Black holes form when stars die. It would seem that nothing can escape from them. However, they may actually have a characteristic such as a charge, and hence a magnetic field. Recent simulations show that they may inherit them from stars.
Mystery of a black hole’s magnetic field
Black holes are some of the most mysterious stellar objects. Best known for sucking everything around them into a gravitational pit from which nothing can escape, but they can also shoot out powerful jets of charged particles, resulting in explosive bursts of gamma rays that can release more energy in a matter of seconds than our Sun has emitted in its entire existence.
In order for such an impressive event to occur, a black hole should carry a powerful magnetic field. However, where this magnetism comes from remains a long-standing mystery.
Using calculations of black hole formation, scientists at the Flatiron Institute and their colleagues have finally found the source of these magnetic fields: the collapsing parent stars of the black holes.
Black holes can form after a star explodes as a supernova, leaving behind a dense residual core called a proto-neutron star.
“Proto-neutron stars are the mothers of black holes in that when they collapse, a black hole is born. What we are seeing is that as this black hole forms, the proto-neutron star’s surrounding disk will essentially pin its magnetic lines to the black hole,” says Ore Gottlieb, first author of the study and a research associate at the Center for Computational Astrophysics (CCA) at the Flatiron Institute in New York City.
Mystery of magnetism in black holes
At first, the team aimed to model a star’s path from birth to collapse and black hole formation. With their simulations, the team planned to study the jet-like expulsions from the black hole that generate bursts of gamma rays. However, Gottlieb’s team ran into a problem with the models.
There have been several theories regarding black holes and their magnetism, but none of them seem to take into account the power of black hole jets and gamma ray bursts.
“What had been thought to be the case is that the magnetic fields of collapsing stars are collapsing into the black hole,” says Gottlieb. “During this collapse, these magnetic field lines are made stronger as they are compressed, so the density of the magnetic fields becomes higher.”
The problem with this explanation is that the strong magnetism in the star causes the star to lose rotation. And without rapid rotation, a newborn black hole cannot form an accretion disk — a flow of gas, plasma, dust and particles around the black hole — and cannot produce the jets and bursts of gamma rays we have observed.
“It appears to be mutually exclusive,” says Gottlieb. “You need two things for jets to form: a strong magnetic field and an accretion disk. But a magnetic field acquired by such compression won’t form an accretion disk, and if you reduce the magnetism to the point where the disk can form, then it’s not strong enough to produce the jets.”
That meant something else was going on, and scientists sought to find out what it was by going directly to the source: the black hole’s progenitor.
Magnetic progenitor of a black hole: a neutron star
To see a more complete picture of the whole process, the scientists used the idea that the accretion disk could possibly preserve the neutron star’s magnetic field. Thus, a black hole is formed with the same magnetic field lines that permeated the neutron star.
The team’s calculations showed that when a neutron star collapses before its entire magnetic field is absorbed by the newly formed black hole, the neutron star’s disk is inherited by the black hole, and its magnetic lines of force become stationary.
“We ran calculations for the typical values that we expect to see in these systems, and in most cases, the timescale for black hole disk formation is shorter than that of the black hole losing its magnetism,” says Gottlieb. “So the disk enables the black hole to inherit a magnetic field from its mother, the neutron star.”
Consequences for the entire space
Gotlieb is excited about the new discovery, not only because it solves a long-standing mystery, but also because it opens the door to further research on jets.
“This study changes the way we think about what types of systems can support jet formation because if we know that accretion disks imply magnetism, then in theory, all you need is an early disk formation to power jets,” he says. “I think it would be interesting for us to rethink all of the connections between populations of stars and jet formation now that we know this.”
Gotlieb thanks the scientific team and CCA capabilities for making this work possible.
Provided by phys.org
