Black holes and neutron stars may collide unseen in dense star clusters

Astronomers have yet to witness a collision between a black hole and a neutron star, but they predict that such a smashup would unleash huge amounts of energy but, unexpectedly, might not generate any detectable light, a new study finds.

These findings reveal how key details about mergers between black holes and neutron stars, such as the amount of detectable light and the mass of the colliding objects, can shed light on contributing factors behind the mergers, such as the dynamics that drove these smashups to happen. 

Both black holes and neutron stars are corpses of stars that died in catastrophic explosions known as supernovas, outbursts that can make a star briefly outshine all of the other stars in its galaxy. When a star goes supernova, the core of its remains collapses under the strength of its own gravitational pull. If this remnant is massive enough, it may form a black hole, which has gravity so powerful that not even light can escape it. A less massive core will form a neutron star, so named because its gravity is strong enough to crush protons together with electrons to form neutrons.

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So far, scientists have witnessed mergers of black holes with other black holes and mergers of neutron stars with other neutron stars. Astronomers now await the first detection of a black hole merging with a neutron star — a collision that might yield a trove of insights regarding the evolution of stars and Einstein's theory of general relativity, the best description yet of how gravity works.

Researchers have two ways to witness these giant mergers. They can look for the types of light, or electromagnetic radiation, that the collisions emit, including radio waves, infrared rays, visible light, ultraviolet rays, X-rays and gamma-rays. Or, they can try to detect ripples in the fabric of space and time known as gravitational waves.

Scientists currently have a solid theoretical framework of what a merger between a neutron star and a black hole might look like if the two objects were born from binary stars (pairs of stars that orbit each other) in relative isolation from anything else around them.

"The two stars go through their evolutionary stages influencing each other — for example, sharing a common envelope [cloud of gas], transferring matter from one to the other [and] increasing or decreasing their mutual distances," study author Manuel Arca Sedda, an astrophysicist at Heidelberg University in Germany, told Space.com.

Previous research suggested that mergers between black holes and neutron stars might occur up to 100 times per year per billion cubic parsecs — a volume equivalent to about 34.7 billion cubic light-years. 

However, much remains uncertain about the way these dead stars might interact when they're surrounded by stars in a dense cluster of up to millions of stars. Now, Arca Sedda finds this scenario can prove significantly different from isolated mergers.

Arca Sedda conducted 240,000 computer simulations of how pairs of neutron stars and black holes behaved within dense clusters. He focused on scenarios where a binary pair consisting of a neutron star and a companion star had an encounter with a black hole, and where a black hole and a companion star had a run-in with a neutron star. He varied the masses and orbits of all of these objects, as well as the basic properties of the other stars in the cluster, such as their elemental composition and velocity.

One unusual discovery was that, in dense clusters, a black hole and a neutron star could merge without generating any detectable light, although the merger would still generate an extraordinary number of gravitational waves. This can happen when a neutron star plunges into a black hole without turning into hot, bright shreds. Such a scenario is easier when the black hole is more than 10 times the mass of the sun — big enough to swallow the neutron star. 

Another way these mergers within dense clusters would differ from isolated mergers is that they would often possess heavier black holes, ones averaging more than 20 times the mass of the sun. In comparison, in isolated mergers between black holes and neutron stars, black holes are often about seven times the mass of the sun and, in general, no more than 20 solar masses, according to a separate study published in the journal Monthly Notices of the Royal Astronomical Society in 2018.

These findings suggest that if mergers between black holes and neutron stars happened in dense clusters, they would have unique qualities that scientists could use to tell these mergers apart from isolated mergers, the study noted. Gravitational-wave observatories such as the European Space Agency's Laser Interferometer Space Antenna (LISA) mission, which is scheduled to launch in 2034, may be able to detect such mergers in dense clusters if they happened about as far from Earth as the Andromeda galaxy, the Milky Way's closest galactic neighbor. In the future, more advanced gravitational-wave observatories, by contrast, may be sensitive to higher frequencies and thus could spot closer mergers.

In the future, Arca Sedda plans to model a heavy star cluster with about a few million stars, "accounting for all the stars simultaneously and checking the formation of neutron star-black hole mergers," he said. However, he noted that it will be a massive undertaking."Up to now," Arca Sedda said, "there exist only five simulations accounting for 1 million stars in the world."

Arca Sedda detailed his findings online March 5 in the journal Communications Physics.

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