Magnetic Inversion as a Mechanism for the Spectral Transition of Black Hole Binaries

A mechanism for the transition between low/hard, high/soft, and steep power law (SPL) spectral states in black hole X-ray binaries is proposed. The low/hard state is explained by the development of a magnetically arrested accretion disk attributable to the accumulation of a vertical magnetic field in a central bundle. This disk forms powerful jets and consists of thin spiral accretion streams of a dense optically thick plasma surrounded by hot, magnetized, optically thin corona, which emits most of the energy in hard X-rays. State transition occurs because of the quasi-periodic or random inversion of poloidal magnetic fields in the accretion flow supplied by the secondary star. The inward advection of the inverted field results in a temporal disappearance of the central bundle caused by the annihilation of the opposed fields and restoration of the optically thick disk in the innermost region. This disk represents the high/soft state. The SPL state develops at the period of intensive field annihilation and precedes the high/soft state. The continuous supply of the inverted field leads to a new low/hard state because of the formation of another magnetically arrested disk. Subject headings: accretion, accretion disks — black hole physics — ISM: jets and outflows — MHD — X-rays: binaries