New simulations from the Center for Computational Astrophysics at the Flatiron Institute suggest that the formation of black holes in the "forbidden" mass range results from a combination of rapid rotation and strong magnetic fields, which eject up to half of the star's mass and create lighter but faster black holes - as in the gravitational wave event GW231123.
Surprising results from a comprehensive set of simulations by astrophysicists at the Flatiron Institute and their colleagues offer an explanation for the “impossible” merger of two supermassive black holes—an event that until recently was considered almost contrary to the known laws of physics. According to the study, strong magnetic fields around exploding giant stars can create black holes in a mass range that for years was considered almost “forbidden territory.”
The results were published in The Astrophysical Journal Letters. And the work was supported by the Simons Foundation.
In 2023, astronomers detected a massive collision in the event GW231123: two extremely massive black holes collided with each other, at an estimated distance of about 7 billion light-years from Earth. In addition to the very high masses, measurements showed that the two black holes were rotating at extreme speeds, almost the speed of light – a combination that was considered almost completely improbable according to conventional models.
Now, a team of researchers from the Flatiron Institute's Center for Computational Astrophysics (CCA) and their colleagues present a detailed formation scenario that reproduces the unusual properties of GW231123. Their simulations—which follow the system from the giant star stage to black hole formation and beyond—point to a physical component that was missing in most previous work: magnetic fields.
“No one has studied these systems the way we have,” says Dr. Or Gottlieb, an astrophysicist at CCA and the first author of the paper. “In the past, they just skipped over the magnetic fields. Once you bring them into the picture, you can explain the origin of this extraordinary event.”
'Impossible' merger in the tax gap
GW231123 was discovered by the LIGO–Virgo–KAGRA collaboration, which measures gravitational waves – ripples in space-time that originate from the motion of very massive bodies. Upon its discovery, it became clear that the event was unusual: the two black holes that crashed into each other are in a mass range where, according to theory, black holes are unlikely to form “directly” from stars.
When very massive stars end their lives, they collapse and explode as a supernova, often leaving behind a black hole. But for a certain mass range, a particularly extreme process called Pair instability supernova (pair-instability supernova). In such an explosion, the entire star is destroyed – there is no heavy core left that can collapse into a black hole.
"As a result of these supernovae, we don't expect to see black holes that are between about 70 and 140 solar masses," Gottlieb says. "So it was very puzzling to discover black holes that happen to be right in that mass range."
One possibility to get around the “mass gap” is repeated mergers: two smaller black holes merge to form a more massive black hole. But in the case of GW231123, even this explanation seems unlikely. Black hole mergers are an extremely violent process, and they often “scramble” or change the spin of the resulting black hole. The black holes in this event were among the fastest ever measured, twisting space-time around them at nearly the speed of light. The appearance of two black holes that large and that fast in the same system seemed like a very unlikely scenario.
That's where the new simulations come into play.
From the giant star to the black hole – in two stages
Gottlieb and his colleagues took a two-step approach in their simulations.
In the first stage, they simulated the evolution of a giant star, with an initial mass of 250 solar masses, from the moment it begins to burn hydrogen in its core until it exhausts its nuclear fuel and reaches final collapse. By the time such a star reaches the supernova stage, it has already lost a large part of its mass, and is left with about 150 solar masses – still above the mass gap, so if a core remains, it can collapse into a massive black hole.
In the second, more complex step, the researchers simulated what happens after the collapse and explosion, taking magnetic fields into account for the first time. They started with the cloud of gas and material left over from the supernova, which surrounds a new black hole at its center and is permeated with magnetic field lines.
Previous work has assumed that most of the mass of this cloud eventually falls into the black hole, so that the final mass of the black hole is very close to the mass of the collapsed star. The new simulations paint a different picture.
If the original star didn't rotate, the material left behind would simply fall rapidly into the black hole. But if the star was rotating rapidly before collapsing, the material forms a rotating disk around the black hole. This disk imparts angular momentum to the black hole, causing it to spin faster as more material falls in.
When magnetic fields come into play, everything changes: the magnetic lines in the disk exert pressure on the gas, and this pressure can accelerate some of the material outward at enormous speeds – close to the speed of light.
These outflows significantly reduce the amount of material that ultimately feeds into the black hole. The stronger the magnetic field, the more mass is ejected rather than falling in. In extreme cases, up to half of the original star's mass can be ejected through the magnetic disk.
Under the conditions used by the researchers in the simulations, the effect of the magnetic fields led to the formation of a final black hole whose mass actually falls. בתוך The mass gap – that is, too heavy to be formed from a "normal" collapse but significantly lighter than the star from which it was born.
"We found that rotation and magnetic fields can fundamentally change the evolution of the star after collapse," Gottlieb explains. "In such cases, the final mass of the black hole can be much lower than the mass of the collapsed star."
Relationship between mass and spin – and observational implications
One of the most interesting elements of the paper is the proposed relationship between the mass of a black hole and its spin. According to the simulations:
- Very strong magnetic fields pull out a large portion of the mass and slow down the black hole's spin – resulting in lighter, slower-spinning black holes.
- Weaker fields allow more matter to be swallowed, thus forming black holes. Heavy More husbands Quick spin.
If there is indeed a consistent relationship between mass and spin, it could be used in the future to better describe the population of black holes in the universe and understand what processes created them. Currently, GW231123 is the only prominent example of a system in which such a relationship can be tested, but the researchers hope that future gravitational wave events will reveal more “impossible black hole mergers” that can be used to test the model.
The simulations also suggest another observational signature: during the formation of black holes in this mass range, Gamma ray burstsIf gamma-ray observations are found to correlate with similar events, they could directly support the scenario proposed by Gottlieb and his colleagues and indicate the prevalence of such "forbidden" black holes in the universe.
If the connection between magnetic fields, mass, and spin is confirmed observationally, it would provide new insight into the fundamental physics of black holes – objects that are not directly visible, but continue to shape the picture of the universe through gravitational waves, gamma radiation, and their effect on the gas and stars around them.
About the Flatiron Institute
The Flatiron Institute is the research arm of the Simmons Foundation. The institute's mission is to advance scientific research through computational methods—data analysis, theory, modeling, and simulations. The institute's Center for Computational Astrophysics develops new computational frameworks for analyzing large astronomical datasets and understanding complex, multiscale physics in cosmological contexts.
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