Magnetically Driven Accretion Flows in the Kerr Metric. II. Structure of the Magnetic Field

Abstract

We present a detailed analysis of the magnetic field structure found in a set of four general relativistic 3D MHD simulations of accreting tori in the Kerr metric with different black hole spins. Among the properties analyzed are the field strength as a function of position and black hole spin, the shapes of field lines, the degree to which they connect different regions, and their degree of tangling. Strong magnetic field is found toward small radii, and field strength increases with black hole spin. In the main disk body, inner torus, and corona the field is primarily toroidal. Most field lines passing through a given radius in these regions wander through a narrow radial range, suggesting an overall tightlywound spiral structure. In the main disk body and inner torus sharp field line bends on small spatial scales are superimposed on the spirals, but the field lines are much smoother in the corona. The magnetic field in the plunging region is also comparatively smooth, being stretched out radially by the infalling gas. The magnetic field in the axial funnel resembles a split monopole, but with evidence of frame dragging of the field lines near the poles of the black hole.

Three-Dimensional Simulations of Vertical Magnetic Flux in the Immediate Vicinity of Black Holes

“…There was no vertical flux through the equatorial plane, near the black hole. This was a similar topology to what was observed generally in the thick tori accretion models of De , Hirose et al (2004), Krolik et al (2005), Beckwith et al (2008), McKinney &Gammie (2004), andMcKinney (2005). This led Komissarov & McKinney (2007) to consider this as a general topology for black hole magnetospheres-approximately the radial flux at the horizon and none through the equatorial plane near the event horizon.…”

“…Unlike the simulations in De , , Hirose et al (2004), Krolik et al (2005), Hawley & Krolik (2006), Beckwith et al (2008), McKinney &Gammie (2004), andMcKinney (2005) that considered a finite toroidal pool of gas, threaded by poloidal loops of like orientation, Igumenshchev (2008) considered an endless supply of gas from the outer boundary. This is a major fundamental distinction from the poloidal loop simulations.…”

“…These works simulated accretion flows around a Schwarzschild black hole using the equations of non-relativistic ideal MHD (Landau & Lifshitz 1987), in which the black hole gravity is approximated using a pseudo-Newtonian potential (Paczyński & Wiita 1980). A major difference between the numerical methods of Igumenshchev et al (2003), Igumenshchev (2008), and the simulations that were presented in De Villiers & , Hirose et al (2004), Krolik et al (2005), Hawley & Krolik (2006), Beckwith et al (2008), McKinney &Gammie (2004), andMcKinney (2005) is that they allow for the astrophysical meaningful condition that material is continuously injected into the system from a distant injection surface, R inj , as opposed to evolving a finite pool of material that was deposited in the initial state.…”

“…The reader should refer to De Villiers & Hawley (2003), De Villiers et al (2003), Hirose et al (2004), Krolik et al (2005), Hawley & Krolik (2006), and Beckwith et al (2008) to get more details on the many simulations that have been performed with this code over the years. This discussion is no more than a brief review.…”

Magnetic Pressure Support and Accretion Disk Spectra

“…Long-wavelength Parker instability modes might be suppressed because they cannot fit inside the box. This is an issue that will need further investigation, but we did a preliminary check by comparing with global, nonradiative, general relativistic MRI simulations (Hirose et al 2004). In the shearing-box simulations, magnetic pressure becomes dominant when the density falls below 0.03 times the midplane density.…”