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High Energy Cosmic Rays -- Powerful Messengers From Beyond Our Galaxy
Highly energetic particles bombard the Earth's atmosphere continuously. The energy of these particles can be a million times higher than the energy of particles accelerated by the most powerful accelerator on Earth, CERN’s Large Hadron Collider (LHC). High energy cosmic rays are truly a mystery. Something out there - no one knows what - is hurling incredibly energetic particles around the Universe. Do these particles come from some unknown super-powerful cosmic explosion? From a huge black hole sucking stars to their violent deaths? From colliding galaxies? Researchers at the Pierre Auger Observatory in Argentina have concluded that the most extreme and energetic of these cosmic ray particles are not originating in our Milky Way Galaxy, but rather in extra-galactic objects far, far away. (Credit: Pierre Auger Observatory) (Image Credit: ESO, L. Calcada)
High Energy Cosmic Rays – Powerful Messengers From Beyond Our Galaxy
A unique observatory in a remote location in Argentina is starting to unravel the mysteries of High Energy Cosmic Rays. There is no scientific consensus on the source of these particles which shower the Earth at energies a million times higher than that produced in particle accelerators like CERN’s Large Hadron Collider (LHC), but the Pierre Auger Observatory is shedding new light on these energetic particles from space and using them as messengers to tell us more about the wider Universe.
Scientists love a mystery, because solving a mystery in nature means the opportunity to learn something new about the Universe. High-energy cosmic rays are just such a mystery. Something out there - no one knows what - is hurling incredibly energetic particles around the Universe. Do these particles come from some unknown super-powerful cosmic explosion? From a huge black hole sucking stars to their violent deaths? From colliding galaxies?
We don't yet know the answers, but we do know that solving this mystery will take scientists another step forward in understanding the universe.
Researchers at the Pierre Auger Observatory report observational evidence, which demonstrates that cosmic rays with energies a million times greater than that of the protons accelerated in the Large Hadron Collider (LHC) come from much further away than from our own Galaxy. Ever since the existence of cosmic rays with individual energies of several Joules was established in the 1960s (1 Joule = approximately 6 x 10^18 electron volts), speculation has raged as to whether such particles are created in our galaxy or in distant extragalactic objects. The 50 year old mystery has been solved using cosmic particles of mean energy of 2 Joules recorded with the largest cosmic ray observatory ever built, the Pierre Auger Observatory in Argentina. The researchers have found that at these energies, the rate of arrival of cosmic rays is about six percent greater from one half of the sky than from the opposite one, with the maximum number of high energy cosmic rays arriving from 120 degrees away from the galactic center.
In the view of Professor Karl-Heinz Kampert (University of Wuppertal), spokesperson for the Auger Collaboration, which involves over 400 scientists from 18 countries, "We are now considerably closer to solving the mystery of where and how these extraordinary particles are created, a question of great interest to astrophysicists. Our observation provides compelling evidence that the sites of acceleration are outside the Milky Way”.
Professor Alan Watson (University of Leeds), emeritus spokesperson, considers this result to be “one of the most exciting that we have obtained and one which solves a problem targeted when the Observatory was conceived by Jim Cronin and myself over 25 years ago”.
Cosmic rays are the nuclei of elements from hydrogen (the proton) to iron. Above 2 Joules, the rate of their arrival at the top of the Earth’s atmosphere is only about one per square kilometer per year -- equivalent to one hitting the area of a football pitch (i.e., soccer field) about once per century. Such rare particles are detectable because they create showers of electrons, photons, and muons through successive interactions with the nuclei in the atmosphere. These showers spread out, sweeping through the atmosphere at the speed of light in a disc-like structure, similar to a dinner plate several kilometers in diameter. They contain over ten billion particles and, at the Auger Observatory, are detected through the Cherenkov light they produce in some of the 1600 detectors, each containing 12 tons of water, spread over 3000 squre kilometers of Western Argentina, an area comparable to that of Rhode Island in the USA. The times of arrival of the particles at the detectors, measured with GPS receivers, are used to find the arrival directions of events to within one degree.
By studying the distribution of the arrival directions of more than 30,000 cosmic particles, the Auger Collaboration has discovered an anisotropy, significant at 5.2 standard deviations (a chance of about two in ten million), in a direction where the distribution of galaxies is relatively high. Although this discovery clearly indicates an extragalactic origin for the particles, the actual sources have yet to be pinned down.
The direction of the excess points to a broad area of sky rather than to specific sources as even particles as energetic as these are deflected by a few tens of degrees in the magnetic field of our Milky Way Galaxy. The direction, however, cannot be associated with putative sources in the plane or center of our Galaxy for any realistic configuration of the Galactic magnetic field.
Cosmic rays of even higher energy than the bulk of those used in this study exist -- some even with the kinetic energy of well struck tennis ball. As the deflections of such particles are expected to be smaller, the arrival directions should point closer to their birthplaces. These cosmic rays are even rarer and further studies are underway using them to try to pin down which extragalactic objects are the sources. Knowledge of the nature of the particles will aid this identification and work on this problem is targeted after the next upgrade of the Auger Observatory.
What Are Cosmic Rays?
Cosmic rays are fast-moving particles from space that constantly bombard the earth from all directions. Most of the particles are either the nuclei of atoms or electrons. Of the nuclei, most are single protons - the nuclei of hydrogen atoms - but a few are much heavier, ranging up to the nuclei of lead atoms. Cosmic ray particles travel at nearly the speed of light, which means they have very high energy. Some of them, in fact, are the most energetic of any particles ever observed in nature. The highest-energy cosmic rays have a hundred million times more energy than the particles produced in the world's most powerful particle accelerator.
