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The Cosmic Battery and the inner edge of the accretion disc




The Poynting–Robertson Cosmic Battery proposes that the innermost part of the accretion disc around a black hole is threaded by a large-scale dipolar magnetic field generated in situ, and that the return part of the field diffuses outwards through the accretion disc. This is different from the scenario that the field originates at large distances and is carried inwards by the accretion flow. In view of the importance of large-scale magnetic fields in regulating the processes of accretion and outflows, we study the stability of the inner edge of a magnetized disc in general relativity when the distribution of the magnetic field is the one predicted by the Poynting–Robertson Cosmic Battery. We found that as the field grows, the inner edge of the disc gradually moves outwards. In a fast spinning black hole with a ≳ 0.8M the inner edge moves back in towards the black hole horizon as the field grows beyond some threshold value. In all cases, the inner part of the disc undergoes a dramatic structural change as the field approaches equipartition.