In theory, a black hole is easy to make. Just take a lump of matter, squeeze it into a ball with a radius smaller than the Schwarzschild radius, and throw up! You have a black hole. In practice, things are not that simple. When you squeeze matter, it is pushed back, so it takes the weight for a star to squeeze tight enough. For this reason, it is widely believed that even the smallest black holes must be at least 5 solar masses. However, a recent study shows that the lower limit may be even smaller.
The work focuses on the red giant star V723 Monoceros. This star has a periodic wobble, which means that it is trapped in orbit with an accompanying object. The companion is too small and too dark to be seen directly. So it has to be either a neutron star or a black hole. On closer inspection, it turns out that the star not only wobbles with its companion in orbit, but is deformed by gravity by its companion, known as a tidal disturbance.
How the distorted shape of V723 Mon affects the light curve. Photo credit: K. Masuda and T. Hirano
Both the orbital wobble and the tidal perturbation of V723 Mon can shift the light coming from it by Doppler. Since these two effects are dependent on the mass of the companion, you can calculate the mass of the companion. It turns out to be about 3 solar masses.
This is strange because it falls within what is known as that [mass gap](/ blog / dark-edge / mass gap) for compact bodies. According to our understanding of nuclear physics, a neutron star should not have more than 2.5 solar masses. The largest neutron star we have observed is about 2.24 solar masses. Since black holes should be larger than 5 solar masses, there is a gap in which no compact bodies are to be expected. And this object is right in the middle.
This graph shows the recent merger compared to known black holes and neutron stars. Photo credit: LIGO-Jungfrau / Frank Elavsky & Aaron Geller (northwest)
This is not the first time we have observed an object in the mass gap. In 2019, LIGO and Virgo discovered gravitational waves from a fusion between an object with 23 solar masses and an object with 2.6 solar masses. While the fusion object could have been a large neutron star, this new object appears to be too large for that. At the moment, the evidence strongly suggests it is a black hole. If that’s true, then it’s the smallest black hole we’ve discovered.
It would also be the closest black hole we discovered just 1,500 light years away. Astronomers have named this object The Unicorn, partly because of its unique properties, partly in the constellation of Monoceros. While we cannot yet confirm that the unicorn is a black hole, we can do so with further studies. I think you might call these future studies unicorn hunters.
Reference: Masuda, Kento and Teruyuki Hirano. “Tidal Effects on the Radial Velocities of V723 Mon: Additional Evidence for a Dark 3 M? Companion. “The Astrophysical Journal Letters 910.2 (2021): L17.
Reference: Jayasinghe, T. et al. “A unicorn in Monoceros: the 3 M? The dark companion of the bright, nearby red giant V723 Mon is a non-interacting candidate for a black hole with a mass gap. “ArXiv preprint arXiv: 2101.02212 (2021).
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