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This artist's impression shows what the binary system
VFTS 243 might look like. The sizes of the two binary components are not to
scale: In reality, the blue star is about 200,000 times larger than the black
hole. |
An elusive type of black hole has been discovered in a neighboring galaxy for the first time, according to a new study based on observations from the European Southern Observatory's (ESO's) Very Large Telescope.
Dormant stellar-mass black holes, which form when
massive stars reach the end of their lives, are particularly hard to spot since
they do not interact much with their surroundings. This is because, unlike most
black holes, dormant ones don't emit high levels of X-ray radiation.
While thought to be fairly common cosmic phenomenon, this
type of black hole previously had not been "unambiguously detected outside
our galaxy," according to the team of US and European researchers involved
in the study.
The newly detected black hole, called VFTS 243, is at
least nine times the mass of our sun, and it orbits a hot, blue star weighing
25 times the sun's mass, making it part of a binary system.
"It is incredible that we hardly know of any
dormant black holes, given how common astronomers believe them to be,"
said study coauthor Pablo Marchant, an astronomer at KU Leuven, a university in
Belgium, in a news release.
Process of elimination
To find the black hole, which can't be observed
directly, the astronomers looked at 1,000 massive stars (each weighing at least
eight times the mass of the sun) in the Tarantula Nebula region of the Large
Magellanic Cloud, a galaxy near the Milky Way.
The discovery was made by a process of elimination,
said coauthor Tomer Shenar, who was working at KU Leuven in Belgium when the
study began and is now a Marie-Curie Fellow at Amsterdam University, in the
Netherlands.
First the researchers identified the stars that were
part of binary systems -- stars moving around a cosmic companion. Then, they
looked for binary systems where the companion was not visible, and careful
analysis ultimately revealed that VFTS 243 was a dormant black hole, he
explained.
"What we see here is a star, weighing about 25
times the mass of our Sun, moving periodically (every 10 days or so) around
something 'invisible,' that we cannot see in the data," Shenar said.
"The analysis tells us that this other 'thing'
must be at least 9 times more massive than our Sun. The main part of the
analysis is elimination: what can weigh nine solar masses, and not emit any
light? A black hole is the only possibility we have got left (this, or a fat
invisible alien...)."
"There might be more in there, but only for this
one we could show the presence of a black hole unambiguously," Shenar
said.
The black hole was found using six years of
observations by the Fibre Large Array Multi Element Spectrograph (FLAMES)
instrument on ESO's Very Large Telescope. FLAMES allows astronomers to observe
more than a hundred objects at once.
Black hole police
Some of the 40 study authors are known in astronomy
circles as the black hole police, according to the news release, because they
have debunked several other discoveries of other black holes.
The paper said that more than 10 discoveries of black
hole binary systems in the past two years were disputed. However, they were
confident that their discovery was not a "false alarm."
"We know what the challenges are, and we did
everything in our capacity to rule out all other options," Shenar said.
The research team said they invited scrutiny of their
latest findings.
"In science, you're always right until someone
proves you wrong, and I cannot know that this would never happen -- I only know
that none of us can spot a flaw in the analysis," Shenar said.
Reference: Research Paper
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