Scientists have discovered one of the smallest black holes in history – and the closest to Earth ever found.
Researchers have called it ‘The Unicorn’, partly because it is so far unique, and partly because it was found in the constellation Monoceros – ‘The Unicorn’. The findings appear today, April 21, in the newspaper Monthly Notices of the Royal Astronomical Society.
“When we looked at the data, this black hole – the Unicorn – just appeared,” said lead author Tharindu Jayasinghe, a doctoral student in astronomy at The Ohio State University and a fellow fellow at Ohio State.
The Unicorn is about three times the mass of our sun – tiny for a black hole. Very few black holes of this mass have been found in the universe. This black hole is 1,500 light-years away from Earth, still inside the galaxy. And, until Jayasinghe started analyzing it, it basically hid itself visibly.
The black hole appears to be a companion to a red giant star, meaning the two are linked by gravity. Scientists cannot see the black hole – they are, by definition, dark, not only visually, but according to the tools that astronomers use to measure light and other wavelengths.
But in this case they can see the companion star of the black hole. This star has been well documented by telescopic systems including KELT, exhausted from Ohio state; ASAS, the predecessor of ASAS-SN, which is now running out of Ohio state, and TESS, a NASA satellite that is looking for planets outside our solar system. Data on it were widely available but have not yet been analyzed in this way.
When Jayasinghe and the other researchers analyzed that data, they realized that something they could not see seemed to orbit around the red giant, causing the light from that star to change in intensity and appearance at various points around the orbit.
Something, they noticed, pulled the red giant and changed its shape. This pulling effect, called tidal distortion, offers astronomers a signal that something is affecting the star. One option was a black hole, but it should be small – less than five times the mass of our sun, falling into a large window that astronomers call the “mass gap.” Only recently have astronomers considered it a possibility that black holes of that mass could exist.
“When you look differently at what we do, you find different things,” said Kris Stanek, co-author of the study, an astronomy professor at Ohio State and a distinguished university expert. “Tharindu looked at this thing that so many other people were looking at, and instead of dismissing the possibility that it could be a black hole, he said, ‘Well, what if it could be a black hole?'”
This tidal interruption is produced by the tidal force of an unseen companion – a black hole.
“Just as the moon’s gravity distorts the Earth’s oceans, causing the seas to swell to and from the moon, producing high tides, so too does the black hole distort the star into a football shape with one axis longer than the other,” said Todd Thompson, co-author of the study, president of the Ohio State Department of Astronomy and a distinguished university expert. “The simplest explanation is that it’s a black hole – and in this case, the simplest explanation is the most likely.”
The speed of the red giant, the period of the orbit, and the way in which the tidal force distorted the red giant told them the mass of the black hole, leading them to conclude that this black hole is about three solar masses, or three times as much as the sun.
For about the last decade, astronomers and astrophysicists have been wondering if they can’t find these black holes, because the systems and systems used weren’t advanced enough to find them. Or, they wondered if they just didn’t exist?
Then, about 18 months ago, many of the members of this Ohio State research team, led by Thompson, published a scientific article in the journal. Science, offering strong evidence that these types of black holes existed. That discovery prompted Jayasinghe and others, both in Ohio state and around the world, to look seriously for small black holes. And that assessment led them to the Unicorn.
Finding and studying black holes and neutron stars in our galaxy is crucial for scientists studying space because it tells them about the way stars form and die.
But finding and studying black holes is, almost by definition, difficult: Individual black holes do not emit the same kind of rays that other objects emit in space. They are, for scientific equipment, electromagnetically silent and dark. Most well-known black holes were discovered because they interacted with a companion star that created many X-rays – and those X-rays are visible to astronomers.
In recent years, larger-scale experiments have been launched to try to locate smaller black holes, and Thompson said he expects to see more “massive gap” black holes discovered in the future.
“I think the field is pushing this, to really map how low mass, how much medium mass and how many high-mass black holes there are, because every time you find one, it gives you a clue as to which stars collapse, which explodes and who’s in between, ”he said.
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