In 2019, researchers announced that something appeared to be off with the sun. After 10 years of observations, they had concluded that the sun’s high-energy radiation was seven times more abundant than expected.
Now a new study based on even higher-energy data has sharpened the picture. Researchers found that the solar gamma-ray excess persists at higher energies. It then drops off at the topmost energies explored. No one can fully explain what’s going on. “It’s just been one fun head-scratcher after another,” said Annika Peter, an astrophysicist at Ohio State University and a co-author of the latest analysis.
In the recent paper, which was posted to the preprint server arxiv.org and is under review at Physical Review Letters, researchers with the High-Altitude Water Cherenkov (HAWC) Gamma-Ray Observatory and collaborators report on the abundance of gamma radiation persisting in a range that is two to 10 times more energetic than that of any previously measured solar gamma rays.
A full theoretical accounting for the abundance of gamma rays at the sun remains hazy, but the new results provide helpful clues for developing an explanation. In particular, the finding of a “cutoff energy” at which the sun ceases to radiate gamma rays could help reveal the complex role of the solar magnetic fields.
These interactions are “fundamentally important,” said Hugh Hudson, an astronomer at the University of Glasgow. “The HAWC data is acknowledged to be the best resource here.”
A leading hypothesis explaining the excesses in the sun’s gamma radiation starts with cosmic rays. These high-energy particles, usually protons, are launched by supernovas, black hole collisions and other extreme events in the universe. As cosmic rays approach our sun, its powerful magnetic fields capture the particles and redirect them outward, away from the sun. The cosmic rays then collide with protons in the sun’s atmosphere to produce unstable particles called pions. As the pions decay, they create gamma rays.
But only some of these gamma rays escape the sun and make it to our detectors. “There’s kind of a ‘just-so’ story for gamma rays,” Peter said. “You need to have the cosmic ray get deep enough in the sun’s atmosphere so that it has a pretty good chance of interacting. But it has to be at the point where the gamma ray can then get out” without interacting with other intervening particles.
The cosmic rays in that sweet spot, researchers think, have been “mirrored” by the sun’s magnetic field lines. A cosmic ray goes in and encounters magnetic field lines that redirect it. On its way out, it collides with a proton, yielding a gamma ray.
One way to test this theory is by measuring how the gamma-ray signal changes over time. In the 2019 study, researchers identified a correlation between the strongest signal and the solar minimum, the phase of the sun’s 11-year cycle when its tangled web of magnetic field lines is weakest. This correlation seems to support the theory. If incoming cosmic rays don’t get deflected by these extended magnetic fields, which can reach far out into the solar system, they can get very close to the sun, where strong magnetic fields spin the particles around at the last moment.
However, the sun’s magnetic pull is only so strong. If an extremely energetic cosmic ray barreled into the sun’s vicinity, it could potentially zip through the fields without any particle collisions.
“At some point, you’d think that cosmic rays are just too high-energy to even be affected by the magnetic field,” said Mehr Un Nisa, an astrophysicist at Michigan State University who is part of the HAWC collaboration. Researchers would see this as a cutoff in the data: Above a certain energy, gamma rays would effectively disappear. The characteristics of any such cutoff could provide clues for how to better understand the gamma-ray excess.
In search of such a high-energy cutoff, Nisa, Peter, and their collaborators turned to the HAWC experiment, a ground-based observatory near Puebla, Mexico, that was completed in 2015. The observatory relies on hundreds of 7.3-meter-wide water-filled tanks camped out at the base of a volcano, covering an area about the size of four football fields. As gamma rays blast through Earth’s atmosphere, they create secondary particles that strike the water in HAWC’s tanks, emitting the electromagnetic equivalent of a sonic boom. HAWC relies on this so-called Cherenkov radiation to reconstruct the incoming gamma rays.
Using HAWC, the scientists managed to probe gamma rays that were over 10 times more energetic than the ones in the 2019 study, which was based on data from the Fermi Gamma-ray Space Telescope. As with the earlier findings, the signal strength was highest at solar minimum. And as hoped, the signal strength dropped off steeply with increasing energies — indicating a cutoff effect. The result provides an important energy scale that helps researchers model the sun’s gamma-ray radiation, Peter said.
Why the cutoff occurs at the energy it does, Nisa and her colleagues can’t say. Nor can they explain why the unexpectedly abundant signal persists at these high energies. “There are no present models that can currently explain [this],” said Elena Orlando, a physicist at the University of Trieste in Italy who is not part of the HAWC collaboration. The signal remains as mysterious as ever.
And HAWC does not probe perhaps the most puzzling aspect of the earlier data: a mysterious narrow dip in the gamma-ray signal at frequencies of 1 trillion trillion hertz.
Peter and her colleagues are working on the problem, developing elaborate simulations of the sun’s magnetic fields and the intricate dynamics of the cosmic particles winding around them.
Beyond solving the gamma-ray mystery, researchers think that the HAWC measurements may lead to broader insights into solar and particle physics. The high-energy particles that penetrate deep into the sun’s atmosphere could help scientists probe an unexplored region of the sun. HAWC is uniquely sensitive to these high-energy particles, as the observatory can measure gamma rays of even higher energies than those created at the Large Hadron Collider. “It gives us a new lab to study new physics out there,” Nisa said.