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Sun blasts out highest-energy radiation ever recorded, raising questions
for solar physics
By Monisha Ravisetti published 1 day ago
"We thought we had this star figured out, but that's not the case."

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An image of the sun.
An image of the sun. (Image credit: NASA/SDO)
In a record-breaking discovery, scientists detected our very own sun
emitting an extraordinary amount of gamma rays — wavelengths of light
known to carry the most energy of any other wavelength in the
electromagnetic spectrum. This is quite a big deal as it marks the
highest-energy radiation to ever be documented coming from our planet's
host star.

Something like 1 trillion electron volts, to be exact.

"After looking at six years' worth of data, out popped this excess of
gamma rays," Meher Un Nisa, a postdoctoral research associate at
Michigan State University and co-author of a new paper about the
findings released Wednesday (Aug. 3), said in a statement. "When we
first saw it, we were like, 'We definitely messed this up. The sun
cannot be this bright at these energies.'"

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Upon deliberation, however, the team realized that such brightness
definitely existed — and it was simply due to the sheer amount of gamma
rays the sun seemed to be spitting out.

"The sun is more surprising than we knew," Nisa said.

Before you start worrying, no, these rays can't harm us. But what they
can do is have a pretty important ripple effect for the future of solar
physics. In fact, they have already raised some important questions
about the sun, such as what role its magnetic field might play in the
newly observed gamma-ray phenomenon.

Related: Scientists may have just cracked the sun's greatest mystery

A diagram that indicates an excess of solar gamma rays seen by HAWC.

What an excess of solar gamma rays looks like to the High-Altitude Water
Cherenkov Observatory Collaboration. (Image credit: Courtesy of the HAWC
Collaboration)
It's all thanks to a unique lens on the cosmos called the High-Altitude
Water Cherenkov Observatory, or HAWC. In short, this observatory,
completed in the spring of 2015, is a facility specifically designed to
observe particles associated with very high-energy gamma rays and cosmic
rays, the latter of which are equally energetic but also mysterious in
that they often travel across the universe without exhibiting a clear
starting point.

"In this particular energy regime, other ground-based telescopes
couldn't look at the sun because they only work at night," Nisa said.
"Ours operates 24/7."

HAWC basically uses a network of 300 large water tanks, a press release
on the new study explains. Each of these tanks is filled with about 200
metric tons of purified water, and they all sit nestled between two
dormant volcano peaks in Mexico more than 13,000 feet (3,962 meters)
above sea level. All of this purified water is important because, as
high-energy particles from space strike the liquid, the collision
results in a phenomenon known as Cherenkov radiation (which you may have
heard of if you've watched the TV show "Chernobyl").

Named after 1958 Physics Nobel Prize laureate Pavel Cherenkov, Cherenkov
radiation essentially refers to a bluish glow that happens when
electrically charged particles move at a certain speed through a certain
medium, in this case water.

Tapping into this concept, HAWC's overall field of view covers 15% of
the sky, allowing it to survey a total two-thirds every 24 hour period
and figure out the roots of various high-energy particles headed to Earth.

An illustration depicting charged particles hitting the water tanks of
the HAWC.

A composite image shows a photograph of the High-Altitude Water
Cherenkov Observatory in Mexico observing particles, whose paths are
shown as red lines, generated by high-energy gamma rays from the sun.
(Image credit: Mehr Un Nisa)
What's normal solar radiation like?
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Even though scientists have observed the sun sending out gamma ray
emissions before, such observations are connected to incredibly extreme
solar events such as super powerful solar flares. The recent gamma-ray
discovery doesn't seem to be associated with that kind of scenario.

Within the sun, nuclear fusion processes are also expected to produce
these strong wavelengths, however, gamma rays created that way don't
exactly make it out of the star — let alone far enough to be detected by
Earth-based instruments.

Instead, most of the time, what we see radiating out from our host star
are infrared wavelengths, ultraviolet wavelengths and, of course,
visible wavelengths that we can see with the unaided eye.

For context, one of those visible wavelengths carries an energy of about
1 electron volt. The gamma rays Nisa and fellow researchers witnessed,
by contrast, exuded about 1 trillion electron volts. And, there were a
lot of them.

The first time scientists observed gamma rays with energies of more than
a billion electron volts, according to the release, was in 2011 with
NASA's Fermi Gamma-ray Space Telescope. But Fermi had a limit. It maxed
out at finding gamma rays with about 200 billion electron volts. So in
2015, the new study's research team started collecting gamma ray data
with HAWC as this observatory didn't seem to have the same restriction.

"They nudged us and said, 'We're not seeing a cutoff. You might be able
to see something," Nisa said.

Which brings us to the present — the first time we've seen sun rays with
energies extending into a trillion electron volts. And, according to
Nisa, that does not appear to be the maximum.

"We thought we had this star figured out, but that's not the case."

The paper was published Thursday (Aug. 3) in the journal Physical Review
Letters

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