The black hole log books have just been rewritten.
A new study reports that a black hole is about 10 times larger than our sun, only 1,560 light-years from Earth. That’s nearly double the previous proximity champ.
The newly discovered object, a stellar-mass black hole called Gaia BH1, lies in a binary system whose other member is a sun-like star. This star is as far from its companion black hole as Earth is from the Sun, which makes Gaia BH1 really special.
“Although there have been many purported discoveries of systems like this, nearly all of these have been refuted,” said study lead author Karim El-Badri, of the Harvard-Smithsonian Center for Astrophysics in Massachusetts and the Max Planck Institute for Astronomy in Germany said in a statement. (Opens in a new tab). “This is the first unequivocal discovery of a Sun-like star in a wide orbit around a stellar-mass black hole in our galaxy.”
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Astronomers believe that our Milky Way galaxy contains about 100 million stellar-mass black holes, which are light-eating objects five to 100 times larger than the Sun.
However, their small size makes these objects relatively difficult to detect, especially by telescope. (Gravity-wave detectors have had more success recently, finding evidence of mergers involving these objects.) The ones that scientists see tend to be “X-ray binaries,” black holes pulling material from a companion star into an accretion disk around themselves. This rapidly rotating dust and gas emits high-energy X-rays and light that some powerful telescopes can observe.
However, not all stellar-mass black holes inhabiting binary systems are actively feeding. Finding these idle objects is more difficult and requires different strategies.
The researchers used one of these alternative techniques in the new study. They meditated on data collected by the European Space Agency’s (ESA) Gaia spacecraft, which accurately maps the locations, velocities and trajectories of nearly two billion Milky Way stars.
One such star is the Gaia BH1 Companion. Its movement shows small anomalies – an indication that something massive and invisible is pulling on it by the force of gravity.
Gaia’s measurements suggested that the black hole could be this jar, but scientists needed more data to know for sure. So they studied the star with a number of ground-based instruments, including the Gemini North and Keck 1 telescopes in Hawaii and the Magellan Clay and MPG/ESO telescopes in Chile.
These follow-up observations, along with the Gaia data, allowed the team to take the system’s scale in detail. The invisible object has a mass of 10 suns, they identified, and orbits the system’s center of mass once every 186 Earth days. And it must be a black hole.
“Our observations of Gemini follow-up confirmed beyond any doubt that the binary contains an ordinary star and at least one dormant black hole,” Al-Badri said. “We were unable to find a plausible astrophysical scenario that could explain the observed orbit of a system that does not include at least one black hole.”
If the invisible object in Gaia BH1 were a star, for example, it would be much brighter than its companion, and therefore easier to see. But none of the team’s observations revealed an indication of a second star in the system.
The Gaia BH1 system is interesting, not only because it is relatively close to us. (Close to the cosmic scheme of things, however; the Milky Way’s famous spiral disk is about 100,000 light-years wide.) The study team is not sure how the star and the black hole would appear in their current positions.
Gaia BH1’s mass suggests that the star that died and gave rise to it must have been massive – at least 20 solar masses or so. These giants only live for a few million years, and swell tremendously before giving up their ghost.
Modeling work suggests that such blowing would likely destroy the companion before it had a chance to evolve into a sun-like star (if the two were born at the same time). If it survived, the researchers said, it would have ended up in an orbit much narrower than the one it currently occupies.
“Interestingly, this system cannot be easily adapted by standard binary evolution models,” Al-Badri said. “It raises many questions about how this binary system formed, as well as how many lurking black holes there are.”
The new study (Opens in a new tab) It was published online today (November 4) in the Journal of the Monthly Notices of the Royal Astronomical Society.
Mike Wall is the author of “Abroad (Opens in a new tab)Book (Great Grand Publishing House, 2018; illustrated by Carl Tate), a book on the search for extraterrestrials. Follow him on Twitter Tweet embed (Opens in a new tab). Follow us on Twitter Tweet embed (Opens in a new tab) or on Facebook (Opens in a new tab).
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