Invisible companion: Astronomers have detected the first dormant stellar black hole outside the Milky Way. The invisible relic of a massive star lies in the Large Magellanic Cloud, a close neighbor of our galaxy. The black hole, which has a mass of around nine solar masses, revealed itself through its gravitational influence on a bright, massive partner star. Also exciting: Without a supernova, this black hole could have arisen directly from the collapse of its progenitor star, as the team reports in “Nature Astronomy”.
stellar black holes are formed when massive stars explode in a supernova at the end of their life cycle and their core collapses. Theoretically, there should be millions of such stellar black holes in our Milky Way alone. So far, however, astronomers have only been able to prove a few of them directly, most of them being revealed by the gravitational wavesreleased when two such black holes collide.
Search for an invisible companion
Such stellar black holes are usually only directly noticeable when they are actively sucking in matter, for example from a companion star. This material then releases high-energy X-rays, revealing the existence of the invisible partner. “But the few known X-ray bright binaries of this type are just the tip of the iceberg,” explain Tomer Shenar from the Catholic University of Leuven in Belgium and his colleagues. Because many more stellar black holes are probably inactive and therefore invisible in the X-ray range.
It is precisely such inactive black holes that Shenar and his team have been looking for. They chose the Tarantula Nebula in the Large Magellanic Cloud as their search area, which contains a particularly large number of massive stars and binary star systems. The astronomers searched there specifically for stars whose movements indicated the presence of an invisible, heavy partner. To do this, they evaluated data from the high-resolution FLAMES spectrograph on the Very Large Telescope of the European Southern Observatory in Chile.
Stellar motion reveals black hole
In fact, the astronomers found what they were looking for: they detected conspicuous, periodic movements in a hot, bluish star with a mass of 25 solar masses. They suggest that this star has an invisible partner and that they orbit each other once every 10.4 days. “While the movement of the primary star is clearly visible in the spectrum, we could not identify a signature of the second object in the spectral data,” reports the team.
The partner of this star can therefore not be a normal star or another more intensely radiating object. However, the movements of the primary star show that its invisible companion must have a mass of at least nine solar masses – it is therefore in the range of a small stellar black hole. According to the astronomers, this is therefore also the most plausible explanation for their observations. “We found a needle in a haystack,” says Shenar.
Extragalactic and about to undergo a second transformation
If confirmed, the system dubbed VFTS 243 would not only be a newcomer among stellar black holes – it would also be the first dormant black hole detected outside the Milky Way. This is because it does not emit any X-rays that would indicate active accretion. “So far, no other X-ray silent black hole outside of our galaxy has been unequivocally detected,” say the astronomers. In the case of VFTS 243, however, everything indicates that such a system is involved.
And their analyzes reveal something else: the massive, blue primary star apparently subtracted a lot of mass from its predecessor star before its partner turned into a black hole. Its current state indicates that it could also become a black hole in the next five million years. A system of two closely orbiting black holes would then remain. Billions more years later, the two could come so close that they collide and merge.
Formed without a supernova?
Also exciting: The newly discovered black hole could have originated differently than many of its “conspecifics”. Traditionally, this is what happens when a dying star first expands and then its core collapses under its own weight. Typically, this core collapse triggers a supernova – at least that is the assumption. However, there is growing evidence that some stars can collapse directly into black holes without exploding.
Exactly this could also have been the case with the recently discovered representative: “The star that formed the black hole in VFTS 243 appears to have completely collapsed, with no evidence of a previous explosion,” reports Shenar. “Recent evidence of this direct collapse scenario has been plentiful, but our study provides arguably one of the most direct evidence.” (Nature Astronomy, 2022; doi: 10.1038/s41550-022-01730-y)
Source: Harvard-Smithsonian Center for Astrophysics, European Southern Observatory (ESO)