"Unicorn" may be smallest and closest black hole to Earth
Astronomers have discovered a black hole that may set a new record or two – it seems to be both the smallest black hole ever detected, and the closest one to Earth found so far.
Nicknamed The Unicorn, the black hole is located right next to a red giant star called V723 Mon in the constellation of Monoceros. That’s only 1,500 light-years from Earth, and the object appears to have a mass just three times that of the Sun.
Both of those are possible records for black holes. The previous record-holder for smallest black hole is 3.3 solar masses, while the previous closest black hole is twice as distant as the Unicorn. The latter record is a little contentious, however – last year it was proposed that the star system HR 6819 housed the closest known black hole, just 1,120 light-years away, but follow-up studies throw doubt on there being a black hole there at all.
Either way, the Unicorn is an intriguing oddity nonetheless. It’s smaller than most of the smallest black holes, a group known as stellar mass black holes, which range between five and about 30 solar masses. Until recently astronomers didn’t really think they came any smaller than that.
Of course, black holes by their nature are tricky to see. Instead the astronomers noticed the Unicorn through its effects on its companion star. Its light appeared to be changing in intensity at different points in its orbit, suggesting it was being stretched into an odd shape by the gravity of something nearby. Since it didn’t have a visible star buddy, a black hole seemed to be the most likely candidate.
“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,” says Todd Thompson, co-author of the study. “The simplest explanation is that it’s a black hole – and in this case, the simplest explanation is the most likely one.”
By analyzing the star’s gravitational distortion, velocity and orbit period, the astronomers were able to calculate that the black hole had the mass of three Suns.
The researchers say that other black holes in this mass gap may be discovered in the next few years, as telescopes become more powerful and astronomers get better at analyzing data.
The research was published in the journal Monthly Notices of the Royal Astronomical Society.
Source: Ohio State University