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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.

Based on the coexistence of the Blandford–Znajek and magnetic coupling processes in black hole (BH) accretion disc, a multi-parameter model for jet powers and radio loudness of active galactic nuclei (AGNs) is studied. It turns out that radio-loudness of AGNs could be governed by five parameters: (i) the BH spin, (ii) a power-law index of the variation of the magnetic field on the disc; (iii) a parameter determining the position of the inner edge of the disc, (iv) the ratio of the pressure of the magnetic field on the horizon to the ram pressure of the innermost parts of an accretion flow, and (v) the ratio of the angular velocity of the open field lines to that of the horizon. The observed dichotomy between radio-loud and radio-quiet AGNs is well interpreted by the effects of the above parameters. Furthermore, we discuss the derivative of radio loudness of AGNs with respect to each parameter separately. In addition, the effect of the screw instability on radio loudness of AGNs is discussed.

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 Paczynski's 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.

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 Maxwell's 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.

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.

It is shown that evolution characteristics of black hole (BH) parameters in the pure disk-accretion are determined uniquely by BH spin parameter a*. Some unusual evolution characteristics of the parameters of BH accretion disks, such as angular velocity, temperature and event horizon radius, are investigated in both thin and thick disk-accretion. And the corresponding turning points, a*c, are derived. In addition, two characteristic functions describing the evolution of BH parameters are derived for thin and thick disks, respectively.

In this paper, we present a simplified model for magnetized neutrino-dominated accretion flow (NDAF) in which effect of black hole (BH) spin is taken into account by adopting a set of relativistic correction factor, and the magnetic field is parameterized as \beta, the ratio of the magnetic pressure to the total pressure. It is found that the disc properties are sensitive to the values of the BH spin and \beta, and more energy can be extracted from NDAF for the faster spin and lower \beta., Comment: 4 pages, 3 figures, accepted for publication in Science in China Series G

The evolution characters of angular velocity Ω h of central black holes of accretion disks (including thin and thick disks) are investigated by considering both accretion and the Blandford\|Znajek (B\|Z) process. It is shown that the rate of change of Ω h with respect to time turns from the positive value to the negative, as the dimensionless angular momentum a\-* evolves to a critical value. Evolving in a non\|monotonous way, Ω h increases at first, and then decreases in a prograde accretion process. Therefore there exists a maximum of Ω h. It is pointed out that dimensionless angular momentum a \-* is more appropriate to describe the spinning state of a central black hole of an accretion disk than angular momentum J and angular velocity Ω h. PACC: 9760L; 0420