HAWC discovers the highest energy light coming from the sun

This story is adapted from the Michigan State University (MSU) press release.

Sometimes, the best place to hide a secret is in broad daylight. Just ask the sun.

In a new paper in the journal Physical Review Letters, the High-Altitude Water Cherenkov Gamma-ray Observatory (HAWC) Collaboration has discovered the highest energy light ever observed from the sun. 

A composite image shows a photograph of the High-Altitude Water Cherenkov Observatory in Mexico. The observatory is a grid of 300 water tanks with steel walls and tan caps. A portion of these tanks are being struck by red lines, showing the trajectory of particles generated by high-energy gamma rays from the sun.
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. Credit: Mehr Un Nisa

The international team behind the discovery also found that this type of light, known as gamma rays, is surprisingly bright. That is, there’s more of it than scientists had previously anticipated.

A picture of Ke Fang, assistant professor in the Department of Physics at the University of Wisconsin–Madison.
Ke Fang

“This new work from HAWC suggests that our sun is still shining even at an energy a trillion times higher than the photons human eyes can see,” says Ke Fang, an assistant professor of physics at the Wisconsin IceCube Particle Astrophysics Center (WIPAC) at the University of Wisconsin–Madison. Fang is the current US spokesperson for HAWC.

Located near Puebla, Mexico, at an altitude of 13,500 feet, HAWC observes gamma and cosmic rays between 100 gigaelectronvolts and a few hundred teraelectronvolts. HAWC uses a network of 300 large water tanks, each filled with about 200 metric tons of water, that is nestled between two dormant volcano peaks. 

From this vantage point, it can observe the aftermath of gamma rays striking air in the atmosphere. Such collisions create what are called air showers, which are a bit like particle explosions that are imperceptible to the naked eye. 

The sun gives off a lot of light spanning a range of energies, but some energies are more abundant than others.

A figure that looks like a heat map shows a bright yellow spot at its center, ringed by “cooler” oranges and purples. This represents the excess of gamma rays observed by the HAWC Collaboration.
What an excess of solar gamma rays looks like from the HAWC observatory. A heat map shows a bright yellow spot at its center, ringed by “cooler” oranges and purples. This represents the excess of gamma rays. Credit: Courtesy of the HAWC Collaboration

The gamma rays HAWC observed were about one trillion electronvolts, or one tera electronvolt. Not only was this energy level surprising, but also the fact that they were seeing so much of it.

In the 1990s, scientists 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, the researchers also hypothesized it would be rare to see these gamma rays reach Earth. 

Now, for the first time, the team has 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.

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. 

“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,” says Mehr Un Nisa, a postdoctoral researcher with MSU and the corresponding author of the new report. “It’s making us see things in a different light. Literally.”


Currently, the international HAWC Collaboration consists of more than 30 institutions in the US, Mexico, Europe, Asia, and South America. The UW–Madison HAWC group at WIPAC, now led by Fang, has been involved with the HAWC experiment from the beginning of the design stage, through its construction, and into the data analysis phase. As part of her research, Fang uses data from HAWC to observe or find gamma-ray sources.