Might the universe’s most extreme black holes owe their existence to magnetism? In 2023, gravitational wave detectors LIGO, Virgo, and KAGRA recorded an event that defied astrophysical logic: GW231123, the collision of two black holes weighing about 100 and 140 solar masses apiece and each spinning nearly at the speed of light. According to established stellar evolution theory, such black holes should be impossible. Yet there they were, 7 billion light-years away, merging in a burst of warped spacetime.
The problem lay in the so-called “mass gap.” Stars between roughly 70 and 140 solar masses are expected to end their lives in pair-instability supernovae-explosions so violent they leave no remnant at all. Black holes in this range can form only indirectly, through mergers of smaller black holes, but that process scrambles spin. The GW231123 pair retained extreme rotation, eliminating the merger-origin hypothesis. Something else had to explain their creation.
Researchers at the Flatiron Institute’s Center for Computational Astrophysics approached the puzzle with end-to-end computational simulations, tracking a 250-solar-mass star from hydrogen burning to collapse. By the time the star reached its terminal supernova, nuclear fusion had reduced its mass to about 150 solar masses-just above the forbidden range. The team then added a missing ingredient: magnetic fields.
In the wake of its collapse, the debris cloud of a nonrotating star falls quickly into the newborn black hole. But when the progenitor is spinning rapidly, the wreckage organizes itself into an accretion disk-a flattened, spinning configuration feeding the black hole. Magnetic fields that thread this disk exert pressure, driving matter outward at nearly light speed. The stronger the magnetic fields, the more material is expelled, leaving less mass available to the black hole. In the most extreme case, as much as half the mass of the star is blown away, launching the final black hole into the mass gap without requiring any violation of supernova physics.
“We found the presence of rotation and magnetic fields may fundamentally change the post-collapse evolution of the star, making black hole mass potentially significantly lower than the total mass of the collapsing star,” said Ore Gottlieb, lead astrophysicist on the study. Strong magnetic fields yield lighter, slower-spinning black holes, while weaker fields produce heavier, faster rotators; this is the striking relation the simulations unveiled. The discovered mass-spin correlation could be a universal pattern in the evolution of black holes.
The implications reach far beyond theory. The team predicts that the creation of mass-gap black holes through this mechanism leads to gamma-ray bursts extremely brief, very powerful releases of high-energy radiation. Observations of such bursts in concert with gravitational waves may confirm the model and specify how frequently these “forbidden” black holes really form. Analogs of these processes have played out in black hole–neutron star mergers, where magnetic field strength and disk mass determine jet luminosity and duration.
The GW231123 detection also tested the limits of gravitational-wave astronomy. Its extreme spins require waveform models capable of handling near–light-speed rotation, pushing computational tools to their limits. The LVK Collaboration’s analysis matched this intricate signal to Einstein’s general relativity with remarkable precision, verifying predictions about rotating black hole deformation. These high-fidelity data sharpen not only astrophysical models but also open avenues to probe exotic physics, including searches for ultralight bosons hypothetical particles that could sap rotational energy from black holes.
By incorporating magnetohydrodynamics into stellar collapse simulations, the CCA team has reframed the origin story of some of the universe’s most enigmatic objects. What once appeared impossible now seems a natural consequence of rotation, magnetism, and relativistic outflows-a reminder that even in the most extreme corners of the cosmos, overlooked physics can rewrite the rules.
