Gamma data reveal surprises about the sun

A decade of solar observation of the telescope has revealed a startling mystery: gamma rays, the highest frequency light waves, radiate from our nearest star seven times more abundantly than expected. Even stranger, despite this extreme excess of gamma rays as a whole, a narrow bandwidth is strangely absent.

Surplus light, space in the spectrum, and other surprises about the solar gamma signal could indicate unknown features of the sun's magnetic field or more exotic physics.

"It's amazing that we've had such a dramatic mistake about something we really should understand: the sun," said Brian Fields, particle astrophysicist at the University of Illinois, Urbana-Champaign.

The unexpected signal appeared in data from the Fermi Gamma Ray Space Telescope, a NASA observatory that analyzes the sky from its outpost in low Earth orbit. While more Fermi data has been accumulated, revealing more and more the spectrum of gamma rays from the sun, the riddles have only proliferated.

"We just kept finding amazing things," said Annika Peter of Ohio State University, co-author of a recent white paper summarizing several years of research into the signal of solar gamma rays. "It's definitely the most amazing thing I've worked on."

Not only is the gamma-ray signal much more powerful than a decades-old theory predicted; it also extends at much higher frequencies than expected and varies inexplicably across the sun's surface and throughout the eleven-year solar cycle. Then there is the gap that researchers call a "trough" – a lack of gamma rays with frequencies of about 10 trillion hertz. "The dip defies all logic," said Tim Linden, a particle astrophysicist at Ohio State who helped analyze the signal.

Fields, who did not participate in the work, said, "They did a great job with the data, and the story told is really amazing."

Probable protagonists of the story are particles called cosmic rays, usually protons that were launched into the solar system by shock waves of distant supernovas or other explosions.

Physicists do not think that the sun emits gamma rays from within. (Nuclear fusions in its nucleus produce them, but they disperse and shift to a low-energy light before leaving the sun.) However, in 1991, physicists David Seckel, Todor Stanev, and Thomas Gaisser of the University from Delaware hypothesized that the sun would nevertheless remain radiant in the gamma rays, because cosmic rays penetrate space and plunge toward it.

From time to time, the Delaware trio declared that a cosmic ray plunging towards the sun "reflected" or turned at the last second thanks to the sinuous and curly magnetic field of the sun. "Remember the Road Runner cartoon?" Said John Beacom, professor at the Ohio State and one of the leaders in signal analysis. "Imagine that the proton is heading straight for that sphere and that it's changing direction at the last minute, and then it's coming back to you." But on its way out, the cosmic ray collides with the gases in the solar atmosphere and dissipates under a burst of gamma rays. .

Based on the speed at which cosmic rays penetrate the solar system, the estimated strength of the sun 's magnetic field, the density of its atmosphere and other factors, Seckel and his colleagues have calculated that the Effectiveness of the symmetry process was about 1%. They predicted a weak glow of gamma rays.

Yet, the Fermi telescope detects an average of seven times more gamma rays from the solar disk than what this theory of cosmic rays predicts. And the signal becomes up to 20 times stronger than expected for gamma rays having the highest frequencies. "We found that the process was compatible with 100% efficiency at high energies," Linden said. "Every cosmic ray that comes in must be reversed." It's curious, because the most energetic cosmic rays should be the most difficult to reproduce.

And the model of Seckel, Stanev and Gaisser did not say anything. According to Seckel, it is difficult to imagine how one could end up with a deep and narrow dive into the gamma ray spectrum starting with cosmic rays, which have a smooth energy spectrum. It's hard to get lows in general, he said, "It's a lot easier to have bumps than hollows. If something comes out of the sun, agree, it's an extra channel. How can I create a negative channel from this? "

The strong glow of gamma rays may reflect a source other than condemned cosmic rays. But physicists had a hard time imagining what. They have long suspected that the sun's core could harbor dark matter – and that dark matter particles, after being sucked and trapped by gravity, could be dense enough to annihilate each other. But how could the gamma rays produced by annihilating the dark matter in the nucleus be able to avoid dispersing before fleeing the sun? Attempts to link the gamma-ray signal to dark matter "seem to be of the Rube Goldberg type," said Seckel.

Some aspects of the signal refer to the cosmic rays and the outlines of the 1991 theory.

For example, the Fermi telescope detects many more gamma rays in the solar minimum, the eleven-year phase of the sun when its magnetic field is the quietest and most orderly. It makes sense, say the experts, if cosmic rays are the source. At the solar minimum, more cosmic rays can reach the powerful magnetic field near the sun's surface and be reflected, instead of being deflected prematurely by the turbulent entanglement of field lines that permeate the inner solar system to others. times.

On the other hand, the detected gamma rays decrease as a function of frequency at a speed different from that of cosmic rays. If cosmic rays are the source, the two rates should match.

Joe Giacalone, a heliospheric physicist at the University of Arizona, says the signal "is probably indicative of something very fundamental about the magnetic structure of the sun," says Joe Giacalone. The most studied star, but its magnetic field – generated by the whirlwind of charged particles inside it – remains poorly understood, leaving us a fuzzy picture of how stars work.