Unicorn: May Be The Closest Black Hole To Earth.

 Monoceros Constellation.

Monoceros is a faint constellation on the celestial equator. It is surrounded by Orion on the West, Gemini to the North, Canis Makor to the south, and Hydra to the east. It is generally best visible at latitudes between 75° and -90°. Alpha Monoceros which the brightest star, has a visual magnitude of 3.931. Beta Monocerotis a triple star system. What are wondering what are triple star systems? Well, the three stars form a fixed triangle, it was discovered by astronomer William Herschel in 1781, calling it “one of the most beautiful sights in the heavens”. Well, apart from the stars, this constellation has many more secrets to tell! Like, it contains the famous Rosette Nebula, the Cone Nebula, two Super-Earth exoplanets, two open clusters -such as M50 and the Christmas Tree cluster (named for its resemblance to a Christmas tree) and what not! Apart from these, astronomers have found evidence for a candidate black hole that exist in this constellation. 

The Monoceros Constellation.

They named it as “The unicorn”, because it resides in the constellation Monoceros. It is amongst the closest black holes to Earth, at a distance of mere 1500 light-years (when compared to billions of light years for stars like Alpha Centauri and all).  Now you may wonder: why “The unicorn”?  Well, the nickname seems to have a double meaning because the black hole lies in Monoceros, and it has incredibly low mass which is about three times that of the Sun makes it nearly one of a kind. "Because the system is so unique and so weird, you know, it definitely warranted the nickname of  'The Unicorn,' said the discovery team leader Tharindu Jayasinghe, an astronomy PhD student at The Ohio State University. Above I mentioned the black hole weighs 3 solar masses. So, how much is “three solar masses”? Let's do some maths. The Sun's mass is about 1.98*10^30 Kg, which is to say that this “small” black hole weighs about 6*10^30 Kg. Just for a term of comparison, let's consider some other well-known black holes. For example, the famous M87 Blackhole – the one black hole that spread all over the world as it was the black which astronomers captured the first, is about 6.5 billion Suns! It's massive. 

You might know about a famous black hole in the heart of our Milky Way, Sagittarius A*, it weighs about 4 million solar masses. So, we say “The unicorn” it's a tiny and peculiar black hole. Only few such super-lightweight black holes are known to us as they're really very hard to find. Black holes swallow up everything, including light, so astronomers have traditionally detected them by noticing the impact they have on near objects. In fact, black holes are black! And we can find them only considering the impact they have on their surroundings (mostly stars). At this point it should be clear that we can't directly observe a black hole, which could lead to the question: so how did astronomers find the evidence for “the unicorn” in the Monoceros constellation? What were they looking at? Which instruments did they use to detect it? 

How was The Unicorn detected?

Most of the time in the universe we find objects that are gravitationally connected.  Binary systems of stars, star systems like the solar system, pulsars that have started a merging process, cannibal black holes, and so on. Even the moon and the Earth are, of course, gravitationally connected.  Connection. So, this is how astronomers found our unicorn black hole. Astronomers team led by Tharindu Jayasinghe, were searching for bright stars accompanied by “non-interacting” black holes, which are black holes that aren’t tugging material off their stellar companions when they suddenly saw signs of the Unicorn around a red giant star in Monoceros. Jaya Singh and his colleagues spotted just this type of gravitational influence, known as a tidal distortion, affecting a bright dying star called V723 Mon. So, this clue had encouraged the team to search for the signs of the star’s potential companion, which had to be both much less luminous and significantly more massive than the red giant, V723 Mon.

Simulation of the tidal tug by The Unicorn.
Image credit: Ohio State illustration by Lauren Fanfer

The tidal tug (it is a gravitational effect that stretches an object along line of centre of mass of the other object, it happens due to gravitational field gradient) of the companion was visible in observations captured by the Kilodegree Extremely Little Telescope (KELT), the All-Sky Automated Survey (ASAS), and NASA's Transiting Exoplanet Survey Satellite (TESS).  The detection was confirmed by the team with follow-up observations from the Remote Observatory Atacama Desert (ROAD), the Neil Gehrels Swift Observatory,and the Keck Observatory. So, they ended up concluding that “the bright red giant V723 Mon has a dark, massive companion that is a good candidate for the closest know black hole,” according to the study. So "The Unicorn" features a companion — a swelled up red giant star that's nearing the end of its life. Just like the gravity of moon distorts the Earth's oceans, causing the seas to bulge toward and away from the moon, causing high tides, so does the black hole distort the star into some ball-like shape with one axis longer than the other. The Unicorn’s low mass also places it in the mysterious mass gap between the most massive neutron stars which is about 2.2 times the Sun and the smallest black holes which is about 5 time the Sun.   

Difficulties Associated With It.

Over the last two years, researchers have found several possible black holes in the mass gap. The unicorn black hole is one of them. That's why is so important for our understanding of the universe! The discovery, if confirmed (since by now the unicorn is simply a candidate black hole), would help illuminate the fine distinction that nature makes at the end of a massive star’s life. After giant star exhausts its fuel, the star’s mass flows inward and its core collapses due to gravity. If the incoming mass can explore and overcome the star’s gravitational force, it bursts into a supernova. But if not if there’s just too much mass the star collapses in on itself and forms a blackhole. What actually determines whether a star explodes as a supernova or collapses into a black hole isn’t clear. The black hole mass gap could be a vital clue to that process. The problem is that at such lower masses, it’s a bit hard to tell the difference between a black hole and a neutron star since the latter can bulk up to a theoretical maximum of 3 solar masses. And depending on the conditions, neutron stars can appear dark as well. In fact, if a neutron star is a pulsar with a beam pointed at you, it does some very obvious things, and you can surely tell that it isa pulsar, but if it’s not, it can be very difficult to separate it from a black hole. If this discovery turns out not to be a black hole and especially if the other candidates don’t hold up to scrutiny either then perhaps black holes simply do not form below 5 solar masses. Such a revelation would carry significant implications for our understanding of supernova physics and for our understanding of the universe in general!

To learn about black holes visit - What is a black hole

Reference -"The Unicorn" paper published in the Royal Astronomical Society. You can read it for free at the online preprint site arXiv.org.