Simulations of the Poynting--Robertson Cosmic Battery in Resistive Accretion Disks

Abstract

We describe the results of numerical "2.5--dimensional" MHD simulations of an initially unmagnetized disk model orbiting a central point--mass and responding to the continual generation of poloidal magnetic field due to a secular source that emulates the Poynting--Robertson (PR) drag on electrons in the vicinity of a luminous stellar or compact accreting object. The fluid in the disk and in the surrounding hotter atmosphere has finite electrical conductivity and allows for the magnetic field to diffuse freely out of the areas where it is generated, while at the same time, the differential rotation of the disk twists the poloidal field and quickly induces a substantial toroidal--field component. The secular PR term has dual purpose in these simulations as the source of the magnetic field and the trigger of a magnetorotational instability (MRI) in the disk. The MRI is especially mild and does not destroy the disk because a small amount of resistivity dampens the instability efficiently. In simulations with moderate resistivities (diffusion timescales up to $\sim$16 local dynamical times) and after $\sim$100 orbits, the MRI has managed to transfer outward substantial amounts of angular momentum and the inner edge of the disk, along with azimuthal magnetic flux, has flowed toward the central point--mass where a new, magnetized, nuclear disk has formed. The toroidal field in this nuclear disk is amplified by differential rotation and it cannot be contained; when it approaches equipartition, it unwinds vertically and produces episodic jet--like outflows. The poloidal field in the inner region cannot diffuse back out if the characteristic diffusion time is of the order of or larger than the dynamical time; it continues to grow linearly in time undisturbed and without saturation, as the outer sections of many poloidal loops are being drawn radially outward.

arXiv comment27 pages, 55 figures

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• Dominance of outflowing electric currents on decaparsec to kiloparsec scales in extragalactic jets
• Numerical simulations of the Cosmic Battery in accretion flows around astrophysical black holes
• On the interpretation of the apparent existence of a preferred magnetic polarity in extragalactic jet sources
• On the origin of the hard X-ray tail in neutron-star X-ray binaries
• The energy distribution of electrons in radio jets
• Helical Magnetic Field in a Massive Protostellar Jet
• A model for 3:2 HFQPO pairs in black hole binaries based on cosmic battery
• Formation and destruction of jets in X-ray binaries
• On the Theoretical Framework of Magnetized Outflows from Stellar-Mass Black Holes and Related Observations