Black holes capture magnetic fields from collapsing stars

Black holes need no introduction. Their ability to pull everything that crosses their boundary into a gravity pit is legendary. But did you know they can also spew out charged particles resulting in explosive bursts of gamma rays?

In an instant, black holes can unleash more energy than our Sun will produce over its entire lifetime. This extraordinary phenomenon owes its intensity to a hidden asset: a formidable magnetic field.

But where this magnetic power originates from has remained an unsolved mystery, until now.

Origin of magnetic fields

Experts at the Flatiron Institute, backed by their collaborators, have managed to identify the source of these powerful magnetic fields.

Through meticulous calculations of black hole formation, they have traced the origin of the magnetic fields back to the collapsing parent stars of the black holes themselves.

Black holes are not born out of the blue. They emerge after a star explodes as a supernova, leaving a dense remnant core known as a proto-neutron star.

The mothers of black holes

Ore Gottlieb, the study’s first author, is a research fellow at the Flatiron Institute’s Center for Computational Astrophysics (CCA) in New York City.

“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,” said Gottlieb. 

“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.”

Dissecting the magnetic enigma of black holes

The journey of a star from birth, collapse, to the formation of a black hole is akin to a cosmic rollercoaster.

The team set out to model this journey in an effort to study outflows like jets that generate gamma ray bursts. However, they hit a roadblock.

The behavior of these magnetic fields during the stellar collapse presented a modeling challenge. There were pre-existing theories about black holes and their magnetic prowess, but none added up when considering the power behind a black hole’s 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,” explained Gottlieb. “During this collapse, these magnetic field lines are made stronger as they are compressed, so the density of the magnetic fields become higher.”

The team determined that a different process had to be at play and focused their attention on a black hole’s progenitor.

Magnetic maestro: The neutron star

The researchers felt that previous simulations may not have presented the complete picture.

“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,” said Gottlieb.

“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’s calculations revealed that before the newly formed black holes gobble up all the magnetic fields of their collapsing neutron stars, the disks of the neutron stars transfer their magnetic field lines to the black holes, anchoring them.

“In most cases, the timescale for a black hole’s disk formation is shorter than it losing its magnetism,” said Gottlieb. “So, by having an accretion disk, the black hole can inherit a magnetic field from its parent, the neutron star.”

Echoes across the universe

Beyond solving a longstanding mystery, this discovery has the potential to expand our understanding of 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,” explained Gottlieb.

“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.”

Black holes and their magnetic mysteries

Gottlieb attributes the success of this endeavor to the collaborative nature of the research and the computational resources at the CCA.

“This was a multidisciplinary collaboration that enabled us to address this question from different directions and form a coherent picture of a star’s evolution post-collapse,” said Gottlieb.

“And the generous computational resources of CCA let us run simulations of the collapse more consistently than has ever been done before. I think these two aspects led to an innovative approach.”

With this leap forward, we inch closer to fully comprehending the enigmatic black holes and their magnetic mysteries, shedding more light on the dark wonders of the universe.

The study is published in The Astrophysical Journal Letters.

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