Researchers from the Flatiron Institute, alongside collaborators, have traced the source of black hole magnetism back to their progenitor stars, as detailed in the November 18 issue of 'The Astrophysical Journal Letters.'
When a star undergoes a supernova, it leaves behind a dense remnant known as a proto-neutron star, which can eventually collapse into a black hole. 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, stated, "Proto-neutron stars are the mothers of black holes; when they collapse, a black hole is born. As this black hole forms, the proto-neutron star's surrounding disk will pin its magnetic lines to the black hole." This understanding sheds light on how black holes power gamma-ray bursts, the most luminous explosions in the universe.
The research team simulated the lifecycle of a star leading to its collapse into a black hole, focusing on outflows such as jets responsible for gamma-ray bursts. They faced challenges in modeling magnetic field behavior during the collapse. Gottlieb remarked, "We were not sure how to model the behavior of these magnetic fields during the collapse of the neutron star to the black hole." This led to further investigation into the origin of the magnetic fields.
Previous theories indicated that a collapsing star's magnetic field lines are compressed upon absorption into the black hole, theoretically amplifying magnetism. However, this compression would halt the star's rotation, preventing the formation of an accretion disk necessary for jets and gamma-ray bursts. Gottlieb noted, "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 allow for disk formation, it won't be strong enough to produce jets."
The breakthrough came from considering the accretion disks of collapsing neutron stars. Gottlieb explained, "Past simulations have only considered isolated neutron stars and isolated black holes, where all magnetism is lost during the collapse. We found that these neutron stars have accretion disks of their own, just like black holes. 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."
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.
Gottlieb emphasized the significant implications for jet formation studies in black holes, stating, "This study changes the way we think about what types of systems can support jet formation. If we know that accretion disks imply magnetism, then all you need is early disk formation to power jets." He expressed interest in rethinking connections between star populations and jet formation in light of this discovery.
Gottlieb highlighted the collaborative nature of the project and the resources from CCA that facilitated the study, noting, "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." The computational resources provided allowed for more consistent simulations of the collapse than ever before, contributing to an innovative approach.