Visualization of black hole magnetic field lines. Credit: A. Bransgrove et al. / Physical Review Letters 2021

Can Black Holes Lose Their Magnetic Fields?

According to a recent study, the answer is yes.

Using computer simulations, American physicists have shown that due to magnetic reconnection, the Kerr black hole, surrounded by a highly magnetized plasma, exponentially loses its magnetic field.

The results of the methods of the kinetics of relativistic plasma and resistive magnetohydrodynamics are consistent with the no-hair theorem, which says that black holes are characterized only by mass, angular momentum, and charge.

In addition, the loss of a strong magnetic field causes potent X-rays from the black hole’s magnetosphere.


The generally accepted no-hair theorem

In general relativity, it is generally accepted that all black holes obey the no-hair theorem: if two black holes have the same mass, charge, and angular momentum, then they cannot be distinguished from each other – all other information about their progenitors and the absorbed matter is hidden from the observer beyond the event horizon.

Black holes, born from the collapse of magnetized stars, are born with a magnetic field piercing the event horizon. Also, a black hole can acquire its own magnetic field as a result of merging with a magnetized neutron star.

Because of this, the black hole has hair in the form of magnetic field lines, but not for long – in a vacuum, any massless field with an integer spin quickly evaporates, leaving the black hole “bald”.

However, magnetized black holes are rarely found in a vacuum: if a black hole was formed as a result of the collapse of a neutron star, plasma will inevitably be present around it, or plasma is formed as a result of the creation of electron-positron pairs near the event horizon.

Due to the presence of highly conductive plasma, the conditions in the no-hair theorem change radically – instead of a vacuum around a black hole, matter appears that is capable of holding a magnetic field and preventing it from jumping off the event horizon.

Can a black hole lose its magnetic field?

In this case, the only possible scenario for the loss of the magnetic field is the reconnection of the magnetic lines, as a result of which the lines of force stretch, break, and reconnect in the form of magnetic loops containing plasma. Formed plasmids either fall beyond the event horizon or fly away from the black hole with relativistic velocities.

In this case, the energy of the magnetic field is converted into kinetic energy of particles and radiation. In 2011, this process was observed when simulating a magnetized black hole in the case of collisional plasma (the authors mistakenly neglected collisionless plasma physics) and in low numerical resolution. This led to an excessively long extinction of the magnetic field and a violation of the no-hair theorem.

Scientists led by Ashley Bransgrove of Columbia University took into account the errors of the previous study and used more accurate numerical simulations of particle kinetics – GRPIC (general-relativistic particle-in-cell) and magnetohydrodynamics – GRRMHD (general-relativistic resistive magnetohydrodynamics) to study the process loss of magnetic field by the Kerr black hole.

As the initial state, physicists chose a black hole with a dipole magnetic field, assuming that it had already absorbed a neutron star surrounded by plasma, but had not yet begun to lose the field.

Both modeling methods showed that the evolution of the magnetosphere takes place in several stages: first, the plasma in the ergosphere revolves around the black hole, carries away its magnetic field, and produces a poloidal magnetic field (whose lines run along the meridians) according to the gimbal rule. As the poloidal magnetic field inflates, the lines of force stretch and thicken at the equator.

As a result, the pattern of field lines resembles the fields of two magnetic monopoles (split-monopole field) – in the northern hemisphere, the lines are directed directly from the black hole, in the southern – to the black hole. The toroidal black hole magnetic field (whose lines are directed along the parallels) is also oppositely directed in the two hemispheres.

From left to right: mean radial plasma velocity, mean zenith (θ) velocity, azimuthal (ϕ) component of the magnetic field in the black hole magnetosphere. Green lines represent poloidal magnetic field lines. Credit: Ashley Bransgrove et al. / Physical Review Letters, 2021
From left to right: mean radial plasma velocity, mean zenith (θ) velocity, azimuthal (ϕ) component of the magnetic field in the black hole magnetosphere. Green lines represent poloidal magnetic field lines. Credit: Ashley Bransgrove et al. / Physical Review Letters, 2021

Such a configuration of magnetic fields in accordance with Maxwell’s first equation gives rise to a current sheet in the equatorial plane, along which magnetic reconnection of fields occurs.

