Cosmic Rays are extremely high energy charged particles (usually protons) that travel the Universe at nearly the speed of light. Scientists have been puzzled by the origin of these high energy particles for decades. It seems now, however, that we are one step closer to knowing their source.
History of Cosmic Rays
In 1912 Victor Hess conducted several experiments where he launched high accuracy electrometers to measure the ionization rate or atoms in our atmosphere. What he discovered was that the ionization rate was much greater the higher you rise in the atmosphere -- a discovery for which he later won the Nobel Prize.
This flew in the face of conventional wisdom. His first instinct on how to explain this was that some solar phenomenon was creating this effect. However, after repeating his experiments during a near solar eclipse he obtained the same results, effectively ruling out any solar origin. Therefore, he concluded that there must be some intrinsic electric field in the atmosphere creating the observed ionization, though he could not deduce what the source of the field would be.
It was more than a decade later before Robert Millikan was able to prove that the electric field in the atmosphere observed by Hess was instead a flux of photons and electrons, that he later termed "cosmic rays", streaming though our atmosphere. He also determined that these particles were not terrestrial in origin, but rather emanated from deep space.
Millikan argued that these photons and electrons where produced by the scattering of gamma-rays by a process known as Compton scattering -- named for physicist Arthur Compton. Ironically, it was Compton in the 1930s that correctly surmised that it was not in fact photons and electrons that comprised cosmic rays, but rather charged particles such as protons. Over the next decade several experiments were carried out to confirm Compton's findings.
Ever since, scientists have sought to understand the nature and origin of cosmic rays. And until recently their origin has remained somewhat of a mystery.
The Source of Cosmic Rays
Because cosmic rays are composed of charged particles their paths can be altered by any magnetic field that it comes into contact with. Naturally, objects like stars and planets have magnetic fields, but interstellar magnetic fields exist. This makes predicting where (and how strong) magnetic fields are extremely difficult. And since these magnetic fields persist throughout all space, they appear in every direction. Therefore it is not surprising that from our vantage point here on Earth it appears that cosmic rays do not appear to arrive from any one point in space.
Determining, then, the source of cosmic rays have proven very difficult. However, there are some assumptions that can be inferred. Fist of all, simply due to the nature of cosmic rays -- being extremely high energy charged particles -- they would need to be produced by a rather powerful mechanism. So events like supernovae or regions around black holes seem to be likely candidates.
In 1949 physicist Enrico Fermi suggested that cosmic rays were simply accelerated particles by magnetic fields in interstellar gas clouds. And since you need a rather large field to create the highest energy cosmic rays, scientists began looking at supernova remnants as the likely source. But, again, with their paths being altered by magnetic fields how can we know for sure?
Well, in June 2008 NASA launched a new gamma-ray telescope known as Fermi -- named, obviously, for Enrico Fermi. While Fermi is a gamma-ray telescope, it has as one of its main science goals to determine the origins of cosmic rays.
Fermi is able to accomplish this because when cosmic rays interact with interstellar gas they produce gamma-rays. So by looking at supernova remnants of various ages, and combining this information with data from ground based gamma-ray observatories that probe even higher energies, scientists hope to either confirm or rule out these objects as the main sources of cosmic rays.
The initial results are promising. Fermi has found that older remnants, like the supernova remnant IC443, are bright in GeV (billions of electron volts) gamma-rays but relatively dim in TeV gamma-rays (trillions of electron volts). However younger remnants, like Cassiopeia A, are bright in both GeV and TeV gamma-rays.
From this information NASA scientists are concluding that the young remnants -- which have the greater magnetic field strengths -- are able to accelerate the protons to the energies observed. While older remnants have magnetic fields that have weakened to the point that the protons can only reach GeV energies with regularity.
Ultimately, the Fermi data seem to validate the notion that supernova remnants act as giant accelerators of charged particles and that they are the primary source of these mysterious cosmic rays.