Mysterious ‘impossible’ merger of two massive black holes explained

(Press-News.org) In 2023, astronomers detected a huge collision. Two unprecedentedly massive black holes had crashed an estimated 7 billion light-years away. The enormous masses and extreme spins of the black holes puzzled astronomers. Black holes like these were not supposed to exist.

Now, astronomers with the Flatiron Institute’s Center for Computational Astrophysics (CCA) and their colleagues have figured out just how these black holes may have formed and collided. The astronomers’ comprehensive simulations — which follow the system from the lives of the parent stars through to their ultimate death — uncovered the missing piece that previous studies had overlooked: magnetic fields.

“No one has considered these systems the way we did; previously, astronomers just took a shortcut and neglected the magnetic fields,” says Ore Gottlieb, astrophysicist at the CCA and lead author of the new study on the work published in The Astrophysical Journal Letters. “But once you consider magnetic fields, you can actually explain the origins of this unique event.”

The collision detected in 2023, now known as GW231123, was observed by the LIGO-Virgo-KAGRA collaboration using detectors that measure gravitational waves, the ripples in space-time caused by the movements of massive objects.

At the time, astronomers couldn’t fathom how such large fast-spinning black holes came to exist. When massive stars reach the end of their lives, many collapse and explode as a supernova, leaving behind a black hole. But if the star falls within a specific mass range, a special type of supernova occurs. This explosion, called a pair-instability supernova, is so violent that the star is annihilated, leaving nothing behind.

“As a result of these supernovae, we don’t expect black holes to form between roughly 70 to 140 times the mass of the sun,” Gottlieb says. “So it was puzzling to see black holes with masses inside this gap.”

Black holes in this mass gap can be formed indirectly, when two black holes merge to form a larger black hole, but in the case of GW231123, scientists thought this was improbable. The merging of black holes is a tremendously chaotic event that often disrupts the spin of the resulting black hole. The black holes of GW231123 were the fastest spinners seen by LIGO, dragging space-time around them at nearly the speed of light. Two black holes of their size and spin are incredibly unlikely, so astronomers thought something else must be at work.

Gottlieb and his collaborators investigated by conducting two stages of computational simulations. They first simulated a giant star 250 times the mass of the sun through the main stage of its life, from when it starts burning hydrogen to when it runs out and collapses in a supernova. By the time such a massive star had reached supernova stage, it had burned through enough fuel to slim down to just 150 times the sun’s mass, making it just above the mass gap and large enough to leave a black hole behind.

A second set of more complex simulations, which accounted for magnetic fields, dealt with the aftermath of the supernova. The model started with the supernova remnants, a cloud of leftover stellar material laced with magnetic fields and a black hole at its center. Previously, astronomers assumed that the entire mass of the cloud would fall into the newborn black hole, making the black hole’s final mass match that of the massive star. But the simulations showed something different.

After a nonrotating star collapses to form a black hole, the cloud of leftover detritus quickly falls into the black hole. However, if the initial star was spinning rapidly, this cloud forms a spinning disk that causes the black hole to spin faster and faster as material falls into its abyss. If magnetic fields are present, they exert pressure on the disk of debris. This pressure is strong enough to eject some of the material away from the black hole at nearly the speed of light.

These outflows ultimately reduce the bulk of material in the disk that eventually feeds into the black hole. The stronger the magnetic fields, the greater this effect. In extreme cases with very strong magnetic fields, up to half of the star’s original mass can be ejected through the black hole’s disk ejecta. In the case of the simulations, the magnetic fields ultimately created a final black hole in the mass gap.

“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,” Gottlieb says.

The results, Gottlieb says, suggest a connection between the mass of a black hole and how fast it spins. Strong magnetic fields can slow down a black hole and carry away some of the stellar mass, creating lighter and more slowly spinning black holes. Weaker fields allow heavier and faster-spinning black holes. This suggests black holes may follow a pattern that ties their mass and spin together. While astronomers know of no other black hole systems on which this connection can be observationally tested, they hope future observations may find more such systems that could confirm this connection.

The simulations also show that the formation of these types of black holes creates bursts of gamma rays, which might be observable. Looking for these gamma ray signatures would help confirm the proposed formation process and reveal how common these massive black holes might be in the universe. Ultimately, if such a connection is confirmed, it would help astronomers gain a deeper understanding of the fundamental physics of black holes.

About the Flatiron Institute

The Flatiron Institute is the research division of the Simons Foundation. The institute's mission is to advance scientific research through computational methods, including data analysis, theory, modeling and simulation. The institute's Center for Computational Astrophysics creates new computational frameworks that allow scientists to analyze big astronomical datasets and to understand complex, multi-scale physics in a cosmological context.

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