New results from the High Altitude Water Cherenkov (HAWC) observatory extend the bright, hard gamma-ray emission from the solar disk observed with NASA’s Fermi Gamma-ray Space Telescope, seemingly due to Galactic cosmic rays showering on nuclei in the solar atmosphere. However, current theoretical models are unable to explain the details of how solar magnetic fields shape these interactions. The new detection thus deepens the mysteries of the solar-disk emission.
Albert et al. report the first detection of a TeV gamma-ray flux from the solar disk, based on 6.1 years of HAWC data. Image credit: NASA’s Goddard Space Flight Center.
“The Sun is more surprising than we knew,” said Dr. Mehr Un Nisa, a postdoctoral researcher at Michigan State University.
“We thought we had this star figured out, but that’s not the case.”
The Sun gives off a lot of light spanning a range of energies, but some energies are more abundant than others.
For example, through its nuclear reactions, the Sun provides a ton of visible light — that is, the light we see. This form of light carries an energy of about 1 electron volt, which is a handy unit of measure in physics.
The gamma rays that Dr. Nisa and her colleagues observed had about 1 trillion electron volts, or 1 tera electron volt, abbreviated 1 TeV.
Not only was this energy level surprising, but so was the fact that they were seeing so much of it.
In the 1990s, physicists predicted that the Sun could produce gamma rays when high-energy cosmic rays — particles accelerated by a cosmic powerhouse like a black hole or supernova — smash into protons in the Sun.
But, based on what was known about cosmic rays and the Sun, they also hypothesized it would be rare to see these gamma rays reach Earth.
At the time, though, there wasn’t an instrument capable of detecting such high-energy gamma rays and there wouldn’t be for a while.
The first observation of gamma rays with energies of more than a billion electron volts came from NASA’s Fermi Gamma-ray Space Telescope in 2011.
Over the next several years, the Fermi mission showed that not only could these rays be very energetic, but also that there were about seven times more of them than scientists had originally expected. And it looked like there were gamma rays left to discover at even higher energies.
When a telescope launches into space, there’s a limit to how big and powerful its detectors can be.
The Fermi telescope’s measurements of the solar gamma rays maxed out around 200 billion electron volts.
“Now, for the first time, we’ve shown that the energies of the Sun’s rays extend into the TeV range, up to nearly 10 TeV, which does appear to be the maximum,” Dr. Nisa said.
“Currently, the discovery creates more questions than answers. Solar scientists will now scratch their heads over how exactly these gamma rays achieve such high energies and what role the Sun’s magnetic fields play in this phenomenon.”
“When it comes to the cosmos, though, that’s part of the excitement. It tells us that there was something wrong, missing or perhaps both when it comes to how we understand our nearest and dearest star.”
“This shows that HAWC is adding to our knowledge of our Galaxy at the highest energies, and it’s opening up questions about our very own Sun. It’s making us see things in a different light. Literally.”
The findings were published in the journal Physical Review Letters.
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A. Albert et al. (HAWC Collaboration). 2023. Discovery of Gamma Rays from the Quiescent Sun with HAWC. Phys. Rev. Lett 131 (5): 051201; doi: 10.1103/PhysRevLett.131.051201
