When LIGO accidentally reported a massive black hole merger earlier this year, physicists were stunned, but confident they would someday find an explanation. They probably didn’t expect an answer it However, soon.
But just as quickly as the supposedly impossible merger occurred, a possible explanation for it has come along surprisingly quickly. Astronomers ran different simulations of how a massive star might collapse into a smaller-sized black hole than expected – including in the “mass gap” where black holes are not thought to exist. The new analysis, published Nov. 10 in The Astrophysical Journal Letters, shows how magnetic fields can strip some of the mass from black holes, meaning that black holes we thought impossible may actually exist and possibly form more frequently than scientists thought.
“No one has considered these systems the way we have; previously, astronomers simply took a shortcut and ignored magnetic fields,” Ore Gottlieb, an astrophysicist at the Flatiron Institute’s Center for Computational Astrophysics and lead author of the study, said in a statement. “But once you consider the magnetic field, you can really explain the origin of this unique phenomenon.”
A “forbidden” merger
Earlier this summer, the LIGO collaboration released information about GW231123, a gravitational wave signal of the collision and merger of two giant galaxies. Gravitational waves – ripples in space-time from cataclysmic cosmic events – allow researchers to understand key properties of black holes without relying on light-based sources.
What was surprising about GW231123 was that the merger produced such a massive black hole – 225 times the mass of our Sun – that its sheer size was “forbidden” according to standard cosmological models, Mark Hannam, LIGO member and physicist at Cardiff University, explained in a previous statement.
It was also not understood how two black holes, each 137 and 103 times the mass of the Sun, managed to keep themselves together while spinning at a speed 400,000 times the Earth’s rotation speed. These masses also lie within the infamous “mass gap” for black holes that erupt from massive stars, adding to the mystery.
The catastrophic collapse of massive stars, known as pair-instability supernovae, rarely leaves anything behind. Gottlieb explained that the resulting stellar graveyard prevents the formation of black holes in the mass range of 70 to 140 times the mass of the Sun. This is known as the “people gap”.
break the impossible
The team tackled the mystery of these large gaps by running simulations at two different stages to test the feasibility of two black holes from merging GW231123. Specifically, the team traced the entire lifetime of a black hole, starting at the birth of a star 250 times the mass of the Sun.
By the time this hypothetical star had burned enough hydrogen to go supernova, it had fallen to about 150 times the mass of the Sun – just above the mass difference. The second phase of the simulation was more complex, figuring out the black hole’s mass, spin, and magnetic field after the supernova. This is when the discrepancy came to light.
As the dying star headed toward an explosive death, the magnetic field surrounding the stellar graveyard threw some debris away from the black hole at almost the speed of light. This minor removal removed some of the mass for the final black hole – leaving the final product within the mass gap. The study said additional simulations showed that in extreme cases, the effects of the magnetic field could destroy half of the star’s original mass, creating a much smaller black hole.
“We found that rotation and the presence of a magnetic field can fundamentally alter the post-collapse evolution of the star, potentially reducing the mass of the black hole to much less than the total mass of the collapsing star,” Gottlieb said.
But wait, there’s more
The findings challenge previous assumptions that the final mass of a black hole typically matches that of the star it came from. The researchers acknowledged in their paper that different stars may have different results, but the simulations nevertheless presented a possible scenario for GW231123.
Yet, as the researchers admit, this is just a simulation – an approximation of real-life conditions. Looking ahead, the team plans to search for actual black holes that formed under conditions similar to GW231123. Supernovae and the black holes that follow are also incredibly energetic events, producing other astronomical phenomena such as gamma-ray bursts or various energy transients. The paper suggests that these could serve as signatures for finding promising black holes.
The discovery is a remarkable mix of something that both proves and refutes the astrophysical consensus: Collapsing stars can form black holes that fit inside the mass gap, and the black hole’s mass need not closely match the mass of the source star. The simulation is based on well-understood theoretical principles, but its results suggest something contrary to what researchers thought about black holes. In a way, it’s a reminder that the universe is much more complex than we can imagine.