Scientists at Michigan State University, led by postdoctoral researcher Mehr Un Nisa, have made a remarkable discovery related to the sun’s highest-energy light, known as gamma rays.
Usually, this high-energy light doesn’t reach the Earth’s surface, but it leaves distinct signals that were spotted by Nisa and her team using a unique observatory called HAWC, short for High-Altitude Water Cherenkov Observatory.
Unlike traditional telescopes that work at night, HAWC operates around the clock and doesn’t look like a regular telescope either. It consists of three hundred large water tanks placed between two dormant volcanoes in Mexico.
Air showers
From this high vantage point, HAWC can observe the results of gamma rays colliding with air in the atmosphere, creating what’s called air showers. These showers release lower-energy particles and light, which HAWC can detect using its instruments.
Nisa and her colleagues have been gathering data since 2015, and, in 2021, they noticed something unexpected. There was an excess of gamma rays coming from the sun, brighter than they had ever imagined.
Mehr Un Nisa said, “After looking at six years’ worth of data, out popped this excess of gamma rays.” She explained, “When we first saw it, we were like, ‘We definitely messed this up. The sun cannot be this bright at these energies.'”
A historic discovery
The sun emits various types of light, each with different energies. One of the most familiar types is visible light, which we see every day. This light has an energy of about one electron volt, a measure used in physics.
However, recently, Mehr Un Nisa and her team made a groundbreaking discovery. They observed gamma rays coming from the sun with an energy level of about one trillion electron volts, or 1 TeV for short. This finding was not only surprising, but it was also astonishing to see so much of this high-energy light.
Sun’s gamma ray prediction and surprising discovery
In the 1990s, scientists had a hunch that the sun might produce gamma rays when high-energy cosmic rays—particles that gain immense speed from cosmic sources like black holes or supernovae—collide with protons in the sun.
However, based on their knowledge of cosmic rays and the sun, they believed it would be quite uncommon for these gamma rays to reach Earth.
At that time, no instrument was capable of detecting such high-energy gamma rays, and it would take a while before technology caught up. The breakthrough came in 2011 when NASA’s Fermi Gamma-ray Space Telescope observed gamma rays with energies of over a billion electron volts.
In the following years, the Fermi mission not only confirmed that these gamma rays could be extremely energetic but also revealed something surprising. There were about seven times more of these gamma rays than scientists had originally anticipated.
Potential of HAWC collaboration
When a telescope is launched into space, there are constraints on the size and power of its detectors. The Fermi telescope, while remarkable, could only measure the sun’s gamma rays up to about two hundred billion electron volts.
However, a group of theorists, led by John Beacom and Annika Peter from Ohio State University, saw potential beyond this limit. They encouraged the HAWC Collaboration to investigate further.
Mehr Un Nisa, part of the HAWC Collaboration and a researcher at Michigan State University, explained, “They nudged us and said, ‘We’re not seeing a cutoff. You might be able to see something.'”
Scientists detect the highest-energy light shining from the Sun. It’s a trillion times brighter than normal pic.twitter.com/cvvnb4Ycmz
— jamilsiddiq (@jamilsiddiq3) August 4, 2023
The HAWC Collaboration is a joint effort involving more than thirty institutions across North America, Europe, and Asia. The recent paper on this discovery lists nearly a hundred authors, representing a significant collective effort.
Among them are Daniel Salazar-Gallegos, a graduate student from Michigan State University, and two professors of physics and astronomy at MSU, James Linnemann and Kirsten Tollefson.
The results are groundbreaking. For the first time, the team has demonstrated that the sun’s gamma rays extend into the TeV range, reaching up to almost 10 TeV. This remarkable finding appears to be the upper limit of the sun’s gamma-ray energies.
More questions following the discovery
The recent discovery has left solar scientists with more questions than answers. They are now pondering how these gamma rays from the sun attain such incredibly high energies and the role that the sun’s magnetic fields may play in this intriguing phenomenon, according to Mehr Un Nisa.
In the vast cosmos, however, this uncertainty is part of what makes the discovery so thrilling. It indicates that there might be something we have misunderstood or overlooked about our closest star.
Nisa highlighted, “This shows that HAWC is expanding our knowledge of our galaxy at the highest energy levels, while also raising questions about our very own sun. It’s providing us with a new perspective. Quite literally, it’s making us see things in a different light.”
This remarkable revelation not only deepens our understanding of the sun but also invites us to explore the universe’s mysteries with fresh eyes.
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