The BZ–MC–BP model for jet production from a black hole accretion disc

Abstract

ABSTRACT Three energy mechanisms invoking large-scale magnetic fields are incorporated ina model to interpret jet production in black hole (BH) systems, i.e., the Blandford-Znajek(BZ) ,the magneticcoupling(MC) andBlandford-Panye(BP)processes.Theseenergy mechanisms can coexist in BH accretion disc based on the magnetic field con-figurations constrained by the screw instability, provided that the BH spin and thepower-law index indicating the variation of the magnetic field at an accretion disc aregreater than some critical values. In this model the jets are driven by the BZ processin the Poynting flux regime and by the BP process in the hydromagnetic regime, beingconsistent with the spine/sheath jet structure observed in BH sources of stellar andsupermassive size.Key words: accretion, accretion discs — black hole physics — magnetic field — jetproduction 1 INTRODUCTIONAs is well known, jets exist in many astronomical cases, suchas active galactic nuclei, quasars and young stellar objects.Different theoretical models have been proposed for acceler-ation and collimation of jets, which can be divided into twomain regimes. Energy and angular momentum are carried byboth the electromagnetic field and the kinetic flux of mat-ter in the hydromagnetic regime, and those are carried pre-dominantly by the electromagnetic field in the Poynting fluxregime (Ustyugova et al. 2000; Lovelace et al. 2002). Bland-ford & Znajek (1977) proposed firstly that jets from AGNscan be powered by a rotating black hole (BH) with a largescale magnetic field threading its horizon. Later, Blandford& Payne (1982, hereafter BP82) suggested that an outflowof matter can be driven centrifugally by large-scale mag-netic fields anchored at the disc surface. These two mech-anisms are usually referred to the BZ and BP processes,respectively. As argued in BP82, an outflow of matter canbe driven centrifugally from the disc, provided that the an-gle of the poloidal magnetic field with the normal of discsurface is greater than a critical value, i.e., α