Positrons, muons, pions, kaons and more were sorted out of the subatomic clutter beginning in the s, adding to a growing zoo of particles. But finding out where cosmic rays originate is a more difficult task, because as the charged particles transit space, their trajectories are scrambled by the powerful magnetic fields that litter interstellar and intergalactic space. In short, there is no straight path back to a source such as a photon of starlight provides astronomers, notes UW-Madison physics professor Justin Vandenbroucke.
So discovering exactly where the more energetic cosmic rays come from has been a grail of physics since their discovery. Suspected sources include some of the most violent phenomena in the universe, objects that act as massive accelerators — far more powerful than any on Earth — to send the particles flying through the universe at light speed.
The most common low-energy cosmic rays come as a stream of charged particles from the sun, a phenomenon known as the solar wind. The highest energy cosmic rays, scientists think, may be emanating from supernova remnants, gamma ray bursters, crashing galaxies and a class of objects known as active galactic nuclei, the black hole cores of massive galaxies.
Blazars, the type of object identified by the IceCube, MAGIC and Fermi observatories as the first identified extragalactic source of cosmic rays, are a type of active galactic nuclei. They are distinguished by the twin jets of energy and matter blasting laser-like from the poles of a rapidly spinning supermassive black hole at the center of the galaxy.
When a jet from an active galactic nucleus is pointing directly at Earth, that classifies the object as a blazar. Moreover, the jets sometimes flare for periods ranging from minutes to months, becoming as much as 10 times brighter. The key to identifying the blazar as a source of cosmic rays is the high-energy neutrino: a nearly massless, uncharged particle that, unlike cosmic rays, travels in a straight line from its place of origin, believed to be the same kinds of distant accelerators that generate cosmic rays.
When a neutrino crashes into a proton it creates a muon, which, in turn, creates a streak of pale blue light when travelling through a medium such as the deep Antarctic ice that makes up the IceCube detector. When IceCube detects the highest energy neutrinos, as it did on Sept.
Astronomers believe anomalous cosmic rays occur when electrically neutral atoms in the heliosheath are ionized and accelerated. When the Voyager 1 spacecraft passed from the heliosheath into interstellar space in , it detected significantly fewer anomalous cosmic rays and a huge increase in those from outside the solar system.
The second type, galactic cosmic rays, flows into the solar system from other parts of the Milky Way. In early , astronomers announced confirmation that supernovae produce most cosmic rays of this type. In the aftermath of a supernova explosion, particles bounce repeatedly among the entangled magnetic fields within the gaseous remnant and accelerate into cosmic rays. At some critical point, they escape into the galaxy. Third are the abundant cosmic rays that originate from the Sun.
Most of these are protons, particles at relatively low energies. The last kind is ultra-high-energy cosmic rays, the type being studied by the Auger Observatory and other projects.
These include the Oh-My-God particles. In , researchers announced compelling evidence that suggests these extremely powerful cosmic rays originate from outside the Milky Way; however, they were unable to pinpoint specific sources. Researchers hope that the future Cherenkov Telescope Array, set to begin operations in , will help them better understand the cataclysmic events that forged these extremely energetic cosmic rays.
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Watch : Mining the Moon for rocket fuel. That means we have to determine where cosmic rays come from by indirect means. Because cosmic rays carry electric charge, their direction changes as they travel through magnetic fields.
By the time the particles reach us, their paths are completely scrambled, as shown by the blue path. We can't trace them back to their sources. Light travels to us straight from their sources, as shown by the purple path. One way we learn about cosmic rays is by studying their composition. What are they made of? What fraction are electrons? Measuring the quantity of each different element is relatively easy, since the different charges of each nucleus give very different signatures.
Harder to measure, but a better fingerprint, is the isotopic composition nuclei of the same element but with different numbers of neutrons. To tell the isotopes apart involves, in effect, weighing each atomic nucleus that enters the cosmic ray detector. All of the natural elements in the periodic table are present in cosmic rays. This includes elements lighter than iron, which are produced in stars , and heavier elements that are produced in violent conditions, such as a supernova at the end of a massive star's life.
ACE launched in August Detailed differences in their abundances can tell us about cosmic ray sources and their trip through the galaxy. Even in this one percent there are very rare elements and isotopes. Elements heavier than iron are significantly more rare in the cosmic-ray flux but measuring them yields critical information to understand the source material and acceleration of cosmic rays.
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