Plasma Lenses

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This finding could help solve the longstanding
mystery of where a major part of the galaxy’s
matter is hiding, the scientists added.
Astronomers first detected clues of these
mysterious structures 30 years ago as they
monitored quasarsquasars, the brightest
objects in the universe. Quasars are the most
energetic form of active galactic nuclei, which
are supermassive black holes in the centers of
distant galaxies that release extraordinarily
large amounts of light as they rip apart stars
and gobble matter.Previous research found
that radio waves from quasars could vary
wildly in strength, a phenomenon technically
known as an extreme scattering event.
Astronomers suggested these events were due
to clouds of plasma ā€” that is, electrically
charged particles. These clouds are essentially
lumps in the thin gas that fills the space
between the stars in the Milky Way.
“Lumps in this gas work like lenses, focusing
and defocusing the radio waves, making them
appear to strengthen and weaken over a
period of days, weeks or months,” study lead
author Keith Bannister, an astronomer at the
Commonwealth Scientific and Industrial
Research Organization (CSIRO) in Australia,
said in a statement.
Previous research suggested these “plasma
lenses” are huge ā€” about 620 million miles (1
billion kilometers) wide, a distance nearly
seven times the distance between Earth and
the sun. Ones detected so far lie about 3,200
light-years away, nearly 800 times farther than
the nearest star to Earth, Proxima
Centauri.Plasma lenses have been difficult to
find, so much about them is a mystery. For
instance, estimates suggested the pressures
within these plasma lenses are about 1,000
times greater than the surrounding interstellar
gas. It was uncertain how these structures
could form and survive long enough for
astronomers to detect as often as they have.
In addition, until now, scientists knew nothing
about the shape of these plasma lenses. This
made it difficult to figure out what these
structures were or what their origins were.
Now astronomers have for the first time
successfully detected a lensing event while it
was happening. This helped them conduct
follow-up analyses that permitted the first
estimates of plasma lens shapes.
Researchers used the Australia Telescope
Compact Array to scan about 1,000 active
galactic nuclei for sudden changes in their
radio waves. They detected a lensing event in
2014 that went on for a year in connection
with the quasar PKS 1939-315, located in the
constellation Sagittarius. Whereas old
analyses of lensing events only monitored two
radio frequencies, “our new method gave us
9,000 frequencies at once,” Bannister told
Space.com. “It was like going from black-and-
white TV to color.”
Based on their findings, the researchers
suggest this plasma lens could neither be a
spherical cloud nor a corrugated or bent
sheet.

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