a new type of black hole discovered outside our galaxy

Like a needle in a haystack. After six years of observation at the European Southern Observatory (ESO), a team of international experts has found the first dormant black hole with star mass orbiting a star in the Magellanic Cloud, a dwarf galaxy near the Milky Way. This new type of black hole, very difficult to observe, confirms a long-standing prediction of theory.

Black holes are by definition difficult to detect because they do not emit light. These massive stars at the end of their course, collapsing into themselves under the weight of their own gravity, have such a high density and gravity that light cannot escape. They are only betrayed by the large amounts of radiation emitted when they swallow the substance around them.

Some are more discreet than others: Sleeping black holes. When they sleep, they interact little with their surroundings and emit almost no electromagnetic waves, and thus passes under the radar of astrophysicists’ instruments. It is an object of this type that has just been unveiled by ESO in the VFTS 243 system in the Magellanic Cloud.

A short-term balance

To find it, this international team of astronomers, by ESO “the black hole police”, studied the Tarantula Nebula, an area where many stars are formed, using a spectrograph installed on ESO’s Very Large Telescope. More than 1,000 massive stars have been sifted through to find those who might have black holes as companions in a binary system where two stars orbit each other.

In the systems observed so far, the star, which has turned into a black hole, is close enough to its companion star to “steal” its substance, explained AFP Hugues Sana from the University of Louvain in Belgium, one of the study’s authors. Once captured, this material emits detectable X-rays.

Conversely, the dormant black hole that scientists have discovered emits none, due to the living star is far enough away not to be eaten. She’s staying now balanced on this track “, which lasts 14 days, explains the astronomer. However, this balance cannot hold because “the living star will grow, and at this point part of its surface will be engulfed by the black hole”, which will then emit X-rays and therefore come out of its dormant state.

To make sure that the phantom object was actually a black hole, the researchers continued to eliminate and exclude several scenarios, such as a star losing its envelope. “The only plausible explanation is that it is a black hole because no other star can reproduce this observational data,” the researcher sums up.

According to the latest models, about 2% of the massive stars in our galaxy are likely to have a black hole around them, or about 100 million. “So far, we only know about ten of them, all registered thanks to their X-rays, so we are missing some!”, Explains Hugues Sana.

New keys to understanding the origins of black hole fusions

This first dormant star-black hole, a dozen times the mass of the Sun, is an important discovery because the scientific community had never discovered one, while astronomers are convinced that they are widespread. Several candidates had presented themselves in recent years, but none had so far been approved.

This discovery opens up new perspectives on the processes that accompany the formation of black holes. In particular, experts wonder if the collapse of the core of a dying massive star is followed by a supernova explosion.

“The star that formed the black hole in VFTS 243 appears to have completely collapsed with no sign of a previous explosion,” said one of the study’s authors. “Evidence for this ‘direct collapse’ scenario has emerged recently, but our study undoubtedly provides one of the most direct indications. This has enormous implications for the origin of black hole fusions in the cosmos.”

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