Where do Cosmic Rays Come From?
Lower-energy cosmic ray particles that strike the earth come from within our own Milky Way Galaxy. They may originate, directly or indirectly, from the supernova explosions that mark the deaths of many stars. These explosions throw out fast-moving magnetic fields which reflect charged particles. Cosmic ray nuclei gain energy when they collide with such a moving reflector. At a magnetic shock, where the magnetic field slows abruptly, particles can become trapped between two reflectors. Like a ping-pong ball caught between two converging paddles, the nuclei make many reflections, and the energy gained in each reflection grows as their energy increases. This "magnetic shock acceleration" model was first proposed by the great physicist Enrico Fermi as an explanation for the acceleration of most cosmic rays. The process has been observed in magnetic shocks in the solar wind that flows out from our sun, producing cosmic rays of modest energy. The stronger moving magnetic fields produced in supernova explosions could provide the energy for most other cosmic rays.
Even these shocks are not strong enough, however, to accelerate the highest-energy cosmic rays. While no one knows their source, there are compelling reasons to believe that they must originate outside our Milky Way galaxy - but where?
Where Do They Get Their Energy?
Wherever they come from, the highest-energy particles hold secrets to the origin of their enormous energies, many millions of times greater than any earthbound particle accelerator can create. Fermi's acceleration mechanism provides an explanation for cosmic ray energies perhaps as high as 1 x 10^15 eV (Electron Volts). Acceleration mechanisms for cosmic rays of higher energies are not understood.
Observational evidence supports the view that cosmic rays with energies up to about 3 x 10^18 eV originate within our galaxy. Above this energy, most cosmic rays may be coming from outside the Milky Way. The highest energy cosmic rays are not deflected much by the weak magnetic fields in our Galaxy, yet they do not arrive preferentially from the disk of the Milky Way or the side of the sky toward the center of the Galaxy. This strongly suggests an extragalactic origin. Although we have not confirmed any source in the cosmos that can produce such energies, several hypotheses have been proposed. These include radio galaxy hot spots and Active Galactic Nuclei (AGN) jets.
Other Cosmological Questions
Because the highest energy cosmic rays are deflected very little by the magnetic fields in our galaxy - and even less by the much weaker fields in intergalactic space - we ought to be able to look back in the direction of the cosmic rays to find their origin. So far, however, none of the cosmic ray events with energies above 1 x 10^20 eV point back to a possible source in the cosmos. Where have they come from? The mystery deepens when we realize that, unless the source is fairly close to our Milky Way Galaxy (within 100 million light years or so), collisions with the low energy microwaves that pervade the universe would reduce cosmic ray energies to levels below 1 x 10^20 eV before they ever reached Earth. The sources must be relatively nearby, but the arrival directions do not point to any known astrophysical powerhouses.
Cosmologists, who study the structure and dynamics of the universe, offer another possible explanation for the mysterious source of the highest-energy cosmic rays. Cosmologists postulate a universe filled with relics left over from the Big Bang - hypothetical objects, called topological defects, with names like "Cosmic Strings," "Domain Walls," and "Monopoles." Although these strange objects figure prominently in theories of the evolution of the universe, we have no experimental evidence to show that they really exist. However, if they did exist, and if they sometimes collapsed, their collapse could produce enough energy to create very high energy cosmic rays. If we could make the connection between high energy cosmic rays and the collapse of topological defects, it would provide experimental evidence for these topological defects and a great step forward in understanding the early universe.
How Do We Learn About Cosmic Rays?
To learn about the nature of high energy cosmic rays, scientists measure their energy and their direction as they arrive from space. Cosmic rays of modest energy are measured directly by sending detectors to heights above most of the Earth's atmosphere, using high-flying balloons and satellites. For high energy cosmic rays, however, it is more efficient to exploit the atmosphere, measuring each cosmic ray indirectly by observing the shower of particles it produces in the air.
An air shower occurs when a fast moving cosmic ray particle strikes an air molecule high in the atmosphere, creating a violent collision. Fragments fly out from this collision and collide with more air molecules, in a cascade that continues until the energy of the original particle is spread among millions of particles raining down upon the earth. By studying the air showers, scientists can measure the properties of the original cosmic ray particles.
"These high energy cosmic rays are messengers from the extreme universe," said Nobel Prize winner Jim Cronin, of the University of Chicago, who conceived the Auger experiment together with Alan Watson of the University of Leeds. "They represent a great opportunity for discoveries."
Watson added: "How does nature create the conditions to accelerate a tiny particle to such a high energy? Tracking these ultra- high energy particles back to their sources will answer that question."
Cosmic rays generally are charged particles. Lower energy rays are greatly affected by galactic magnetic fields, taking twisted and distorted paths to earth. High-energy rays, less affected by magnetic fields, take a more direct path to Earth. If experimenters see more rays from one direction than from another, they can refine their observations to include point source searches, tracking back fairly closely to a point source or an object in the sky.
"Once more science stands at the threshold of resolving a fundamental question that has so far eluded mankind - the source of high energy cosmic rays," the Chief Executive of the UK's Particle Physics and Astronomy Research Council (PPARC), Professor Keith Mason. "And I look forward with great interest to Auger's quest to unravel one of Nature's most intriguing mysteries."
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