Stellar black holes are formed when a massive star reaches the end of its life cycle and explodes in a supernova. The closest example of such a black hole to Earth has now been detected by astronomers at a distance of 1560 light years. It is an inactive and therefore invisible black hole of ten solar masses orbited by a sun-like star. Subtle changes in his movements betrayed the presence of the "silent" partner. So far, however, it is unclear how this pair came about, because the distance between the star and the black hole does not fit with common ideas.
There are an estimated hundred million stellar black holes in our home galaxy, the Milky Way. So far, however, astronomers have only been able to track down a tiny fraction of them – a few dozen specimens. These black holes were mostly part of binary systems and gave themselves away because they sucked material from their companion star. The gas then circling around the event horizon as an accretion disk heats up so much that it emits high-energy X-rays and the entire system becomes visible as an X-ray binary star. Some other stellar black holes have been discovered from the gravitational waves released when they merge with another black hole or with a neutron star.
Search for "silent" black holes
However, it is much more difficult to find inactive stellar black holes. Because they don't suck in any matter, no radiation emanates from their immediate surroundings. At the same time, these holes absorb all radiation that gets into the area of their attraction. It is true that in recent years there have been several reports of supposed discoveries of dormant black holes. Mostly they were based on anomalies in the orbit or the spectrum of companion stars of these "dark" partners. But most of these candidates have been put into perspective or even refuted by follow-up studies. Others, such as an inactive black hole in the binary star system V723 Mon, discovered in 2021 just 1500 light-years away, have not yet been confirmed.
Astronomers led by Kareem El-Badry from the Max Planck Institute for Astronomy in Heidelberg and the Harvard-Smithsonian Center for Astrophysics in the USA have now taken a slightly different approach. For their search for silent black holes, they evaluated the recently published third dataset from the European space telescope Gaia. This also contains the movement data and positions for a good 800,000 double star systems in our galaxy. In this data, El-Badry and his team specifically searched for systems in which one of the stars shows the orbit changes typical of the presence of a more massive partner, but telescope observations do not find any visible partners at this point. The team found six candidates whose movements could indicate a nearby inactive black hole. To check these candidates, the astronomers also observed the six star systems with several ground-based telescopes, including the Gemini North telescope in Hawaii, in order to use changes in radial velocity and the light spectra to narrow down the movements and masses of the objects involved.
Invisible partner of a sun-like star
One candidate stood up to all scrutiny: Gaia BH1, a binary system about 1560 light-years away, made up of a sun-like star and a heavier invisible companion - an inactive black hole. "Our follow-up analyzes with Gemini have unequivocally confirmed that this binary contains a normal star and at least one inactive black hole," the astronomers report. "We could not find a plausible astrophysical scenario that could explain the observed orbit of the system without the participation of at least one black hole." Gaia BH1 is thus the first clearly detected silent stellar black hole in the Milky Way - and at the same time the stellar black hole closest to us , as the astronomers explain. “I've been looking for a system like Gaia BH1 for the last four years, trying all sorts of methods - but none of them worked. I am all the more pleased that this search has finally been successful,” says El-Badry.
According to more detailed analyses, the invisible black hole in Gaia BH1 weighs around ten solar masses, while the sun-like companion star only weighs around one solar mass. Both objects orbit each other at a distance equal to that of the Earth from the Sun. "With an orbital period of 185.6 days, the orbit is further than any other known stellar black hole binary," report El-Badry and his colleagues. But this is exactly what poses problems for the astronomers, because current educational models do not fit this system. According to calculations, the progenitor star of the black hole had a mass of at least 20 solar masses and was therefore correspondingly short-lived. It should therefore have inflated to become a supergiant and devoured it in the early days of the smaller companion star. If both partners were closer, interactions with the companion star could have prevented this inflation. But that was obviously not the case, as El-Badry and his team explain.
So far, astronomers can only speculate as to how this black hole-star pair came about. It would therefore be conceivable that the system might have originated in a star cluster. Then gravitational turbulence could have subsequently brought the smaller companion star, which was previously orbiting further out, into its orbit. It would also be possible that Gaia BH1 was a triple star system with two closely orbiting massive stars. These prevented each other from becoming supergiants, and then formed two black holes that now orbit each other so closely that they appear as one. Closer study of this unusual system could determine if this is the case.
Source: Kareem El-Badry (Center for Astrophysics | Harvard & Smithsonian, Cambridge, USA) et al., Monthly Notices of the Royal Astronomical Society, doi: 10.1093/mnras/stac3140