Magnetic Connection Model for Launching Relativistic Jets from a Kerr Black Hole

We present an alternative model for launching relativistic jets in active galactic nuclei (AGN) from an accreting Kerr black hole (BH) by converting the accretion disc energy into jet energy, when the rotational energy of the BH is transferred to the inner disc by closed magnetic field lines which connects the BH to the disc (BH-disc magnetic connection). In this way, the available disc energy is increased by the BH rotational energy. We assume that the BH may undergo recurring episodes of its activity with: (i) a first phase when accretion power dominates, and (ii) a second phase when BH spin-down power dominates. In both cases the jet is driven by a low-luminosity, (geometrically) thin accretion disc, as the disc energy is used to launch the jet. We use the general relativistic conservation laws to calculate the mass flow rate into the jets, the launching power of the jets, and the angular momentum transported by the jets. We consider BHs with a spin parameter a∗ > 0.95, so that the jets are launched from the region inside of the BH ergosphere. The angular momentum removed from the accretion disc is carried away by the disc particles that ultimately form the jets. As far as the BH is concerned, it can (i) spin up by accreting matter and (ii) spin down due to the magnetic counter-acting torque on the BH. We found that a stationary state of the BH (a∗ = const) can be reached if the mass accretion rate is larger than ṁ ∼ 0.001. Here, the mass accretion rate Ṁ is specified in units of Eddington accretion rate ṀEdd by ṁ = Ṁ/ṀEdd. The maximum value of the BH spin parameter depends on ṁ being less but close to 0.9982 (Thorne’s model). For ṁ < 0.001, the BH spins down continuously, unless a large amount of matter is provided. In addition, the maximum AGN lifetime can be much longer than ∼ 10 yr when using the BH spin-down power. This result is consistent with the estimation of the maximum AGN lifetime when the AGN output power is provided by the Blandford–Znajek mechanism.