Scientific Method / Science & Exploration

Supernova observations solve the mystery of cosmic-ray origins

Explosion's shockwave accelerate particles to the highest energies we've seen.

Supernova remnant IC 443, seen in X-ray light. Proton collisions in this remnant produced pions, which decayed to gamma rays - which allowed astronomers to solve the mystery of cosmic ray origins.
NASA/DOE/Fermi LAT Collaboration, Chandra X-ray Observatory

On October 15, 1991, a high-energy proton from deep space struck Earth's upper atmosphere. Known as the "Oh My God Particle", this proton was by far the highest energy cosmic ray ever seen. This one proton's energy was equivalent to a regulation soccer ball traveling at 15 meters per second (34 miles per hour). In the two decades following, observers spotted several similarly energetic cosmic rays, which left a big question: what was accelerating these protons to higher speeds than anything we can achieve in on Earth?

The most likely candidates are supernova explosions, some of the most violent events in the Universe. Until now, however, no one could establish a clear association between cosmic rays and the remains of a supernova. Now a new study used the Fermi Large Area Telescope to make the connection. M. Ackermann and colleagues established the source of at least some of the highest energy cosmic rays beyond reasonable doubt.

Enlarge / Supernova remnant W44, seen in X-ray light.
ESA Herschel/XMM-Newton
In a core-collapse supernova, a massive star (greater than 8 times the mass of the Sun) blows off its outer layers in a huge cataclysm. According to models for high-energy cosmic rays, the cosmic rays don't come from the star. Instead, the shock wave from the explosion accelerates atoms in interstellar gas to nearly light speed. Since most of this gas is hydrogen—a proton and electron—the result is very high energy protons, some of which reach Earth as cosmic rays.

Testing that model would require showing that these high-energy protons were produced in supernovas. When protons collide at high velocity, either with other free protons or atomic nuclei nuclei, the collisions often produce neutral pions. Pions consist of a quark and antiquark; neutral pions (written as π0) are extremely unstable, decaying rapidly into pairs of gamma rays. (The Large Hadron Collider slams two beams of protons into each other, so neutral pions are also produced in large quantities in that experiment.) By looking for the distinctive gamma ray signature from pion decay in supernova remnants, astronomers hoped to find evidence of high-energy protons.

The problem is that electrons in supernovas also produce gamma rays as they accelerate. (Electrons are much less massive than protons, so they tend to undergo interactions that prevent them from reaching Earth.) Therefore, high resolution observations are required to distinguish between pion-produced gamma rays and those from electrons.

The pion-decay spectrum exhibits a distinctive pattern in energy: a sharp rise to a peak, then a slow fall off at higher energies, which is a product of the energy of the protons that made them.

The authors of the current study observed two supernova remnants in our galaxy—IC 443 and W44—for 4 years, using the orbiting Fermi gamma ray observatory's Large Area Telescope (LAT). They focused on a portion of the gamma ray spectrum where there's very little electron-produced contamination. The researchers used a sophisticated algorithm to determine whether other sources could create a false positive signal, but found the most likely possibility was pion-produced gamma rays from the supernova remnants.

These results were clear enough as to exclude other reasonable possibilities: the gamma ray signals were definitely from pion decay, and pions could only be produced by very energetic proton-proton collisions. Comparing the spectrum of light to the observed cosmic ray energy spectrum, the researchers conclude that the mystery is solved. Many, if not all, of the high-energy cosmic ray protons bombarding Earth must come from supernova explosions and their remnants.

Science, 2013. DOI: 10.1126/science.1231160 and DOI: 10.1126/science.1228297 (About DOIs).

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