There should actually be millions of stellar black holes in the Milky Way – relics of massive stars. So far, however, only a few are known because they are invisible and difficult to find. Now astronomers may have detected such a stellar black hole in the Large Magellanic Cloud. It was revealed by its gravitational pull on a bright, massive partner star. From its movements it can be seen that the invisible partner is an inactive black hole with a mass of nine solar masses. If confirmed, it would be the first detection of such a dormant stellar black hole outside the Milky Way.
Stellar black holes are formed when massive stars reach the end of their life cycles and collapse under their own gravity. The fact that such black holes exist is proven, among other things, by the detection of gravitational waves that are released when two such black holes collide and merge. But detecting these massive star relics directly is much more difficult. Because they are invisible, such stellar black holes are only noticeable when they suck in matter, for example from a companion star. This material then emits high-energy X-rays, revealing the existence of the black hole. So far, however, only a few of such binary systems consisting of a star and an active black hole are known. It is even more difficult to detect such stellar black holes when they are dormant and no X-rays are emitted from their vicinity.
Search for invisible companions
“We have been looking for such black holes and binary star systems for more than two years,” says co-author Julia Bodensteiner from the European Southern Observatory (ESO) in Garching. For their study, the astronomers chose the Tarantula Nebula, an area in the Large Magellanic Cloud where a particularly large number of massive stars and binary star systems exist. Using six years of observational data from ESO’s Very Large Telescope in Chile, they specifically searched for stars whose movements indicated the presence of an invisible, heavy partner. Using the high-resolution FLAMES spectrograph, the researchers led by first author Tomer Shenar from the Catholic University of Leuven in Belgium were able to identify these movements based on tiny fluctuations in the light spectrum of the stars.
The team found what they were looking for in one of the stars: a hot, bluish star with a mass of around 25 solar masses is apparently part of a binary star system with a hidden partner, as the astronomers determined. Both partners orbit each other with an orbital period of 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,” the astronomers report. Therefore, this partner cannot be a normal star or any other more intensely radiating object. However, the movements of the visible star show that its partner 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.
Inactive black hole beyond our galaxy
If confirmed, the system dubbed VFTS 243 would not only be a new addition to the few star-black hole binary systems known so far – it would also be the first dormant stellar black hole detected outside the Milky Way. “When Tomer asked me to check his results, I had my doubts. But I couldn’t find a plausible explanation for the data that didn’t involve a black hole,” says co-author Kareem El-Badry of the Center for Astrophysics | Harvard & Smithsonian in the US. He has already exposed several supposed finds of black holes as a mistake. This time, however, he too is convinced, as he explains. So there is no evidence that it is a different object, at the same time the small outflow of matter from the primary star also fits with a dormant black hole as a partner.
The discovery also offers new insights into the processes by which such black holes form from massive progenitor stars. Conventional wisdom has it that this happens when the dying star first expands and then its core collapses under the weight of 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.”
Source: Tomer Shenar (KU Leuven) et al., Nature Astronomy, doi: 10.1038/s41550-022-01730-y