Astronomers trace cosmic rays to supernovae

Long-standing mystery solved by examining shock waves of blasts

AFP-JIJI, The Observer

Cosmic rays — fast-moving particles that constantly pummel our planet — come from the explosion of supernovae, new research confirmed Thursday, resolving an astronomical mystery.

Protons make up 90 percent of these rays, which pelt Earth’s atmosphere and were discovered a century ago by the Austrian-born physicist Victor Franz Hess.

Scientists had suggested two possibilities for the origins of these protons: supernovae within our Milky Way galaxy or strong jets of energy from black holes elsewhere in the universe.

The recent consensus among scientists has pointed to supernova remnants as the source, but this remained unproven, said Stefan Funk, an astrophysicist at Stanford University and a coauthor of the new findings.

As Earth’s upper atmosphere is bombarded by cosmic rays, they create cascades of lighter particles that shower down on the planet’s surface. This background radiation accounts for about one-tenth of the natural background radiation that people are exposed to.

Cosmic rays travel far faster than anything that man-made accelerators can achieve. The “Oh-My-God” particle that was detected in 1991 over Utah carried kinetic energy equal to that of a baseball traveling at about 100 kph. It was most likely a proton traveling extremely close to the speed of light.

The report was presented at the annual conference of the American Association for the Advancement of Science in Boston and will also appear in Friday’s issue of the journal Science.

“In the last century, we’ve learned a lot about cosmic rays as they arrive here,” Funk said in a statement announcing the findings.

“We’ve even had strong suspicions about the source of their acceleration, but we haven’t had unambiguous evidence to back them up until recently.”

Using NASA’s Fermi Gamma-ray Space Telescope, researchers over the course of four years analyzed data from two supernova remnants thousands of light-years away and found the proof they were looking for.

“For the first time, we were able to detect the smoking-gun feature of the accelerated protons,” Funk told reporters.

“We are talking about the most gigantic explosions in our galaxy — and they give energy to the tiniest things we know.”

The supernova remnants that led to the discovery are known as IC 443 and W44 and are located 5,000 and 9,500 light-years away, according to NASA.

The researchers found that shock waves from the supernovae accelerated protons to nearly the speed of light, turning them into cosmic rays, said the statement.

“When these energetic protons collided with static protons in gas or dust, they gave rise to gamma rays with distinctive signatures, giving scientists the smoking-gun evidence they needed to finally verify the cosmic-ray nurseries,” it said.

Still, as humans spend more time in the higher atmosphere, questions remain.

“While we have demonstrated that supernova remnants accelerate cosmic rays, the next step will be to determine exactly how they do it, and also up to what energies they can do so,” Funk said.

In addition, he noted, “There are suggestions that cosmic rays have provided early mutations that make life possible” and provide condensation droplets that create clouds.