Stellar-mass black holes form when massive stars reach the end of their lives and collapse under their own gravity. In a binary system (a system of two stars revolving around each other), this process leaves an orbiting black hole with a luminous companion star.
Sometimes this type of black hole goes 'dormant', not emitting the usual high levels of X-ray radiation, which is how they are normally detected. Now one has been found outside the Milky Way.
An international team of astronomers known as 'the black hole police', which has refuted several discoveries of this type of object, has detected an inactive stellar-mass black hole in the Large Magellanic Cloud, a neighboring galaxy to ours. In addition, the star that gave rise to this hole disappeared without any sign of a powerful explosion, as published in the magazine Nature Astronomy.
“For the first time, our team came together to publicize the discovery of a black hole rather than refute it; we identified a needle in a haystack”, emphasizes the main author, Tomer Shenar, who began the study at the KU Leuven center (Belgium) and now has a Marie-Curie fellowship at the University of Amsterdam (The Netherlands).
Although other similar black hole candidates have been proposed, the team claims that this is the first inactive stellar-mass black hole to be unambiguously detected outside our galaxy. It is part of the binary system VFTS 243, made up of a hot blue star with 25 times the mass of the Sun and a black hole, which is at least nine times the mass of the Sun. The star is about 200,000 times larger than the hole.
"It's amazing that we hardly know about these dormant black holes, given how common the astronomical community assumes they are," explains co-author Pablo Marchant of KU Leuven. Dormant black holes are particularly difficult to detect as they don't interact much with their surroundings.
“For more than two years, we have been looking for these kinds of binary black hole systems,” adds co-author Julia Bodensteiner, a researcher at the European Southern Observatory (ESO) in Germany, who was excited when she learned of the data on VFTS 243, “ who in my opinion is the most compelling candidate reported to date.”
In the Tarantula Nebula
To find this binary system, the collaboration searched for nearly 1,000 massive stars in the Tarantula Nebula region of the Large Magellanic Cloud, focusing on those that might have black hole companions. Identifying these as black holes is extremely difficult, as there are so many alternative possibilities.
"As a researcher who has refuted possible black holes in recent years, I was extremely skeptical about this discovery," insists Shenar. The skepticism was shared by co-author Kareem El-Badry, from the Harvard & Smithsonian Center for Astrophysics (USA), whom Shenar calls the “black hole destroyer”. “When Tomer asked me to review his findings, I had my doubts. But I couldn't find a plausible explanation for the data that didn't involve a black hole," explains El-Badry.
The finding also gives the team a unique insight into the processes that accompany the formation of black holes. The astronomical community believes that a stellar-mass black hole forms as the core of a dying massive star collapses, but it remains unclear whether or not this process is accompanied by a powerful supernova explosion.
"The star that formed the black hole in VFTS 243 seems to have completely collapsed, with no signs of a previous explosion," says Shenar, who recalls that evidence for "this 'direct collapse' scenario has only recently emerged, but it could be said that our study provides one of the clearest indications; And this has huge implications for the origin of black hole mergers in the cosmos.”
Six years of observations with the VLT
The black hole in VFTS 243 was found using six years of observations of the Tarantula Nebula by the Fiber Large Array Multi Element Spectrograph (FLAMES) instrument on the Very Large Telescope ( VLT) of ESO in Chile.
Despite the 'black hole police' moniker, the team actively encourages scrutiny, debate with alternative models, and hopes their work will lead to the discovery of other stellar-mass holes orbiting massive stars, thousands of which are predicted. that exist in the Milky Way and in the Magellanic Clouds.