Where Do Cosmic Rays Come From?
March 1, 2013

Cosmic Rays Originate From Supernovae Shock Waves

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John P. Millis, Ph.D. for redOrbit.com — Your Universe Online

As we search the heavens, one of the striking observations is that we are constantly bombarded by a stream of extremely high energy charged particles, traveling at nearly the speed of light. Known as Cosmic Rays — composed mostly (90%) of protons with the rest mostly heavy nuclei and a small number of electrons — these particles appear to come from all directions.

But where do they come from? Pinpointing this origin is a bit of a challenge. Because they are charged particles their motion is influenced by magnetic fields. And it doesn´t take the powerful fields of stars or planets to alter their trajectory, the river of magnetic field that flows throughout interstellar space can spread these particles throughout the cosmos.

Not all hope is lost however, as there are some clues to their progenitors. Being such high-energy particles, it is necessary that there be a powerful event to accelerate them to their observed velocities. Very few events have such power. This is why, for some time, researchers have looked to supernovae as the most likely candidate for their creation.

The trick is that to prove their hunch an indicator is necessary that can correlate the creation of these cosmic rays with these events. To do so, the evidence would have to come from a neutral particle — something that would not be affected by the interstellar magnetic field.

The most obvious candidate is light, specifically gamma rays — the highest form of electromagnetic radiation. When high-energy particles collide with lower energy particles (usually protons) an elementary particle known as a pion is produced. These are short lived and quickly decay into gamma rays.

To investigate the possible link between supernovae and cosmic rays, researchers used data from the Fermi Gamma-ray Telescope to study supernova remnants. Scientists expect that following the initial supernova explosion, a shock wave pushes outward into the interstellar medium, interacting with protons and other charged particles and nuclei, accelerating them to high energy.

Most of these particles will travel across the cosmos, a tiny fraction of which eventually finding their way to Earth. But a fraction of the particles swept up in the shock wave will quickly bump into a nearby particle or nuclei that has yet to be accelerated. The ensuing process, as outlined above, produces gamma rays, which can be detected by Fermi.

The problem is that there are other processes in supernova events that can also produce gamma rays, so scientists have to be very careful to isolate the flux of light coming only from the pion decay process.

Luckily, because pions are rather massive particles there is a minimum amount of energy imparted to the resulting gamma rays. So scientists predicted that below a certain threshold there would be a steep cutoff in the gamma ray flux, owing to the fact that most of the radiation would come from pion decay (and by extension, cosmic ray interactions).

By studying two supernova remnants — W44 and IC 443 — a team led by Dr. Stefan Funk, a professor at Stanford University and part of the Kavli Institute for Particle Astrophysics and Cosmology, found the “smoking gun”.

The flux of gamma rays from these two objects indicated that the dominant process for producing the emission was from the decay of neutral pions. Since these particles are only produced from the collision of very high energy particles with lower energy particles — a process that has been well studied for decades in terrestrial particle accelerators — this result confirms, what scientists have long believed, that cosmic rays originate from the shock waves of supernovae.