Studies of supernova remnant RCW 86 have revealed the origin of cosmic rays. (Image courtesy of ESO/E. Helder and NASA/Chandra.)
An international team of researchers has discovered strong evidence that extremely energetic cosmic rays are born in supernova remnants.
"Cosmic rays constantly bombard the earth's atmosphere but, until now, we didn't have proof of where in our galaxy they originated," said co-author Stefan Funk of the Kavli Institute for Particle Astrophysics and Cosmology at SLAC National Accelerator Laboratory. "That's because cosmic rays are almost entirely made of protons, which as charged particles are bent by magnetic forces as they travel to Earth. So we can't just trace a straight line back to know where they originated, like we can with light."
Instead, the researchers traced the sources of cosmic rays by indirect means.
In a paper published last week in Science Express, the researchers describe measurements made with the European Southern Observatory's Very Large Telescope and NASA's Chandra X-ray Observatory. These measurements, of a star that exploded in the year A.D. 185, compare the temperature of the gas immediately behind the shockwave created by the stellar explosion with the speed of the shockwave itself. If the energy of the stellar explosion was converted solely into heat and motion, these two measurements should have been directly related by a very well-known and well-tested equation. Yet when the researchers plugged their measurements into the equation, it didn't balance. Something else was being energized by the explosion.
"When a star explodes in what we call a supernova, a large part of the explosion energy is used for accelerating some particles up to extremely high energies," said co-author Eveline Helder of the Astronomical Institute Utrecht in the Netherlands. "The energy that is used for particle acceleration is at the expense of heating the gas, which is therefore much colder than theory predicts."
The researchers concluded that the missing energy goes into accelerating protons to nearly the speed of light-creating the cosmic rays that continually pummel our solar system, creating flashes of light behind the eyelids of astronauts and causing glitches in electronic components on Earth.
"Our observations reveal the smoking gun," said Helder.
This story was first published in SLAC Today on June 30, 2009.