Researchers from the Flatiron Institute, in collaboration with others, have now traced the source of black hole magnetism back to their progenitor stars. This discovery was reported in the November 18 issue of 'The Astrophysical Journal Letters'.
When a star explodes as a supernova, a dense remnant known as a proto-neutron star is left behind. This remnant can eventually collapse into a black hole.
"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," explained Ore Gottlieb, the study's first author and a research fellow at the Flatiron Institute's Center for Computational Astrophysics (CCA) in New York City. "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."
The study was co-authored by CCA researchers Brian Metzger, Jared Goldberg, Matteo Cantiello, and Mathieu Renzo.
Unraveling the Magnetism Mystery
The research team set out to simulate the lifecycle of a star from its formation to its eventual collapse into a black hole, aiming to investigate outflows such as jets responsible for gamma ray bursts. However, they encountered challenges in modeling magnetic field behavior during the collapse.
"We were not sure how to model behavior of these magnetic fields during the collapse of the neutron star to the black hole," said Gottlieb. This prompted a deeper investigation into the origin of the magnetic fields.
Previous theories suggested that a collapsing star's magnetic field lines are compressed as they are absorbed into the black hole, which should theoretically amplify the magnetism. However, this compression would halt the star's rotation. Without rotation, an accretion disk - necessary for the formation of jets and gamma ray bursts - would not form.
"It appears to be mutually exclusive," Gottlieb noted. "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."
Discovering a New Perspective
The team's breakthrough came from considering the accretion disks of collapsing neutron stars. "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," explained Gottlieb. "This led us to propose that an accretion disk could preserve the magnetic field of the neutron star, allowing the black hole to inherit these magnetic field lines."
The researchers' calculations confirmed that, in most cases, the formation of an accretion disk around a black hole occurs more quickly than the loss of the neutron star's magnetic field. This finding supports the idea that black holes can retain the magnetic field of their parent neutron star.
Broader Implications for Astrophysics
Gottlieb emphasized that this discovery has significant implications for the study of jet formation in black holes. "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 stated. "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."
Gottlieb highlighted the collaborative nature of the project and the resources provided by CCA that made this study possible. "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," he said. "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."
Research Report:She's Got Her Mother's Hair: Unveiling the Origin of Black Hole Magnetic Fields through Stellar to Collapsar Simulations
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