Like a needle in a haystack. After six years of observations at the European Southern Observatory (ESO), an international team of experts has discovered the first dormant stellar-mass black hole 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 by the theory.
Black holes are by definition difficult to detect because they emit no light. At the end of their course, these massive stars collapse in on themselves under their own gravitational weight, having such a high density and gravitational force that light cannot escape. They are betrayed only by the huge amount of radiation that is emitted when consuming the matter around them.
Some are more discreet than others: dormant black holes. when they sleep They interact very little with their environment and emit almost no electromagnetic waves, thus moving under the radar of astrophysicists’ instruments. It is one such object that was discovered by ESO in the VFTS 243 system in the Magellanic Cloud.
A short run equilibrium
To find out, this international team of astronomers, dubbed the “black hole police” by ESO, used a spectrograph installed on ESO’s Very Large Telescope to study the Tarantula Nebula, a region where many stars form. More than 1,000 massive stars have been extracted A binary system where two stars orbit each other may have black holes as companions to find them.
In the systems observed so far, the star that has become a black hole is close enough to “steal” matter from its companion star, explained AFP Hughes Sanake of the University of Leuven in Belgium, one of the study’s authors. This material, once captured, emits X-rays, which can be detected.
In contrast, researchers discovered that dormant black holes do not emit any, because “The living star is far enough away to eat. He stays for now Balanced in this orbit”, lasting 14 days, explains the astronomer. This equilibrium may not last, however, because “the living star will grow, and during this time, part of its surface will be consumed by the black hole”, which will then emit X-rays and therefore come out of its dormant state.
To confirm that the phantom object was indeed a black hole, the researchers proceeded by elimination, ruling out several scenarios such as a star losing its envelope. “The only reasonable explanation is that it is a black hole, because no other star can reproduce this observational data”, summarizes the researcher.
According to recent 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 detected for their X-ray emission, so we’re missing something!”, explains Hughes Sana.
New keys to understanding the origin of black hole mergers
This first dormant interstellar black hole, a dozen times the mass of the Sun, is an important discovery because the scientific community has never detected one while astronomers are sure they are very widespread. Several candidates have presented themselves in recent years but none have been validated so far.
This discovery opens up new perspectives on the processes that accompany black hole formation. In particular, experts wonder if a dying massive star undergoes core collapse after a supernova explosion.
“The star that produced the black hole in VFTS 243 appears to have completely collapsed, with no signs of a previous explosion,” said one of the study’s authors. “Evidence for this ‘direct collapse’ scenario has only recently emerged, but our study provides arguably one of the most direct indications. It has huge implications for the origin of cosmic black hole mergers.”