Effect of the toroidal magnetic field on the runaway instability of relativistic tori

Abstract

Aims. Runaway instability operates in fluid tori around black holes. It affects systems close to the critical (cusp overflowing) configuration. The runaway effect depends on the radial profile l(R) of the angular momentum distribution of the fluid, on the dimension-less spin a of the central black hole (|a| ≤ 1), and other factors, such as self-gravity. Previously it was demonstrated that for the power-law dependence of the radial angular momentum profile, l(R) ∝ R q , non-magnetized tori always become runaway stable for a sufficiently high positive value of q. Here we discuss the role of runaway instability within a framework of an axially symmetric model of perfect fluid endowed with a purely toroidal magnetic field. Methods. The gradual accretion of material over the cusp transfers the mass and angular momentum onto the black hole, thereby changing the intrinsic parameters of the Kerr metric. We studied the effect of the plasma parameter β (ratio of gas to magnetic pressure) and other parameters of the model on the evolution of critical configurations that are just on the verge of cusp overflow. Results. By contributing to the total pressure, the magnetic field causes small departures from the corresponding non-magnetic configuration in the early phases of accretion. However, we show that the toroidal magnetic component inside an accretion torus does not change the frequency of its oscillations significantly. We identify these oscillations as the radial epicyclic mode in our example. Nevertheless, these weak effects can trigger the runaway instability even in situations when the purely hydrodynamical regime of the torus is stable. On the other hand, in most cases the stable configuration remains unaffected, and the initial deviations gradually decay after several orbital periods. We show examples of the torus evolution depending on the initial magnetization β, the slope q, and the spin a. Conclusions. The toroidal magnetic field plays a more important role in the early phases of the accretion process until the perturbed configuration finds a new equilibrium or disappears because of the runaway instability.

Relativistic thick accretion disks: Morphology and evolutionary parameters

“…In this article we investigate thick accretion disks orbiting a Kerr black hole attractor. We model the accreting toroidal matter within the so called "Polish doughnut" (P-D) hydrodynamic model introduced and detailed in a series of works [1][2][3][4][5][6][7][8][9][10][11][12], and then developed for many different attractors and contexts [13][14][15][16][17][18][19][20][21][22][23][24][25][26]. This is a fully general relativistic model of an opaque and super-Eddington, pressure supported disk, based on the Boyer theory of the equilibrium and rigidity in general relativity [27].…”

Oblique Magnetic Fields and the Role of Frame Dragging Near Rotating Black Hole

“…[3] argued that tori with radially increasing angular momentum density are more stable. To check on this assertion, our algorithm of the numerical experiment proceeds as follows [20]. At the initial step the mass of the black hole was increased by a small amount, typically by about few percent.…”

“…Oblique Magnetic Fields and the Role of Frame Dragging ised magnetic fields and fluids during the accretion process, within the framework of relativistic MHD limit, see [19,20].…”

Effects of magnetic field on the runaway instability of relativistic accretion tori near a rotating black hole

“…Within the framework of an axially symmetric magnetized fluid torus model we have extended the previous results on the onset of runaway instability of relativistic configurations near a rotating black hole. Based on our recent paper (Hamerský & Karas 2013), these calculations are relevant for understanding the accretion process in the inner regions of accretion flows. We employ the critical (overflowing) solution as an initial configuration, which we perturb and evolve by using a 2D numerical scheme.…”