Is the “Unicorn” the Closest Black Hole?

Closest black hole?
This artist’s illustration exhibits how a large companion, presumably a low-mass black hole, distorts the red big star right into a teardrop form.
Lauren Fanfer / Ohio State University

If Ohio State University astronomers are proper, the invisible companion of the red big star V723 Monocerotis (in the constellation the Unicorn) is the nearest black hole in the universe, simply of 1,500 light-years away. But not everyone seems to be satisfied.

V723 Mon is an 8.3-magnitude variable red big star about as large as our Sun however some 25 occasions bigger. Astronomers had beforehand catalogued it as an eclipsing binary, by which a second, unseen star passes over it from Earth’s perspective about each 60 days.

However, when Ohio State PhD candidate Tharindu Jayasinghe took a detailed take a look at archival information from three automated telescopes — the All-Sky Automated Survey (ASAS), the Kilodegree Extremely Little Telescope (KELT), and the Transiting Exoplanet Survey Satellite (TESS) — he discovered that the brightness variations noticed earlier than as an alternative indicated that tidal forces from a extra large companion had distorted the star right into a tear-drop form.

In a research that can seem in Monthly Notices of the Royal Astronomcial Society (preprint available here), Jayasinghe’s staff makes use of the system’s interval, the orbital velocity, and the inferred distortion of the red big to derive the mass of its invisible companion: three solar plenty. They declare {that a} single compact physique, that’s, a low-mass black hole, is the probably rationalization.

Based on an impartial evaluation, Kento Masuda (Osaka University, Japan) and Teruyuki Hirano (NINS, Japan) arrive at an analogous conclusion in a paper posted on the arXiv preprint server. If confirmed, the “Unicorn,” as Jayasinghe and his colleagues have dubbed the object — each for its location on the sky and its uniqueness — can be the closest identified black hole.

However, Ed van den Heuvel (University of Amsterdam) says he’s skeptical about the purported proof for such “non-interacting” low-mass black holes, the place there’s no mass switch, accretion disk, and X-ray emission to assist the declare.

When a staff led by Jayasinghe’s coauthor Todd Thompson (additionally at Ohio State) printed the discovery of one other non-interacting 3.3-solar-mass black hole companion to a distant red big star in Science, van den Heuvel and Thomas Tauris (Aarhus University, Denmark) had been fast to level out in a follow-up , additionally in Science, that the invisible object may very effectively be a detailed pair of faint companions as an alternative. Earlier claims of non-interacting black holes – together with one in the binary star system HR 6819, at a mere 1,120 light-years away – also didn’t stand the test of time.

So it stays to be seen what number of scientists will probably be satisfied of the existence of the Unicorn. Van den Heuvel, for one, believes it’s extra doubtless that right here, too, the invisible companion is a detailed pair of low-luminosity stars. “I don’t think they have presented rock-solid proof,” he says.

In the Mass Gap

If V723 Mon does harbor a bona fide black hole, it might be the smallest one ever discovered. So far, the lowest-mass black hole that astronomers are certain of weighs in at simply over 6 solar plenty, leaving a “mass gap” between black holes and neutron stars, the latter of that are about twice as large as the Sun at most.

A plot of the masses of the different components of observed compact binary mergers
Black holes and neutron stars are greatest seen in binary methods, whether or not they’re found through their gravittational waves (blue and orange for black holes and neutron stars, respectively) or through their electromagnetic radiation (purple and yellow, respectively). There seems to be a “mass gap” between neutron star and black holes, and astronomers are actively trying to see if it is actual.
LIGO-Virgo / Northwestern U. / Frank Elavsky & Aaron Geller

Finding black holes on this mass hole would shed new mild on their formation mechanisms. According to van den Heuvel, present theories can solely type them when a neutron star accretes sufficient materials to additional collapse right into a black hole.

Meanwhile, Jayasinghe and his colleagues suppose their Unicorn will not be fully distinctive in spite of everything. Based on a tough statistical evaluation, they estimate that our Milky Way Galaxy would possibly include just a few hundred related methods. Future massive spectroscopic surveys like APOGEE (a part of the ongoing Sloan Digital Sky Survey) and the Chinese LAMOST survey (carried out with the Large-sky Area Multi-Object fibre Spectroscopic Telescope) are anticipated to yield extra compact-object binaries and black hole candidates, they write. According to Jayasinghe, V723 Mon stands out due to its proximity. “It is very bright, so it is relatively cheap to follow up with ground-based facilities,” he says.


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