According to the simulation data, for the first time, magnetic reconnection appears near the so-called stagnation surface, outside of which the plasma moves from the black hole, and inside it is absorbed by it.

Black hole magnetosphere in the GRPIC (top) and GRRMHD (bottom) models, the color represents the plasma magnetization. Credit: Ashley Bransgrove et al. / Physical Review Letters, 2021
Black hole magnetosphere in the GRPIC (top) and GRRMHD (bottom) models, the color represents the plasma magnetization. Credit: Ashley Bransgrove et al. / Physical Review Letters, 2021

Thus, plasmoids born outside the stagnation surface fly away along with the current sheet at a speed close to the speed of light, while those born inside slowly, at a speed of less than a tenth of the speed of light, move to the event horizon. The rate of magnetic reconnection in the GRPIC model turned out to be equal to one-tenth the speed of light, which is 10 times higher than the rate of reconnection in GRRMHD.

Because of this, the plasmoids in GRPIC have time to grow more than in GRRMHD, before they are thrown away at a relativistic speed. This discrepancy is due to the fact that a simplified particle diffusion model is used in GRRMHD, while plasma in GRPIC is modeled from the first principles.

Left: 3D modeling of magnetospheres, green tubes are magnetic field lines that penetrate the event horizon, rope tubes are reconnecting magnetic field lines. Right: 2D slice of the magnetosphere, color represents plasma magnetization. Credit: Ashley Bransgrove et al. / Physical Review Letters, 2021
Left: 3D modeling of magnetospheres, green tubes are magnetic field lines that penetrate the event horizon, rope tubes are reconnecting magnetic field lines. Right: 2D slice of the magnetosphere, color represents plasma magnetization. Credit: Ashley Bransgrove et al. / Physical Review Letters, 2021

Scientists also carried out magnetohydrodynamic modeling in three-dimensional mode (GRRMHD2). An axisymmetric picture of magnetic field reconnection was no longer observed in it: three-dimensional plasmoids resemble tangled tubes of finite length with a more complex topology than those of two-dimensional plasmoids.

In both models, the magnetic flux through the black hole surface decreases exponentially quickly, regardless of the strength of the field at the beginning of the experiment (in the case of a strongly magnetized plasma and a small Larmor radius ) – and this confirms the fulfillment of the no-hair theorem.

Time dependence of the magnetic flux at the event horizon for vacuum (power law decay), in the GRRMHD (slow exponential decay) and GRPIC (exponential decay) models. Credit: Ashley Bransgrove et al. / Physical Review Letters, 2021
Time dependence of the magnetic flux at the event horizon for vacuum (power-law decay), in the GRRMHD (slow exponential decay) and GRPIC (exponential decay) models. Credit: Ashley Bransgrove et al. / Physical Review Letters, 2021

Physicists also found out that the final charge of the black hole is zero, that is, as a result of demagnetization, the black hole again became a Kerr black hole. Scientists discovered radiation when the lines reconnected and calculated the total dissipative power seen by an observer at infinity.

As expected, in a black hole magnetic field above a million gauss and in the limit of high plasma magnetization, almost all magnetic energy is converted into radiation in the hard X-ray range, which from the outside may look like a galactic magnetar flash.

The authors also note that during the “balding” of the black hole, coherent radio emission can be observed, as well as maser emission arising from the collision of giant plasmoids with plasma streams.


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Sources:

Bransgrove, A., Ripperda, B., & Philippov, A. (2021, July 27). Magnetic hair and Reconnection in black Hole magnetospheres. Physical Review Letters.
Martineau, K., & Martineau, K. (2021, July 27). In virtual outer space, a black HOLE sheds its magnetic hair. Columbia News.
Sumner, T. (2021, July 28). Magnetic ‘balding’ of black holes saves general relativity prediction. Simons Foundation.

Written by Vladislav Tchakarov

Hello, my name is Vladislav and I am glad to have you here on Curiosmos. My experience as a freelance writer began in 2018 but I have been part of the Curiosmos family since mid-2020. As a history student, I have a strong passion for history and science, and the opportunity to research and write in this field on a daily basis is a dream come true.

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