For the first time, scientists have detected light
from behind a black hole, and it fulfills a prediction rooted in Albert
Einstein's theory of general relativity.
Stanford University astrophysicist Dan Wilkins and
his colleagues observed X-rays that were released by a supermassive black hole
located at the center of a galaxy that is 800 million light-years from Earth.
These bright light flares are not unusual because
although light can't escape a black hole, the enormous gravity around it can
heat up material to millions of degrees. This can release radio waves and
X-rays. Sometimes, this super-heated material is hurled out into space by rapid
jets -- including X-rays and gamma rays.
But Wilkins noticed smaller flashes of X-rays that
occurred later and were different colors -- and they were coming from the far
side of the black hole.
"Any light that goes into that black hole
doesn't come out, so we shouldn't be able to see anything that's behind the
black hole," said Wilkins, study author and research scientist at the
Kavli Institute for Particle Astrophysics and Cosmology at Stanford University
and SLAC National Accelerator Laboratory, in a statement.
However, the black hole's strange nature actually
made the observation possible.
"The reason we can see that is because that
black hole is warping space, bending light and twisting magnetic fields around
itself," he said.
X-ray flares have been seen from the far side of a
black hole for the first time, as depicted in this rendering.
The study published last Wednesday in the journal
Nature.
"Fifty years ago, when astrophysicists starting
speculating about how the magnetic field might behave close to a black hole,
they had no idea that one day we might have the techniques to observe this
directly and see Einstein's general theory of relativity in action," said
Roger Blandford, study coauthor and the Luke Blossom Professor in the School of
Humanities and Sciences and professor of physics at Stanford University, in a
statement.
Einstein's theory, or the idea that gravity is
matter warping space-time, has persisted for a hundred years as new
astronomical discoveries have been made.
Some black holes have a corona, or a ring of bright
light that forms around a black hole as material falls into it and becomes
heated to extreme temperatures. This X-ray light is one way scientists can
study and map black holes.
As gas falls into a black hole, it can spike to
millions of degrees. This extreme heating causes electrons to separate from
atoms, which creates magnetic plasma. The powerful gravitational forces of the
black hole cause this magnetic field to arc high above the black hole and twirl
until it breaks.
This isn't unlike the sun's corona, or hot outer
atmosphere. The sun's surface is covered in magnetic fields, which cause loops
and plumes to form as they interact with charged particles in the sun's corona.
This is why scientists refer to the ring around black holes as a corona.
"This magnetic field getting tied up and then
snapping close to the black hole heats everything around it and produces these
high energy electrons that then go on to produce the X-rays," Wilkins
said.
While studying the X-ray flares, Wilkins spotted
smaller flashes. He and his fellow researchers realized the larger X-ray flares
were being reflected and "bent around the black hole from the back of the
disk," allowing them to see the far side of the black hole.
"I've been building theoretical predictions of
how these echoes appear to us for a few years," Wilkins said. "I'd
already seen them in the theory I've been developing, so once I saw them in the
telescope observations, I could figure out the connection."
The observations were made using two space-based
X-ray telescopes: NASA's NuSTAR and the European Space Agency's XMM-Newton.
More observation will be needed to understand these
black hole coronas and the European Space Agency's upcoming X-ray observatory,
called Athena, will launch in 2031.
"It's got a much bigger mirror than we've ever
had on an X-ray telescope and it's going to let us get higher resolution looks
in much shorter observation times," Wilkins said. "So, the picture we
are starting to get from the data at the moment is going to become much clearer
with these new observatories."
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