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WASHINGTON — Astronomers have spotted what they’re calling a cosmic “needle in a haystack” in a galaxy next to our Milky Way — a black hole that’s not only classified as dormant but appears to have been born without the explosion of a dying star. The researchers said Monday that it differs from all other known black holes in that it is “X-ray quiet” — it does not emit strong X-ray radiation that indicates it is gobbling up nearby material with its strong gravitational pull — and that it did not give birth to a single star explosion called a supernova. Black holes are unusually dense objects with gravity so strong, not even light can escape. It, with a mass at least nine times that of our sun, was spotted in the Tarantula Nebula region of the Large Magellanic Cloud galaxy and lies about 160,000 light-years from Earth. A light year is the distance light travels in one year, 5.9 trillion miles (9.5 trillion kilometers). An extremely bright and hot blue star with a mass about 25 times that of the sun orbits this black hole in a stellar marriage. This so-called binary system is called VFTS 243. Researchers believe that the companion star will eventually also become a black hole and could merge with the other. Quiescent black holes, thought to be relatively common, are difficult to detect because they interact so little with their surroundings. Many previously suggested candidates have been debunked by further study, including members of the team that uncovered this. “The challenge is to find these objects,” said Tomer Shenar, a researcher in astronomy at the University of Amsterdam, lead author of the study published in the journal Nature Astronomy. “We’ve found a needle in a haystack.” “It’s the first object of its kind to be discovered after astronomers searched for decades,” said astronomer and study co-author Kareem El-Badry of the Harvard & Smithsonian Center for Astrophysics. It is the first object of its kind to be discovered after astronomers searched for decades.
–Kareem El-Badry, astronomer and co-author of the study
The researchers used six years of observations from the European Southern Observatory’s Very Large Telescope based in Chile. There are several classes of black holes. The smallest, like the one recently detected, are so-called stellar mass black holes formed by the collapse of individual massive stars at the end of their life cycle. There are also intermediate-mass black holes as well as the massive supermassive black holes that reside at the center of most galaxies. “Black holes are intrinsically dark objects. They don’t emit light. So to detect a black hole, we usually look at binary systems in which we see a bright star moving around a second, undetected object,” he said. study co-author Julia Bodensteiner, a postdoctoral researcher at the European Southern Observatory in Munich. The collapse of massive stars into black holes is typically thought to be associated with a powerful supernova explosion. In this case, a star perhaps 20 times the mass of our sun threw some of its material into space in its death vortex and then collapsed in on itself without an explosion. The shape of its orbit with its companion provides clues to the lack of an explosion. “The system’s orbit is almost perfectly circular,” Shenar said. If a supernova had occurred, the force of the explosion would have kicked the newly formed black hole in a random direction and it would have followed an elliptical rather than a circular orbit, Shenar added. Black holes can be ruthless predators, gobbling up any material—gas, dust, and stars—that wanders into their gravitational pull. “Black holes can only be relentlessly predatory if there is something close enough to them that they can devour. Typically, we detect them if they receive material from a companion star, a process we call accretion,” said Bodensteiner. Shenar added, “In so-called quiescent black hole systems, the companion is far enough away that material does not accrete around the black hole to heat up and emit X-rays. Instead, it is immediately swallowed by the black hole.” x
