Monday, October 10, 2016

Untangling the Mystery of Cosmic Ray Sources

The north star indicates north. Seeing the moon overhead means...that the moon is overhead. It sounds obvious, right? But not everything works this way. Cosmic rays are high energy particles produced in astronomical events. They careen through space at very high speeds, some eventually making their way to Earth. Studying the cosmic rays that hit Earth and our atmosphere can tell us a whole lot about what’s happening out there, but there is a big challenge: unlike light, cosmic rays don’t travel through space in a straight line.

This composite image shows supernova remnant IC 443, also known as the Jellyfish Nebula.
This is one of the remnants scientists studied with the Fermi space telescope to show
 that supernova remnants produce cosmic rays.
Image Credit: NASA/DOE/Fermi LAT Collaboration, NOAO/AURA/NSF, JPL-Caltech/UCLA.
Space is full of objects with magnetic fields. That’s no problem if you are a particle of light, but if you are a particle with electric charge, like a cosmic ray, the whole direction of your life can be set by magnetic fields. They exert forces that can send you winding through the cosmos like a tourist from North Dakota navigating Boston with no GPS and no map.

This means that when we detect cosmic rays here on Earth, we can’t just look straight back to see where they came from. If some crazy astronomical event produced a ton of cosmic rays, we might see an increase in the number detected on Earth, but they would be arriving from every direction.

Understanding cosmic rays and their origins isn’t just important for understanding the night sky better. Cosmic rays can damage microelectronics. They can harm people—particularly astronauts traveling outside of protective shield of the Earth’s atmosphere and magnetic field, but there's also a good deal of evidence to support the prospect that cosmic rays are responsible for lightning. Cosmic rays may reveal secrets of the fundamental particles that make up the universe. Astronomers have detected cosmic rays with many different energies—some with energies millions of times higher than particles in the Large Hadron Collider.

The source of cosmic rays has puzzled scientists for over a century. In the last few years, data from NASA’s Fermi Gamma-ray Space Telescope has indicated that many, but not all, cosmic rays come from supernovae—giant explosions of dying stars. (For a great description of how cosmic rays are produced by supernovae and the Fermi results, check out this NASA video.) This is an important advancement, but most of our cosmic ray observatories are located on Earth. Finding a model that helps us trace cosmic rays detected on Earth back to their source would be a very exciting discovery.

Like the smell of pumpkin spice diffusing through the air around a latte, cosmic rays produced by a supernova remnant diffuse into the surrounding space under the influence of magnetic fields. Based on diffusion theory, scientists can predict the distribution of arrival directions of cosmic rays—almost evenly from every direction. Measurements of cosmic rays at most energies support this model, but they also show that a small percentage of cosmic rays prefer particular directions. This preference varies with the energy of the cosmic rays. Something funny seems to happen right around 0.1-0.3 peta electron volts (or PeV, 1 PeV=1015 eV)—these few special cosmic rays seem to completely reverse their direction.

To better understand this, Markus Ahlers, a scientist with the Wisconsin IceCube Particle Astrophysics Center (WIPAC) and the University of Wisconsin-Madison, analyzed recent ground-based data on cosmic rays with energies surrounding this point. In the work, published last week in the journal Physical Review Letters, Ahlers shows that the distribution of cosmic rays in this energy range can still be well described by the diffusion model if the cosmic rays are from a young, local source and if you consider three things:

  • the motion of the solar system relative to the source,
  • a strong ordered magnetic field outside of our solar system, but still relatively close by, and
  • the limited reconstruction capabilities of ground-based cosmic ray detectors.
“Young, local source” refers to sources less than about 100,000 years old and within something like 1015 miles of us. These kinds of sources produce cosmic rays in the appropriate energy range. You can think of the “strong ordered magnetic field” as the magnetic field in the area outside of our heliosphere, but much closer to us than the cosmic ray source. Since this field is ordered, it causes charged particles to move in a predictable way that can be incorporated into the model.

The model turns out to match cosmic ray data pretty closely. Taking things one step further, Ahlers explored the area of the sky where his model suggests the cosmic rays might be originating. In just the right spot sits the Vela supernova remnant, the remains of a star that exploded about 11,000 years ago. Vela is not only in the right place, but it’s the right age to explain what astronomers are seeing.

“If my speculations about the Vela supernova remnant are correct, it would be the first time that we are able to ‘see’ a cosmic ray source directly, instead of with indirect observations,” says Ahlers. If his model is correct, that would also indicate that we can learn more about the magnetic field outside our solar system by observing cosmic rays—an exciting prospect. Upcoming measurements from the HAWC cosmic ray and gamma ray observatory in Mexico will be another opportunity to test whether this model matches data and can help untangle some of the mystery surrounding cosmic rays.

Kendra Redmond

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