Black hole magnetic field
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Black Hole Magnetic Field: Key Insights and Phenomena
Black Hole in a Uniform Magnetic Field
When a stationary, axisymmetric black hole is placed in a uniform magnetic field aligned along its symmetry axis, it selectively accretes charges until its charge reaches a specific value, ( Q = 2B_0J ), where ( B_0 ) is the magnetic field strength and ( J ) is the black hole's angular momentum. This configuration also reveals that the gyromagnetic ratio of a slightly charged, stationary, axisymmetric black hole is ( g = 2 )1.
Electromagnetic Fields in Black Hole Environments
The interaction between black holes and magnetic fields can lead to various electromagnetic phenomena. For instance, in a vacuum cavity between a neutral black hole and a surrounding plasma shell, the electromagnetic fields tend to be nearly uniform. The magnetic flux across the surface of a neutral black hole decreases as the black hole's angular momentum increases2. Additionally, magnetized black holes in an external gravitational field exhibit unique properties, such as the Meissner effect in extremal rotating solutions, which occurs only in the zero-charge limit3.
Charged Particle Motion in Magnetic Fields
The presence of an external magnetic field significantly affects the motion of charged particles around a rotating black hole. The magnetic field enlarges the region of stability for circular orbits towards the event horizon. This effect is particularly pronounced in the case of a maximally rotating black hole, where strong magnetic fields enable relativistic motions in both direct and retrograde innermost stable circular orbits4.
Eddy Currents and Dissipative Effects
Black holes can also generate eddy currents when placed in an external magnetic field. These currents, which satisfy Ohm's law with a surface resistivity of approximately 377 ohms, lead to Joule heating. The rotation of a black hole in an oblique uniform magnetic field induces these currents, and the resulting ohmic losses provide a simple way to calculate the torque exerted on the black hole5.
Toroidal Magnetic Fields in Self-Gravitating Disks
In the context of self-gravitating disks around black holes, toroidal magnetic fields play a crucial role. These fields influence the stability and mass distribution of the disks. There are typically two branches of solutions: one for relatively light disks and another for disks that can be more massive than the black hole itself. The magnetic field in the disk affects these properties, leading to a bifurcation in the parameter space of solutions6.
Gravimagnetic Phenomena and High-Energy Activity
Magnetized black holes are central to explaining high-energy activities in galactic cores and quasars. These black holes exhibit unusual gravimagnetic phenomena, such as the creation of an inductive potential difference during rotation in a magnetic field, drift in an external electromagnetic field, and changes in the chemical potential of the event horizon. These effects result from the interplay between electrodynamics and gravitation9.
Magnetic Hair and Reconnection
Despite the no-hair theorem, which states that black holes are characterized only by mass, spin, and charge, black holes with highly magnetized plasma-filled magnetospheres exhibit complex magnetic field structures. Simulations show that a dipole magnetic field on the event horizon can open into a split monopole and reconnect in a plasmoid-unstable current sheet. This reconnection process is a powerful source of hard X-ray emission when the magnetic field is strong10.
Conclusion
The interaction between black holes and magnetic fields leads to a variety of fascinating phenomena, from charge accretion and stability of particle orbits to eddy currents and high-energy emissions. These interactions are crucial for understanding the complex behaviors of black holes in different astrophysical environments and their role in high-energy astrophysical processes.
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Most relevant research papers on this topic
Black hole in a uniform magnetic field
A stationary, axisymmetric black hole in a uniform magnetic field selectively accretes charges until its charge becomes Q=2mathrmBb_0J$, resulting in a gyromagnetic ratio of 2$.
Highly CitedBlack Holes and Magnetic Fields
Near-uniform magnetic fields in a vacuum cavity between a neutral black hole and a surrounding plasma shell decrease as the hole's angular momentum increases.
Magnetized black holes in an external gravitational field
Magnetized black holes in an external gravitational field exhibit thermodynamical properties similar to isolated black holes, with the Meissner effect observed in extremal rotating solutions.
Motion of charged particles around a rotating black hole in a magnetic field
An external magnetic field can significantly enhance the stability of circular orbits around a rotating black hole, potentially allowing relativistic motions in innermost stable orbits.
Highly CitedBlack-hole eddy currents
Black-hole eddy currents can exist even in the absence of external currents, and their rotation in an oblique uniform magnetic field can be used to calculate the torque exerted on the hole.
Highly CitedToroidal magnetic fields in self-gravitating disks around black holes
Magnetic fields in self-gravitating disks around black holes can influence the bifurcation in parameter space, allowing for relatively light disks and massive disks, depending on the geometry of the spacetime.
Black hole perturbations and electric-magnetic duality
This paper extends perturbation theory based on harmonic expansion to have manifest invariance under electric-magnetic duality, enabling simple computation of gravitational and electromagnetic radiation emitted by black holes interacting with electromagnetically charged matter.
Magnetized Black Holes
The "magnetized" black hole model predicts unusual "gravimagnetic" phenomena, including inductive potential differences, drift, and changes in chemical potential, and influences on quantum-mechanical processes in black holes.
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