Magnetic fields solve mystery of 'impossible' black hole merger

In November 2023, three gravitational wave detectors on Earth picked up a signal from a collision that shouldn't have happened. Two black holes, one a hundred times the mass of the sun and the other a hundred and forty times heavier, had smashed together seven billion light-years away. They were also spinning at nearly the speed of light. According to everything astronomers understood about how stellar mass black holes form—through the collapse and explosion of massive stars—these objects should not exist. Yet there they were, their merger rippling through space-time in a way that instruments could measure.

The puzzle was immediate and vexing. Stars massive enough to leave behind black holes of this size should end their lives in what's called a pair-instability supernova, an explosion so violent that nothing remains behind, not even a black hole. This creates what physicists call the mass gap: a range between roughly seventy and a hundred and forty times the sun's mass where stellar mass black holes simply shouldn't form. The two black holes detected in the signal, designated GW231123, fell squarely inside this forbidden zone. Researchers considered whether they might have formed through earlier mergers, but that didn't work either. Mergers scramble the spin of the resulting black hole, yet these two were still rotating at maximum speed. Something else had to explain their existence.

Ore Gottlieb and his colleagues at the Flatiron Institute's Center for Computational Astrophysics in New York decided to rebuild the problem from the beginning. They started with a simulation of a giant star weighing around two hundred and fifty times what the sun weighs and tracked it through its entire life cycle until the moment it exploded. What they found was straightforward: by the time the star reached the end of its life, it had burned through so much fuel that it had shrunk to about a hundred and fifty solar masses. That was light enough to leave behind a black hole when the supernova occurred.

But the real breakthrough came when they added one element that previous research had largely ignored: magnetic fields. In their second simulation, they modeled what happens in the aftermath of a supernova—a cloud of leftover stellar material tangled with magnetic fields, with a newborn black hole at its center. The conventional assumption had been that all of this debris would eventually fall into the black hole, making it as massive as the original star. What Gottlieb's team observed instead was far more nuanced. When the progenitor star was spinning rapidly, the leftover material formed a rotating, flattened disk around the black hole. As more material spiraled inward, the black hole spun faster and faster. But here was the crucial part: the magnetic fields in that disk generated pressure strong enough to blast some of the stellar wreckage away from the black hole at nearly the speed of light. The stronger the magnetic fields, the more material got ejected. If the fields were powerful enough, half the star's original mass could be stripped away entirely.

This mechanism solved the mystery. A black hole that formed from a rapidly spinning star in the presence of strong magnetic fields would end up lighter and slower-spinning than theory had predicted. Weaker magnetic fields would allow more material to accumulate, resulting in more massive and faster-spinning black holes. The two black holes that merged in 2023 could have formed in the mass gap after all, their properties shaped by the magnetic environment surrounding them at the moment of their birth.

The implications extend beyond solving a single puzzle. Gottlieb's simulations suggested that the formation of these mass-gap black holes should be accompanied by a burst of gamma rays—high-energy radiation that astronomers can detect. If such a detection is made, it would confirm the mechanism and open a new window into understanding how black holes form and evolve. The connection between a black hole's mass, its spin, and the strength of the magnetic fields around it may prove to be one of the fundamental relationships in astrophysics, waiting to be tested against the next generation of observations.