Origin of Cosmic Rays Confirmed

NASA's Chandra X-ray Observatory image shows the edge of RXJ1713.7-3946, a supernova remnant. On the right, X-ray hot spots are appearing and disappearing. The rapid rise and fall of the spots indicate that electrons are being accelerated to near-light speed in the presence of strong magnetic fields. Credit: CXC/Yasunobu Uchiyama/HESS/Nature

Cosmic rays constantly bombard the Earth as tiny, extremely energetic particles traveling close to the speed of light, yet their origins have eluded scientists for nearly 100 years. A new study, however, brings the mystery a step closer to resolution.

Supernova remnants—the leftovers of massive stellar explosions—possess magnetic fields much stronger than previously thought, recent observations of pulsating X-ray hot spots reveal. Scientists said the discovery serves as some of the first direct evidence for a system powerful enough to accelerate particles into cosmic rays.

"Magnetic field strength lies at the heart of cosmic-ray acceleration theory," said Yasunobu Uchiyama, an astrophysicist with the Japan Aerospace Exploration Agency (JAXA).

Uchiyama and his colleagues' detail their findings in the Oct. 4 issue of the journal Nature.

Energetic mystery

Cosmic rays were first discovered in 1912, and since the 1960s scientists have suspected supernova remnants as their breeding grounds.

Such remnants travel through interstellar gas as they expand, producing high-speed shockwaves that can generate powerful magnetic fields. As protons, electrons and other charged particles from interstellar gas bounce around in the magnetic fields, they're accelerated to blinding speeds to create cosmic rays.

Cosmic ray factories in space work similar to Earth's particle accelerators, yet can pump particles with energies tens of thousands of times greater than the largest man-made machines.

Until Uchiyama and his team's discovery, however, magnetic fields strong enough to create cosmic rays had never been directly detected.

"Previous estimates of magnetic fields in supernova remnants were based on indirect arguments," Uchiyama said. "In our study, we determine the magnetic field in a direct manner."

X-ray hot spots

To make the discovery, Uchiyama and his team focused NASA's Chandra telescope on X-ray hot spots in a supernova remnant called RXJ1713.7-3946, located a few thousand light-years from Earth in the constellation Scorpius.

The hot spots brightened and faded in less than a year, a variability that is the hallmark of cosmic ray generation. Because the hot spots barely moved, the astrophysicists were also able to peg the speed of the supernova shockwave at 10 million mph (16 million kph).

The measurement allowed the team to gauge the strength of the remnant's particle-accelerating magnetic fields.

"This is an extremely important paper," said physicist Don Ellison of North Carolina State University, who was not involved in the study. "This is the first time such rapid X-ray variability has been seen in a supernova remnant."