Normally, black holes emit X-rays and can thus be located. But now, for the first time, researchers are discovering a very special specimen outside of the Milky Way. It’s surprising for two reasons.
Astronomers have found a strange black hole in a galaxy neighboring the Milky Way. The special thing about it: The black hole in the Large Magellanic Cloud is “quiet” – it does not reveal itself through radiation, but only through its gravitational pull. According to the researchers, it would be the first discovery of a quiet black hole outside of the Milky Way. In addition, this stellar black hole was apparently created by a “direct collapse”, i.e. without an accompanying supernova explosion, according to the scientists in the journal “Nature Astronomy”.
“Stellar black holes are the remnants of large stars originally more than 15 times the mass of our Sun,” explains team leader Tomer Shenar from the University of Louvain in Belgium. When such a star has exhausted its nuclear energy supply, it collapses under its own gravitational pull into a black hole. Most of the time, this catastrophe is accompanied by a supernova explosion, in which some of the stellar matter is ejected into space.
Astronomers suspect that there are billions of such stellar black holes in the Milky Way and surrounding neighboring galaxies, the “local group”. However, only a few are actually known – namely those that form a double system with another star. The black holes then often suck off matter from their partner star with their strong gravitational pull. Before this gas falls into the black hole, it heats up and then emits radiation in the high-energy X-ray range – thus revealing the presence of the black hole.
In fact, there should also be many quiescent black holes that are not inundated with matter. To search for such quiet black holes, Shenar and his colleagues have targeted almost 1000 massive stars in the region of the Tarantula Nebula in the Large Magellanic Cloud with the Very Large Telescope of the European Southern Observatory ESO in Chile.
The team finally struck gold: they found a telltale periodic motion around hot blue star VFTS 243. The star, which itself contains 24 times the mass of the sun, apparently forms a close binary system with another object with at least nine solar masses. The two objects orbit each other every 10.4 days.
The second object itself remains completely invisible. The researchers searched for the radiation of another star in the light of VFTS 243 – without success. “It follows from the mass of the invisible object that it must be a black hole,” state Shenar and his team. And this black hole is also quiet, it doesn’t emit any X-rays either. This would have made it possible for the first time to detect such a black hole outside the Milky Way – an important step in estimating how many such objects there are.
But the dark companion of VFTS 243 had another surprise in store: The orbits of the binary system are apparently almost circular. However, when a stellar black hole is formed, when the collapsing star ejects its outer envelope into space, it does not happen symmetrically. This gives the black hole a powerful push – putting it on a highly elliptical orbit. In extreme cases, the black hole can even be catapulted out of the binary.
The surprisingly circular orbit suggests that VFTS 243’s black hole formed without a supernova explosion. “Recently, there has been repeated evidence of such a direct collapse scenario,” explains Shenar. “But our study probably provides one of the most direct indications of this so far.”
