a compact object compared to its gravitational sphere of influence on non-relativistic winds has prevented numerical simulations to converge to a b-h accretion solution around a compact object up to now.…”

Why Do AGN Lighthouses Switch Off?

“…If the outflow is strong enough, then the amount of mass reaching the BH could be a great deal less than the mass supplied on the outside [107], [108] (but see [109], [110]). [105]. Note the obvious turbulent eddies which are due to convection.…”

“…The density profile of a magnetized spherical flow was found to differ significantly from the R −3/2 behavior predicted by the Bondi model ( §4), and the mass accretion rate was found to be much less than the value given in (2). The reason for the discrepancy is as follows [105].…”