Astronomers discover first ever ‘silent’ black hole orbiting a Be star MWC 656

Astronomers have discovered a new first: a silent black hole orbiting around a fast-spinning blue star. This new discovery hints at a much higher population of these unusual binary star systems in our galaxy.

Roughly 8,500 light years from Earth, there's a star called MWC 656, which is about 10 times the mass of our sun. MWC 656 is what's known as a Be star — hot, bright and blue. It's spinning very quickly, roughly one million kilometres per hour. That's about 140 times faster than our sun spins, which is impressive for a star so massive and it's fast enough that the star throws off material from its surface to form a wide disk around its equator.

Astronomers gazing at MWC 656 using two of the optical telescopes at the Canary Island's Roque de los Muchachos Observatory were not only able to see this disk around the star, but they also spied another disk right next to it, which was spinning around a stellar companion. However, they didn't see any light source at the centre of the disk, and measurements of the disk showed it had to be something massive, which pointed to one thing — a black hole.

"It turned out to be an object with a mass between 3.8 and 6.9 solar masses," said Ignasi Ribas of CSIC at the Institute of Space Sciences, according to Astronomy. "An object like that, invisible to telescopes and with such large mass, can only be a black hole because no neutron star with more than three solar masses can exist."

There are two strange thing about this, though. In all the Be stars in our galaxy that astronomers have found in binary systems, the companion has always been a small, dense neutron star. Also, although all neutron star and black hole binaries emit X-rays as a tell-tale marker of their presence, this black hole is quiet, emitting no X-ray radiation at all.

The video below is an animation showing what this binary system looks like:

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Both black holes and neutron stars emit X-rays as matter from their companion star draws close to them, heats up and emits intense radiation. However, this isn't happening with MWC 656 and researchers believe they know why. It has to do with how fast MWC 656 is spinning and how fast its disk of matter spins as a result.

"The absence of X-ray emission from this system is evidence that material is not channelled into the black hole; rather, it must be retained in a holding pattern within the accretion disk," said Virginia McSwain, of Lehigh University in Pennsylvania, in a piece in Nature.

"Gas in the outer regions of the Be star's disk will have high angular momentum, which will be transferred to the accretion disk during the mass transfer," she wrote.

"Without an efficient mechanism to remove this angular momentum, accretion will be suppressed and the black hole will remain quiet."

McSwain calls the discovery of this black hole "like finding a needle in a haystack." Given how common Be stars are, this find implies that there could be a lot more binary pairs like this in our galaxy.

(Image and video courtesy: Gabriel Pérez/SMM (IAC))

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