An artist’s impression of what the binary star system VFTS 243 — containing a black hole and a large luminous star orbiting each other — might look like if observed up close. (REUTERS)
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WASHINGTON — Astronomers have spotted what they’re calling a cosmic “needle in a haystack” in a galaxy neighboring 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.
Researchers said Monday that this one differs from all other known black holes in that it is “X-ray silent” — it does not emit powerful X-ray radiation indicative of engulfing nearby material with its strong gravitational pull — and that it does not was born in a stellar explosion called a supernova.
Black holes are extremely dense objects with such intense gravity that not even light can escape.
This one, with a mass at least nine times that of our sun, was found in the Tarantula Nebula region of the Large Magellanic Cloud galaxy and is located 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 km).
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 named VFTS 243. Researchers believe that the companion star will eventually also become a black hole and may merge with the other.
Dormant black holes, thought to be relatively common, are difficult to detect because they interact very little with their surroundings. Multiple previously proposed candidates have been debunked by further study, including by members of the team that uncovered this one.
“The challenge is finding these objects,” said Tomer Schenar, a research associate in astronomy at the University of Amsterdam and lead author of the study, published in the journal Nature Astronomy. “We have identified a needle in a haystack.”
“This is the first object of its kind to be discovered after astronomers searched for decades,” said astronomer and study co-author Karim El-Badri of the Harvard-Smithsonian Center for Astrophysics.
It is the first object of its kind to be discovered after astronomers searched for decades.
– Karim El-Badri, astronomer and co-author of the study
The researchers used six years of observations from the Chile-based European Southern Observatory’s Very Large Telescope.
There are different categories of black holes. The smallest, like the newly discovered one, are so-called stellar-mass black holes, formed by the collapse of massive individual stars at the end of their life cycle. There are also intermediate-mass black holes, as well as the massive supermassive black holes found at the center of most galaxies.
“Black holes are inherently dark objects. They do not emit any light. Therefore, to detect a black hole, we usually look at binary systems in which we see a luminous star orbiting a second, undetected object,” said study co-author Julia Bodensteiner, a postdoctoral fellow at the European Southern Observatory in Munich.
It is generally assumed that the collapse of massive stars into black holes is associated with a powerful supernova explosion. In this case, a star with a mass about 20 times the mass of our sun ejected some of its material into space in its death throes, then collapsed in on itself without an explosion.
The shape of its orbit with its companion offers evidence for the lack of an explosion.
“The system’s orbit is almost perfectly circular,” Chenard said.
If a supernova had occurred, the force of the explosion would have kicked the newly formed black hole in a random direction and given it an elliptical rather than a circular orbit, Chenard added.
Black holes can be ruthlessly voracious, gobbling up any material—gas, dust, and stars—that wanders within their gravitational pull.
“Black holes can only be relentlessly hungry if there’s something close enough to them that they can swallow. Normally, we detect them if they receive material from a companion star, a process we call accretion,” Bodensteiner said.
Chenard added: “In so-called dormant black hole systems, the companion is far enough away that material does not accumulate around the black hole to heat up and emit X-rays. Instead, it is immediately absorbed into the black hole.”
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