Magnetic flux captured by an accretion disk

We suggest a possible mechanism of efficient transport of the large-scale external magnetic flux inward through a turbulent accretion disk. The outward drift by turbulent diffusion which limits the effectiveness of this process is reduced if the external flux passes through the disk in concentrated patches. Angular momentum loss by a magnetocentrifugal wind from these patches of strong field adds to the inward drift. We propose that magnetic flux accumulating in this way at the center of the disk provides the ‘second parameter’ determining X-ray spectral states of black hole binaries, and the presence of relativistic outflows. Subject headings: black hole physics — MHD — accretion, accretion disks — magnetic fields — galaxies: active