Black holes are probably the most extreme objects in the Universe. Supermassive versions of these highly dense objects are quite rare and likely reside only at the centers of large galaxies. However, stellar-mass black holes – which weight from five to 100 times the mass of our Sun – are much more common, with an estimated 100 million in the Milky Way alone. Only a few of these have been found to date, and nearly all of them were “active” – meaning that, unlike dormant black holes, they shine brightly in X-rays while consuming material from a nearby stellar companion.
Now, by using the Gemini North telescope in Hawai‘i – one of the twin telescopes of the InternationalGemini Observatory – operated by National Science Foundation’s NOIRLab, a team of researchers has discovered the closest black hole to Earth. The so-called Gaia BH1 is a dormant black hole located just 1,600 light-years away from our planet in the constellation Ophiuchus. Its discovery was made possible through observations of the motion of the black hole’s companion, a Sun-like star orbiting the black hole at about the same distance as the Earth orbits the Sun.
“While there have been many claimed detections of systems like this, almost all these discoveries have subsequently been refuted. This is the first unambiguous detection of a Sun-like star in a wide orbit around a stellar-mass black hole in our Galaxy,” said study lead author Kareem El-Badry, an astrophysicist at Harvard University.
“I’ve been searching for dormant black holes for the last four years using a wide range of datasets and methods. My previous attempts — as well as those of others — turned up a menagerie of binary systems that masquerade as black holes, but this is the first time the search has borne fruit.”
By measuring the velocity of the companion star as it orbited the black hole, the scientists managed to identify the central body as a black hole about ten times more massive than our Sun. However, astronomers’ current models of the evolution of binary systems cannot clearly explain how the peculiar configuration of the Gaia BH1 system could have arisen. According to the scientists, the progenitor star which later turned into this newly detected black hole should have been at least 20 times as massive as the Sun, meaning that it would have lived only a few million years. If both stars formed at the same time, this star would have quickly turned into a supergiant, supposedly engulfing the other star before it had time to become a proper, hydrogen-burning, main-sequence star like our Sun. However, this did not appear to have happened, suggesting that there are major gaps in our understanding of how black holes form and evolve in binary systems.
“It is interesting that this system is not easily accommodated by standard binary evolution models. It poses many questions about how this binary system was formed, as well as how many of these dormant black holes there are out there,” El-Badry concluded.
An in-depth description of the black hole is published in the journal Monthly Notices of the Royal Astronomical Society.
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