August 30, 2013
Breakthrough Data Point To Origin Of Extra-Galactic Cosmic Rays
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John P. Millis, PhD for redOrbit.com - Your Universe Online
For more than 100 years astronomers have been aware of their existence, but their origins have remained something of a mystery. But new reports may finally be bringing this mystery to a close.
The majority of cosmic rays are now believed to originate from supernova explosions, the result of the collapse of a massive star. Such cosmic rays are known as Galactic Cosmic Rays, since their associated cosmic rays come from supernovae within the Milky Way.
But above about 100-billion-billion electron volts – one electron volt is the energy gained by an electron moving through a one volt potential – it is believed the cosmic rays come from extra-galactic events. The leading theories have involved certain gamma-ray burst (GRB) events – similar to supernovae, but even more intense as they are tied to only the most massive stars – or perhaps even from active supermassive black hole cores.
The defining point in the cosmic ray spectrum is known as the knee, because the rate at which we observe the cosmic rays, as a function of energy, bends sharply at the point, similar to how a human knee bends a leg.
“The spectrum steepens at the ‘knee,’ which is generally interpreted as the beginning of the end of the galactic population. Below the knee, cosmic rays are galactic in origin, while above that energy, particles from more distant regions in our universe become more and more likely,” explains University of Delaware physicist Bakhtiyar Ruzybayev.
However, energies above this 'knee' are difficult to probe with current experimentation, so establishing the exact shape of the cosmic ray spectrum has lacked precision. Now, however, using the IceCube Neutrino Observatory at the South Pole, researchers have been able to place the most accurate constraints on the shape of the cosmic ray spectrum above the knee. They have found that the 'bends' are even more complicated than previously thought, revealing extra features such as spectral hardening around 20 PeV and steepening around 130 PeV.
“These measurements provide new constraints that must be satisfied by any models that try to explain the acceleration and propagation of cosmic rays,” explains Ruzybayev.