‘We identified a "needle in a haystack",’ says Shenar who started the study at KU Leuven in Belgium and is now a Marie-Curie Fellow at Amsterdam University, the Netherlands. Though other similar black hole candidates have been proposed, the team claims this is the first ‘dormant’ stellar-mass black hole to be unambiguously detected outside our galaxy.
Stellar-mass black holes are formed when massive stars reach the end of their lives and collapse under their own gravity. In a binary, a system of two stars revolving around each other, this process leaves behind a black hole in orbit with a luminous companion star. The black hole is "dormant" if it does not emit high levels of X-ray radiation, which is how such black holes are typically detected. ‘It is incredible that we hardly know of any dormant black holes, given how common astronomers believe them to be’, explains co-author Pablo Marchant of KU Leuven. The newly found black hole is at least nine times the mass of our Sun, and orbits a hot, blue star weighing 25 times the Sun’s mass.
Most convincing candidate
Dormant black holes are particularly hard to spot since they do not interact much with their surroundings. ‘For more than two years now, we have been looking for such black-hole-binary systems,’ says co-author Julia Bodensteiner, a research fellow at ESO in Germany. ‘I was very excited when I heard about VFTS 243, which in my opinion is the most convincing candidate reported to date.’
To find VFTS 243, the collaboration searched nearly 1000 massive stars in the Tarantula Nebula region of the Large Magellanic Cloud, looking for the ones that could have black holes as companions. Identifying these companions as black holes is extremely difficult, as so many alternative possibilities exist.
‘As a researcher who has debunked potential black holes in recent years, I was extremely skeptical regarding this discovery,’ says Shenar. The skepticism was shared by co-author Kareem El-Badry of the Center for Astrophysics | Harvard & Smithsonian in the USA, whom Shenar calls the ‘black hole destroyer’. ‘When Tomer asked me to double check his findings, I had my doubts. But I could not find a plausible explanation for the data that did not involve a black hole,’ explains El-Badry.
The discovery also allows the team a unique view into the processes that accompany the formation of black holes. Astronomers believe that a stellar-mass black hole forms as the core of a dying massive star collapses, but it remains uncertain whether or not this is accompanied by a powerful supernova explosion.
‘The star that formed the black hole in VFTS 243 appears to have collapsed entirely, with no sign of a previous explosion,’ explains Shenar. ‘Evidence for this ‘direct-collapse’ scenario has been emerging recently, but our study arguably provides one of the most direct indications. This has enormous implications for the origin of black-hole mergers in the cosmos.’
Six years of observations
Despite the nickname ‘black hole police’, the team actively encourages scrutiny, and hopes that their work, published today in Nature Astronomy, will enable the discovery of other stellar-mass black holes orbiting massive stars, thousands of which are predicted to exist in Milky Way and in the Magellanic Clouds.
‘Of course I expect others in the field to pore over our analysis carefully, and to try to cook up alternative models,’ concludes El-Badry. ‘It's a very exciting project to be involved in.’