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The smallest black hole discovered – a “dark unicorn” three times the mass of the Sun 1,500 light-years away

Invisible little thing: Astronomers have discovered the smallest black hole in the Milky Way to date. It is about three times the mass of the Sun and is only 1,500 light-years away from us – so it may be the closest black hole to Earth. Researchers have discovered the invisible and dormant “dark unicorn” because its gravitational pull affects the path of a red giant.

Star Black holes A giant star arises when it explodes in a supernova at the end of its life cycle. The stellar nucleus then collapses into a much denser neutron star or becomes a black hole depending on its mass. But where Border Is between the two and it is not yet clear how small a black hole will become.

One of the main reasons for this is that small black holes that are not actively absorbed by matter are invisible to astronomers and their measuring instruments. Although these stellar black holes are part of a dual system, their existence is largely unknown in the behavior of their partners. How many of them are in the Milky Way, and what these invisible, unrelated black holes are, is largely unknown.

Red giant with fluctuating brightness

But now astronomers have discovered one of these hidden little creatures – very close to us. The object around a bright red giant about 1,500 light-years away in the galaxy Monosaurus (unicorn) is hidden in orbit. This gigantic giant V723 Moon has long been known as a volatile variable star. However, it is not clear why its light curve is constantly changing.

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To clarify this, researchers led by Tarindu Jayasinghe evaluated observation data at different wavelengths from different telescopes. Among other things, they used motion data from ISA’s Kia satellite, spectral data from NASA’s DES and Swift space telescopes, and many terrestrial spectrographs.

Influence of the invisible comrade

Both the light spectrum and the resulting radial velocity and the shape of the red giant represent the gravitational influence on a subset. “However, we have not found any evidence of a light-emitting companion, and can exclude a main sequence star or a pair of stars as an associated object,” the research team said. Instead, it should be a small but dark object.

According to astronomers, the simplest explanation for such a small, dark companion is a black hole. “When we looked at the data, this black hole really caught our eye,” Jayasinghe says. From the characteristics of the light curve, he and his colleague conclude that this black hole, named after its “unicorn” galaxy, contains only three solar masses.

The smallest and closest black hole

This will make this “dark unicorn” the smallest and closest black hole to date. Because a year ago astronomers had a binary star system 1,000 light years away HR 6819 He also discovered evidence for a star black hole. Subsequent observations could not confirm this because Jayasinghe and his team reported.

In contrast, the “unicorn” of the V7233, 1,500 years away, has already been confirmed by a second, independent measurement. Kento Masooda of Osaka University and his colleague Derryuki Hirano re-examined the red giant’s radial velocity. With the help of a model, they explored how to better explain some of the irregularities in the light curve.

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Confirmed by independent measurement

Effect: Observed features can be seen in the black hole wave effect on the red giant. The gravitational pull of the invisible companion pulls the star in a wandering manner – like the tidal waves that cause the Moon’s gravitational pull in the Earth’s oceans. “Occasional effects on the radiation velocity of 723 moons can be quantified by the wave distortion of the star and the associated decay of the absorption lines,” the researchers say.

Their calculations also confirm the lowest mass of the “dark unicorn”: out of 2.93 solar masses, it belongs to the smallest black hole ever seen. It moves in what is called the mass gap – the area where the boundary between black holes and neutron stars is supposed to lie. So far only a few black holes are known in this mass range, one of which may be beyond him Gravity Betrayed.

Most of them are there

Jayasinghe and his team estimate that there should be many more dormant and invisible small black holes in the Milky Way. According to this, there may be about 100 to 1,000 dual systems in the galaxy alone, which, like 723 moons, contain giant stars and dormant black holes.

Astronomers hope that in the future more accurate spectrographs and telescopes will help find such “dark unicorns”. It may also answer the question of what happens when a large star collapses – and what happens when a neutron star or black hole forms. (Royal Astronomical Society Monthly Announcements, Magazines, arXiv: 2101.02212; Letters from the Journal of Astronomy, 2021; doi: 10.3847 / 2041-8213 / abecdc)

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They are: Ohio State University