Black holes are not what they eat. Einstein’s general theory of relativity predicts that no matter what a black hole consumes, its external properties depend only on its mass, rotation, and electric charge. All other details of its diet disappear.
Astrophysicists call this nonsense disbelief. (Black holes are said to be “no hair.”)
However, there is a potentially hairy threat to the assumption. Black holes can be created by a strong magnetic field or obtained by tapping the magnetized material. Such a field has quickly disappeared so that non-hair conjectures should take place. However, there are no real black holes in isolation. They can be surrounded by plasma – a gas so energized that electrons have detached from its atom – that can maintain a magnetic field and possibly refute the assumption.
Using simulations of plasma intruded black hole supercomputers, researchers at Computational Astrophysics (CCA) at the Flatiron Institute in New York, Columbia University, and Princeton University found that no-hair assumptions were true. The group reports its findings 27. July in Physical Review Letters.
“The non-hair assumption is the cornerstone of the overall relativity industry,” says research author Bart Ripperda, a CCA researcher and Princeton postdoctoral researcher. “If a black hole has a long-lived magnetic field, the hairless assumption is broken. Plasma physics, fortunately, became the solution that saved the non-hair assumption from breaking.”
The team’s simulations showed that the lines of the magnetic field around the black hole break quickly and reconnect, creating plasma-filled pockets that open into space or fall into the blind spot of the black hole. This process quickly clears the magnetic field and may explain the flares visible near the supermassive black holes, the researchers report.
“Theorists didn’t think about this because they usually put their black holes in a vacuum,” Ripperda says. “But in real life, plasma is often, and plasma can maintain and bring in magnetic fields. And it has to be consistent with your non-hair conjecture.”
Ripperda has co-authored the study with Columbia graduate student Ashley Bransgrove and CCA research worker Sasha Philippov, who is also a Visiting Researcher at Princeton.
A 2011 study of the problem revealed that non-hair assumptions were in difficulty. However, the study looked at these systems only at low resolution and treated plasma as a liquid. However, the plasma around the black hole is so diluted that the particles rarely collide with each other, so handling it as a liquid is a simplification.
In a new study, the researchers performed high-resolution plasma physics simulations with a general-relativistic model of a black hole magnetic field. It took 10 million CPU hours to complete all the calculations. “We couldn’t have done these simulations without the computational resources of the Flatiron Institute,” Ripperda says.
The resulting simulations showed how the magnetic field around the black hole develops. Initially, the field extends in an arc from the north pole of the black hole to the south pole. Then the interactions inside the plasma cause the field balloon outward. This opening causes the field to disintegrate into individual lines of magnetic field radiating outward from the black hole.
The field lines alternate in a direction either toward or away from the event horizon. The nearby magnetic field lines form a connection, creating a braided pattern of field lines that come together and split. There is an opening between two such connection points that fills the plasma. The plasma is powered by a magnetic field that fires outward into space or inward into a black hole. As the process continues, the magnetic field loses energy and eventually withers.
Critically, the process happens quickly. The researchers found that a black hole consumes its magnetic field at a rate of 10 percent of the speed of light. “The quick reconnection saved the undoubted speculation,” Ripperda says.
The researchers suggest that the mechanism by which the observed flares of the supermassive black hole in the center of the Messier 87 galaxy are activated can be explained by the balding process observed in the simulations. Preliminary comparisons between them seem promising, although they feel a more accurate assessment is needed. If they do line up, the energetic flares operating by magnetic reconnection in the black horizon event horizons may be a widespread phenomenon.
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