Solar gamma-rays with energies >1TeV surprise observers
Baffled Scientists Detect Massive Unexplained Radiation From the Sun, Study Reports


Scientists have captured the most energetic radiation ever detected from our Sun, an unexplained discovery that challenges models of our star’s inner workings, reports a new study.  



Astronomers witnessed our star emitting gamma rays, the highest-energy form of light, that are one trillion times more intense than the visible sunshine we experience here on Earth. This constant gamma-ray flux—which does not pose a threat to life on our planet—is far brighter than expected based on models of the Sun’s behavior, raising new questions about the mechanisms that are fueling the radiant glow.

			
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The Sun mostly shines in the infrared, visible, and ultraviolet bands of the light spectrum, though recent observations from pioneering gamma-ray telescopes have hinted that our star may be active at much higher energies. To follow up on these clues, scientists observed the Sun for six years with the High Altitude Water Cherenkov (HAWC) observatory, which sits at an altitude of 4,100 meters between the peaks of dormant volcanoes near Pueblo, Mexico. 





To their surprise, the HAWC researchers discovered the first solar gamma-rays that surpass a tera-electron volt (TeV), a unit of energy that is only seen in the highest-energy realms of the universe. The team said that their observations “deepen the mysteries of the solar-disk emission” and “highlight the need for a revised framework that can explain the anomalous excess of gamma rays from the Sun also in the TeV range,” according to a study published on Thursday in Physical Review Letters. 

“The Sun was studied by the Fermi gamma-ray space telescope, mostly as a foreground object that needed to be accounted for when they were looking at galaxies much further away,” said Mehr Un Nisa, an astroparticle physicist at Michigan State University who co-authored the study, in an email to Motherboard. “In that process, they found the puzzling observations” of high-energy gamma rays. 

“In particular, it looked like the emission would continue to high energies where Fermi could not observe but HAWC could so we decided to give it a look,” she added, noting that “as particle physicists we were interested in the Sun as a window to new physics—particularly dark matter. Many theories predict that dark matter could be captured in the Sun and annihilate to produce signals that might be detectable by our neutrino and gamma-ray telescopes.” 





With that in mind, the HAWC team sought to probe the upper limits of the Sun’s energy range with their unique observatory. Unlike traditional telescopes, HAWC is made of 300 large water tanks, each containing 200 metric tons of purified water, which are monitored by a complex network of sensors. 

This unusual detector is designed to capture gamma rays and other messengers from the most energetic realms of the universe, including cosmic rays, which are mysterious super-charged particles that race across space. 

“HAWC is among the few detectors capable of observing the Sun in the TeV range,” the team said in the study. “Its large field of view and high livetime fraction allow continuous exposure as the Sun transits across the sky.”





The HAWC collaboration is mainly focused on high-energy phenomena that originates far beyond the solar system, but the team decided to study the Sun after NASA’s Fermi spotted gamma-ray emission in the giga-electron volt (GeV) range, which is 1,000 times lower than TeV emission. Scientists think these bright gamma rays are produced when cosmic rays from distant frontiers slam into the Sun and interact with particles under its surface, though the exact details of these encounters remain unclear.

The new study is based on an exposure that elapsed between November 2014 and January 2021, resulting in a six-year dataset of high-energy solar observations. The Sun goes through cycles that last 11 years, which include a minimum period of activity as well as a maximum period that is marked by powerful flares, frequent solar storms, and abundant sunspots. Even though the Sun was in the quietest phase of its solar cycle during HAWC’s observation period, it was still emitting record-breaking gamma-rays with energies of up to 10 TeV, opening up an entirely new window into our familiar star.

“The main mystery of the Sun is that there is a noticeable mismatch between what we observe at such high energies and what was predicted by theorists,” said Nisa. “As observers, we plan on continuing taking more data particularly now that the Sun is becoming more active in its solar cycle. We would like to see how the emission changes from one solar maximum to the next.” 





“At the same time, our theorist colleagues are actively working on new models and simulations of the Sun,” she continued. “HAWC measurements will feed into these simulations and paint a more accurate picture of what is going on.”

In particular, the mysterious origin of the gamma rays might be unraveled by future studies that focus on the Sun’s powerful magnetic fields, which likely play a significant role in the production of the high-energy emission.

“The observed flux is almost two orders of magnitude higher than the flux expected from the solar limb alone, indicating the important role of magnetic fields in modulating and enhancing the flux,” the team said.





“Probing the Sun at the highest energies is key to understanding the propagation of cosmic rays in the heliosphere, and in particular to solving the puzzles of the unexpectedly bright GeV gamma ray emission seen by Fermi,” the researchers concluded. “Our TeV observations with HAWC show that the Sun continues to be an anomalously bright gamma-ray source at very high energies.”

Update: This article has been updated to include comments from study author Mehr Un Nisa.




