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."
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