Despite decades of study, black holes are still one of the most puzzling objects in the Universe. As we know from Einstein’s Theory of General Relativity, the gravitational force of these stellar remnants alters the curvature of spacetime around them. This causes gas, dust, and even photons (light) in their vicinity to fall inwards and form disks that slowly accrete onto their faces, never to be seen again. However, astronomers have also noted that they can produce powerful jets that accelerate charged particles to close to the speed of light (aka. relativistic jets).
These jets lead to powerful gamma-ray bursts (GRBs), which have been observed with black holes that have powerful magnetic fields. However, where these magnetic fields come from has remained a mystery to astrophysicists for some time. According to new research led by scientists from the Flatiron Institute, the source of these fields may have finally been revealed. Based on a series of simulations they conducted that modeled the life cycle of stars from birth to collapse, they found that black holes inherit their magnetic fields from the parent stars themselves.
The research was led by Ore Gottlieb, a Research Fellow from the Theoretical High Energy Astrophysics (THEA) group at the Flatiron Institute’s Center for Computational Astrophysics (CCA) and Columbia University’s Astrophysics Laboratory. He was joined by colleagues from the CCA and CAL and researchers from the University of Arizona, the Steward Observatory, and Princeton University. The paper that details their findings was published on November 18th in the Astrophysical Journal Letters.
Black holes form from the collapse of proto-neutron stars, which are essentially what remains after massive stars have blown off their outer layers in a supernova explosion. While there have been a few theories about where black holes get their magnetism, none could account for the power of black hole jets or GRBs. Through their simulations, the team initially planned to study outflows from black holes, including the jets that produce GRBs. However, as Gottlieb’s explained in a Simons Foundation press release, the team ran into a problem with the models:
“We were not sure how to model the behavior of these magnetic fields during the collapse of the neutron star to the black hole. So, this was a question that I started to think about for the first time. What had been thought to be the case is that the magnetic fields of collapsing stars are collapsing into the black hole. During this collapse, these magnetic field lines are made stronger as they are compressed, so the density of the magnetic fields become higher.”
The only problem with this theory is that the strong magnetic fields of neutron stars cause them to lose angular momentum (their rotation). Without this, the gas, plasma, and dust surrounding newly formed black holes will not form an accretion disk around them. This, in turn, would prevent black holes from producing the jets and gamma-ray bursts that astronomers have observed. This suggests that previous simulations of collapsing neutron stars didn’t provide a complete picture. Said Gottlieb:
“It appears to be mutually exclusive. 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. Past simulations have only considered isolated neutron stars and isolated black holes, where all magnetism is lost during the collapse. However, we found that these neutron stars have accretion disks of their own, just like black holes. And so, the idea is that maybe an accretion disk can save the magnetic field of the neutron star. This way, a black hole will form with the same magnetic field lines that threaded the neutron star.”
The team ran calculations for neutron stars collapsing to form black holes and found that, in most cases, the timescale for black hole disk formation is often shorter than that of the black hole losing its magnetism. In short, before a newly formed black hole swallows a proto-neutron star’s magnetic field, its magnetic field lines become anchored in the neutron star’s surrounding disk passes to the black hole. As Gottlieb characterized it:
“So the disk enables the black hole to inherit a magnetic field from its mother, the neutron star. 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. It’s very exciting to finally understand this fundamental property of black holes and how they power gamma ray bursts — the most luminous explosions in the universe.”
This discovery resolves the long-standing mystery of where black holes get their magnetic fields. It also presents astronomers with new opportunities to study relativistic jets and gamma-ray bursts, one of the most powerful phenomena in the Universe. If confirmed, these results suggest that forming an early accretion disk is the only thing needed for powerful jets to emerge. Gottlieb and his team are excited to test this theory with future observations.
Further Reading: Simons Foundation, Astrophysical Journal Letters