Missing link' black hole discovered in unusual binary system with red giant star

 

The binary system G3425 exhibits some unusual and difficult-to-explain features.

Astronomers recently discovered an exciting new black hole in the G3425 binary system, located about 5,800 light-years away. This discovery is even more thrilling because the black hole may belong to the "missing link" category of stellar-mass black holes. The visible star in this system is a red giant, a type of star formed when a star runs out of hydrogen fuel, causing its outer layers to expand dramatically. Our sun is expected to undergo a similar process in about 5 billion years, swelling to the orbit of Mars. However, unlike G3425, it is highly unlikely to have a black hole as a companion.

In G3425, the red giant star has a mass around 2.7 times that of our sun. What truly fascinates scientists, though, is the black hole's mass, estimated to be between 3.1 and 4.4 times the sun's mass. This makes it one of the lightest black holes ever found and places it in the so-called "mass gap"—a category of black holes that are rarely observed.

Wang Song, from the National Astronomical Observatories of the Chinese Academy of Sciences (NAOC), led the team that weighed the black hole. He noted that the discovery confirms the existence of mass-gap black holes and demonstrates that such low-mass black holes can exist in binary systems that survive supernova explosions.

The team detected the black hole using data from the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) and the Gaia space telescope. These tools allowed scientists to observe the black hole's gravitational pull on the red giant, which exposed its presence. The research has been published in the journal Nature Astronomy.

Mystery of the missing masses

Black holes have only a few defining characteristics, a fact highlighted by physicist John Wheeler’s famous remark, "Black holes have no hair." Aside from electric charge and angular momentum, one key feature that distinguishes black holes is their mass.

The most massive black holes are the supermassive ones, which reside at the centers of most large galaxies. These giants have masses ranging from millions to billions of times that of our sun. A step down from them are the elusive intermediate-mass black holes, with masses between 100 and 100,000 solar masses.

Neither of these types of black holes can form directly from a collapsing star. Instead, they grow over time by voraciously consuming matter from their surroundings or through repeated mergers with other black holes.

At the smaller end of the scale are stellar-mass black holes, the smallest astrophysical black holes we know of. These have masses up to 100 times that of the sun and are formed when a star at least eight times the sun’s mass runs out of fuel. As the star can no longer fuse heavier elements to counteract the inward pull of gravity, it collapses to form a stellar-mass black hole. 

Over the past six decades, astronomers have discovered many stellar-mass black holes, generally ranging from 5 to 25 times the mass of our Sun. This fits well with our current understanding, but there's a puzzle.

Theories about black hole formation suggest that stars retaining just three times the mass of the Sun, after shedding most of their mass in a supernova, should still form stellar-mass black holes. This raises the question: where are the 3 to 5 solar mass black holes? One explanation might be that during a supernova, some unknown mechanism prevents the formation of black holes in this mass range.

Another possibility is that lower-mass black holes could be more prone to being "kicked" away by the explosive forces of a supernova, making them less likely to remain in binary systems with visible companions, which are the systems we most easily observe.

All black holes, regardless of size, are surrounded by an event horizon, a boundary beyond which not even light can escape, rendering them invisible. However, black holes in binaries, like the one in the system G3425, can be detected through the influence they exert on their companion stars. A "supernova-kicked" black hole without a companion would be much harder to detect.

This brings up another question: how did the black hole in G3425 manage to avoid this kick and stay bound to its companion?

Additionally, G3425's binary system presents another mystery. It has a circular orbit, with a period of about 880 Earth days and zero eccentricity, meaning the orbit is almost perfectly round. Current binary evolution models struggle to explain this.

As astronomer Song put it, "The wide, circular orbit of this binary is unexpected, especially involving a low-mass black hole. It presents a real challenge to our current theories of binary evolution and supernova explosions."

Although G3425's red dwarf and black hole binary is puzzling, it also offers valuable insights. It shows that quiet, invisible black holes can be detected through their effects on companion stars, and studying these elusive low-mass black holes could shed light on the evolution of binary systems.