Lei Wei-Hua

Magnetic Coupling of a Rotating Black Hole with the Surrounding Accretion Disc

The evolution characteristics and energy extraction of a rotating black hole are investigated by considering the magnetic coupling with the surrounding accretion disc. It is found that both the mass and spin of the black hole might be reduced by the joint effects of disc accretion and magnetic coupling, provided that the latter is stronger than the former. The efficiencies of the two energy mechanisms are calculated and compared to a variety of parameters. In addition, the validity of the laws of black hole thermodynamics is discussed.

A Toy Model for Magnetic Field Configurations in Black Hole Accretion Discs

We discuss the feature of the magnetic field configuration arising from double counter oriented electric current-rings in the accretion disc around a Kerr black hole (BH). We discuss the relevant physical quantities corresponding to this configuration: (1) the power and torque transferred by the large-scale magnetic field, (2) the angular momentum and energy fluxes transferred from the BH to the inner disc, (3) the radiation flux from the disc. In addition, we discuss the possibility that the closed magnetic field anchored at the disc probably evolves to the open magnetic field, which is helpful to produce the jet from the disc.

Effects of Magnetic Coupling on Profile of Emission Lines and Images of an Accretion Disc Around a Black Hole

The profiles of emission lines and images of an accretion disc around a black hole (BH) are simulated by considering the effects of the magnetic coupling (MC) of a central BH with the disc. The MC effects are discussed for both slow- and fast-spinning BHs, and the following results are obtained. Firstly, the width of the emission lines and the brightness of the disc are reduced and augmented for slow- and fast-spinning BHs, respectively. Secondly, the image of the disc becomes dimmer and brighter near the inner edge of the disc for slow- and fast-spinning BHs, respectively. It turns out that all these results arise from the MC effects on the position of the dominant emission region of the accretion disc.

A Unified Model of Magnetic Extraction of Spin Energy from a Black Hole

A unified model of magnetic extraction of spin energy from a black hole is discussed based on the theory of black hole magnetosphere. The magnetic extracting power is expressed by a unified formula, which is applicable to both the Blandford-Znajek (BZ) process and the magnetic coupling (MC) process. The strength and characteristics of the BZ power and the MC power are compared in detail. In addition, the impedance matching condition for the BZ power is extended to the more general case.

Investigation on the Quasi-Cycle of Black Hole Spin

A reverse condition of accretion flow and its constraints on black hole spin in the quasi-cycle are proposed. It is shown that the necessary condition for the quasi-cycle is that the closed magnetic field lines must dominate over the open ones at the horizon. The efficiency of converting accreted rest mass into radiation energy in the quasi-cycle is proved to increase with the strength of the magnetic transfer process. In addition, a general expression for the efficiency is derived.

Electromagnetic Quantities in Black Hole Magnetosphere

Some electromagnetic quantities in the black hole (BH) magnetosphere are discussed by considering the coexistence of the Blandford–Znajek process and the magnetic coupling process. These quantities are (i) flux of electromagnetic energy and angular momentum transferred between the BH and the disc, (ii) poloidal currents flowing on the horizon and disc, (iii) poloidal electric field on the horizon, (iv) toroidal magnetic field in the BH magnetosphere, and (v) voltage drop across the magnetic coupling region on the horizon. It turns out that these quantities are determined mainly by three parameters: (i) the positions relative to the corotation magnetic surface, (ii) the BH spin, and (iii) the power-law index for the variation of the magnetic field on the disc.

Cycle of Black Hole Spin due to Disc Accretion Alternating with Magnetic Transfer

The cycle of black hole (BH) spin proposed by Li and Paczynski (henceforth CYCLP) is compared with a more natural model (henceforth CYC03), in which energy and angular momentum are transferred from a rotating BH to a region of some widths by the closed magnetic field lines. It turns out that the efficiency of converting the accreted mass into the radiation energy in the CYC03 is less than that estimated in the CYCLP, while the BH mass and entropy in the CYC03 are greater than those in the CYCLP. It is shown that the features of the CYC03 are insensitive to the power-law index indicating the variation of the magnetic field in the disc.

Coexistence of two mechanisms for extracting energy from a rotating black hole

Evolution characteristics of a rotating black hole (BH) are discussed in coexistence of the Blandford-Znajek (BZ) process and the magnetic coupling (MC) process in the parameter space consisting of the BH spin and the power-law index of the magnetic field on the disc. The condition for the coexistence of the two energy mechanisms are derived by using the mapping relation between the angular coordinate on the BH horizon and the radial coordinate on the disc. It is shown that not only the two mechanisms can coexist, but also the power and the rate of change of BH entropy in the BZ process will dominate over those in the MC process, provided that the BH spin and the power-law index are great enough.

A Toy Model for Advection Dominated Accretion Flows

A toy disc model with advection dominated accretion on to a black hole is presented. The advection dominated accretion flows (ADAFs) are assumed to exist in the inner thick disc with rin rout. Compared with Paczynskis toy model the thick disc is not limited to be 100% advective. It turns out that the inner radius rin depends not only on the outer radius rout but also on the ADAF parameters f and e. The effects of the inner thick disc on the radiation efficiency and the temperature profile of the outer thin disc are discussed in details.

Temperature Profile of Black Hole Accretion Disc with Magnetic Coupling

Two new mapping relations between the angular coordinate on the black hole (BH) horizon and radial coordinate on the disc are given according to the requirement of general relativity and Maxwells equations, and the effects of magnetic coupling (MC) on temperature of accretion disc are investigated by comparing with pure accretion. It is shown that the MC effects on the temperature profile are related intimately to the BH spin, and the influence on the peak value of disc temperature based on the modified mapping relations is not as great as that based on the linear mapping. The peak value and the corresponding radius of peak value ring of disc temperature do not increase monotonically as the increasing spin of BH, each containing a maximum for the fast-spinning BH. The value ranges of the bolometric luminosity and color temperature of the disc are both extended by the MC effects.