They discover a strange black hole that formed without an apparent explosion of its star

This is also the first unambiguously detected inactive stellar-mass black hole outside our galaxy.

An experienced international team, renowned for refuting several black hole discoveries, has discovered a stellar-mass black hole in the Large Magellanic Cloud, a neighboring galaxy to our own. As the study’s lead author Tomer Shenar says, “For the first time, our team came together to publicize the discovery of a black hole rather than refute it.” Furthermore, they found that the star that gave rise to the black hole disappeared without any sign of a powerful explosion. An explosion of such power, known as a supernova, normally accompanies the death of the massive star and the birth of a black hole from the stellar corpse. The discovery was made thanks to six years of observations obtained with the Very Large Telescope (VLT) of the European Southern Observatory (ESO).

“We identified a ‘needle in a haystack,'” confirms Shenar, who began the study at the Catholic University of Leuven (KU Leuven) in Belgium and is now a Marie Curie fellow at the University of Amsterdam in the Netherlands. Although other similar black hole candidates have been proposed previously, the team claims that this is the first “dormant” stellar-mass black hole to be unambiguously detected outside our galaxy.

Stellar-mass black holes form when massive stars reach the end of their lives and collapse in on themselves 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. The black hole is “dormant” if it is not emitting high levels of X-ray radiation, which is how such black holes are normally detected. “It is incredible that we hardly know about the existence of these dormant black holes, given how common the astronomical community assumes they are,” explains Pablo Marchant of KU Leuven and a member of the research team. The newly found black hole has at least nine times the mass of our Sun and orbits a hot blue star whose mass is about 25 times that of the Sun.

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 such binary black hole systems,” says Julia Bodensteiner, a co-author of the study and a researcher at ESO in Germany. “I was very excited when I learned the data on VFTS 243, which in my opinion is the most compelling candidate reported to date.

This artist’s rendering shows what the binary system VFTS 243 would look like if we were looking at it closely. The system, which is located in the Tarantula Nebula in the Large Magellanic Cloud, is made up of a hot blue star with 25 times the mass of the Sun and a black hole, which is at least 9 times the mass of the Sun. The sizes of the two components of this binary system are not to scale: the blue star is actually about 200,000 times larger than the black hole. Note that the “lensing” effect we see around the black hole is shown for illustrative purposes only, in order to make this dark object more noticeable in the image. The tilt of the system means that when looking at it from Earth, we cannot see the black hole eclipsing the star. (Image: ESO / L. Calçada)

To find VFTS 243, the research team searched nearly 1,000 massive stars in the Tarantula Nebula region of the Large Magellanic Cloud, looking for ones that might have black hole companions. Identifying these companions as black holes is extremely difficult, as there are so many alternative possibilities.

“As a researcher who has disproved possible black holes in recent years, I was extremely skeptical of this discovery,” says Shenar. The skepticism was shared by Kareem El-Badry, from the research team and from the Center for Astrophysics that depends on Harvard University and the Smithsonian Institution, in the United States. Shenar colloquially calls Kareem El-Badry 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 discovery also offers 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 this process is always accompanied by a powerful supernova explosion.

“The star that formed the black hole in VFTS 243 appears to have completely collapsed, with no signs of a previous explosion,” explains Shenar. “Evidence for this ‘direct collapse’ scenario has emerged only recently, but our study arguably provides one of the clearest indications. This has huge implications for the origin of black hole mergers in the cosmos.”

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 ESO’s VLT.

The study is titled “An X-ray quiet black hole born with a negligible kick in a massive binary of the Large Magellanic Cloud.” And it is published in the academic journal Nature Astronomy (Source: ESO. CC BY 4.0)