Gamma ray detection marks highest energy light from the Sun

Scientists have discovered that the Sun produces higher energy light than was thought possible. An unusual type of telescope detected gamma rays with energies of over 1 tera electron volt (TeV), at least five times more energetic than previously known.

The Sun emits light spanning a wide range of energies, from infrared through visible light and up to ultraviolet. It was previously predicted that the Sun could produce gamma rays – electromagnetic radiation with the highest energy – through interactions with cosmic rays from distant sources, but these would rarely reach Earth to be detectable.

A few decades later, these gamma rays were eventually detected by NASA’s Fermi Gamma-ray Space Telescope in 2011. With more and more observations over the years since, Fermi found that the Sun was producing around seven times more gamma rays than had been predicted. Their energies were detected at up to 200 giga electron volts (GeV), which is the upper limit that Fermi can pick up. So for the new study, scientists used a different instrument that’s sensitive beyond that limit.

The instrument in question is called the High-Altitude Water Cherenkov Observatory (HAWC), and it works in a way unlike your everyday telescope. It’s made up of a series of 300 big tanks filled with 200 tons of water each. When gamma rays hit molecules in the Earth’s atmosphere, they create a cascade of lower energy particles, and these can interact with the water molecules in those big tanks. Sensitive instruments keep watch for these interactions, and scientists can work backwards to calculate the energy of the original gamma ray.

Using HAWC data gathered between 2015 and 2021, the researchers discovered that the Sun was producing gamma rays with energies well beyond that which Fermi detected. They were reaching energies on the scale of TeV, with some spiking to almost 10 TeV.

“After looking at six years’ worth of data, out popped this excess of gamma rays,” said Mehr Un Nisa, corresponding author of the study. “When we first saw it, we were like, ‘We definitely messed this up. The Sun cannot be this bright at these energies’.”

But the sheer amount of data over those six years showed that it was the case. Exactly how the Sun produces them remains a mystery, the team says, but further research will investigate how their energy gets so high and what role the Sun’s magnetic field might play.

The research was published in the journal Physical Review Letters.

Source: Michigan State University