The Cosmic Battery Revisited

Abstract

We reinvestigate the generation and accumulation of magnetic flux in optically thin accretion flows around active gravitating objects. The source of the magnetic field is the azimuthal electric current associated with the PoyntingRobertson drag on the electrons of the accreting plasma. This current generates magnetic field loops that open up because of the differential rotation of the flow. We show through simple numerical simulations that what regulates the generation and accumulation of magnetic flux near the center is the value of the plasma conductivity. Although the conductivity is usually considered to be effectively infinite for the fully ionized plasmas expected near the inner edge of accretion disks, the turbulence of those plasmas may actually render them much less conducting due to the presence of anomalous resistivity. We have discovered that if the resistivity is sufficiently high throughout the turbulent disk while it is suppressed inside its inner edge, an interesting steady state process is established: accretion carries and accumulates magnetic flux of one polarity inside the inner edge of the disk, whereas magnetic diffusion releases magnetic flux of the opposite polarity to large distances. In this scenario, magnetic flux of one polarity grows and accumulates at a steady rate in the region inside the inner edge and up to the point of equipartition when it becomes dynamically important. We argue that this inward growth and outward expulsion of oppositely directed magnetic fields that we propose could account for the $30 minute cyclic variability observed in the Galactic microquasar GRS 1915+105.

Numerical simulations of the Cosmic Battery in accretion flows around astrophysical black holes

“…As time progresses, the CB contribution becomes more and more significant. In a previous study, Contopoulos et al (2006) and Christodoulou et al (2008) simulated the CB-induced magnetic field growth in a Newtonian simulation under the assumption of a prescribed central isotropic radiation field. As a result, magnetic field loops were generated mostly in the vicinity of the central object, namely at the position of the innermost stable circular orbit (hereafter ISCO).…”

Formation and destruction of jets in X-ray binaries

“…The PRCB was originally proposed as a means to account for the origin of magnetic fields in the accretion flows surrounding astrophysical compact objects (Contopoulos & Kazanas 1998; see also Contopoulos et al 2006;Christodoulou et al 2008). This physical mechanism has found observational support in observations of the Faraday rotation-measure gradients across extragalactic jets (Contopoulos et al 2009;Christodoulou et al 2011).…”

The Cosmic Battery in Astrophysical Accretion Disks

“…We followed the evolution of the magnetic field by solving the induction equation in one dimension under the assumption that the generated magnetic field is purely vertical. We extended this analysis in axisymmetric configurations (two dimensions; Contopoulos et al 2006;Christodoulou et al 2008) under the assumption that the accretion disk is connected to a steady-state force-free magnetosphere above and below it. Our work faced some criticisms.…”