Generation and Transport of Magnetic Flux in Accretion–Ejection Flows

Abstract

Astrophysical accretion flows are associated with energetic emission of radiation and outflows (winds and jets). Extensive observations of these two processes in X-ray binary outbursts are available. A convincing understanding of their dynamics remains, however, elusive. The main agent that controls the dynamics is believed to be a large scale magnetic field that threads the system. We propose that during the quiescent state, the field is held in place by a delicate balance between inward advection and outward diffusion through the accreting matter. We also propose that the source of the field is a growing toroidal electric current generated by the aberrated radiation pressure on the innermost plasma electrons in orbit around the central black hole. This is the astrophysical mechanism of the Cosmic Battery. When the return magnetic field outside the toroidal electric current diffuses through the surrounding disk, the disk magnetic field and its associated accretion rate gradually increase, thus leading the system to an outburst. After the central accretion flow approaches equipartition with radiation, it is disrupted, and the Cosmic Battery ceases to operate. The outward field diffusion is then reversed, magnetic flux reconnects with the flux accumulated around the central black hole and disappears. The magnetic field and the associated accretion rate slowly decrease, and the system is gradually driven back to quiescence. We conclude that the action (or inaction) of the Cosmic Battery may be the missing key that will allow us to understand the long-term evolution of astrophysical accretion-ejection flows.Galaxies 2019, xx, 5 2 of 18 hole. The observational fact that accretion disks indeed generate outflowing winds and jets from their surfaces nevertheless remains rather surprising.Morover, it has been observed that, in many cases, the kinetic power of the outflow exceeds the total luminosity of the disk by several orders of magnitude [2,3,4]. This may be explained by the presence of an external agent that removes angular momentum and thus also gravitational energy from the accretion flow. In the SS73 picture, angular momentum is removed through viscous stresses in the disk, and, therefore, gravitational energy is released locally at all radii as high-energy radiation. Recent observations suggest that angular momentum is removed via a large scale magnetic field that threads the accretion and ejection flows. In doing so, gravitational energy is transformed into the kinetic energy of the outflow, thus there is no need for it to be radiated locally in the disk. This action of the large scale magnetic field suppresses the total radiation from the disk and modifies its high-energy spectrum [5,6,7].The study of accretion flows is very complicated by itself. Now that we have concluded that accretion flows co-exist and as it seems do depend strongly on ejection flows, one needs to study accretion and ejection as one coherent process. Over the past couple of decades, several works contributed to the development of our unde...

Striped Blandford/Znajek jets from advection of small-scale magnetic field

“…Closed field lines in this kind of magnetosperic topology allow for the exchange of angular momentum between the BH and the AD. Uzdensky (2005) further identifies the possibility of a combination of these closed field lines and open field lines extending to a region far away from the central object in BH/AD systems, effectively extracting part of the energy by the BZ process (see also Contopoulos 2019, on the coexistence of electromagnetic accretion and ejection flows). Parfrey et al (2015) argue that, since magnetic field lines can remain closed only up to r close in stationary and axisymmetric magnetospheres, magnetic topologies composed by flux tubes with widths l < l crit = r close − r ISCO ∼ O(r g ) will not produce jets.…”