Astronomers have found what is thought to be the closest black hole to Earth, and if confirmed, it's also among the smallest black holes ever found. This cosmic object is the dark companion of the red giant star V723 Mon, in the constellation of Monoceros, the Unicorn. For this, and the fact it's so rare, it has been nicknamed "The Unicorn".
The Unicorn is estimated to be about three times the mass of our Sun – very small for black hole standards – and is located just 1,500 light-years from the Solar System, still inside the Milky Way.
Its companion star is roughly the same mass but the two don’t have the strong interaction seen in black hole binaries such as Cygnus X-1. In that case, the black hole is stealing material from its companion, and in doing so it produces X-rays. In this case, the researchers could see something tugging at V723 Mon, affecting its shape, and suspect it's the gravitational pull of the Unicorn gently tugging at its companion, changing its shape and the light it emits.
“Just as the moon’s gravity distorts the Earth’s oceans, causing the seas to bulge toward and away from the moon, producing high tides, so does the black hole distort the star into a football-like shape with one axis longer than the other,” co-author Todd Thompson, chair of Ohio State’s astronomy department explained. “The simplest explanation is that it’s a black hole – and in this case, the simplest explanation is the most likely one.”
The variability of the star has been noted by many observatories both on the ground and in space before but it was never attributed to the possible effect of a small non-interacting black hole. It was only recently that astronomers considered black holes with such a small mass existed.
“When we looked at the data, this black hole – the Unicorn – just popped out,” said Tharindu Jayasinghe, lead author of a paper describing it accepted for publication in the Monthly Notices of the Royal Astronomical Society.
The discovery of the Unicorn is important as it is located in the so-called “lower mass gap.” Based on our physics, black holes created in supernovae have a certain range of possible masses. Astronomers have reported a distinct lack of observations of black holes of this small size, with some suggestions they may not even exist. The Unicorn now shows they are out there.
“When you look in a different way, which is what we’re doing, you find different things,” added co-author Professor Kris Stanek. “Tharindu looked at this thing that so many other people had looked 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?'"
Welcome to the neighborhood, Unicorn.