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Cosmic rays are particles -- mostly protons -- that bombard Earth at nearly the speed of light. They are not rays in the sense that they may be considered a part of the electromagnetic spectrum, however, they provide an important research vector for scientists to follow. Not much is known about the origin of Cosmic Rays, however several Scientists have put forth the theory that Cosmic Rays are created in supernovae events. They create the elements Beryllium and Boron when they strike atoms of the interstellar medium (ISM), breaking them down into lighter elements. It is within these elements that the history of cosmic rays are written, and investigators have not yet found a natural end to the cosmic-ray spectrum - yet.

The cosmic abundances of the three elements -- hydrogen, helium and lithium -- were initially set in the big bang. By taking a closer look at two of the lightest elements in the universe, it may be possible to solve a mystery that lies at the intersection of cosmology, cosmic rays and chemical evolution.

While astronomers generally agree that cosmic rays originate in supernova explosions, they differ over the mechanism responsible.^43a The traditional view is that cosmic rays are particles of the interstellar medium that were ionized and accelerated by a supernova shock wave. An alternate view, however, suggests that cosmic rays are pieces of the star blown off in the explosion. Supernova remnants are thought to enrich the interstellar medium with heavy elements that eventually condense into new stars, although the space between the stars contains only about one atom per cubic centimeter, a far lower density than the best artificial vacuums we can create.

These volumes are filled with vast electrical and magnetic fields, intimately connected to a diffuse population of charged particles even less numerous than the neutral atoms. The low densities allow electrical and magnetic forces to operate over large distances and timescales. Galactic space is filled with an energetic and turbulent plasma of partially ionized gas.

Stars are net producers of certain elements - in particular, it is thought that the amounts of the elements iron and oxygen will slowly increase in the interstellar medium. This provides a method of measuring the cosmic abundances of beryllium and boron over time and space. The abundance of these two elements should scale with iron in predictable ratios. When measured, however, these ratios were found to be off significantly, leading many astronomers to conclude that the standard cosmic ray scenario was either incomplete or incorrect.

Scientists at the University of Minnesota, decided to re-examine the data. Instead of comparing how the abundances of beryllium and boron change with iron, the scientists chose to compare them with oxygen. Oxygen nuclei are succeptible to being broken up by cosmic rays to make the lighter elements.

By carefully analyzing the abundances of beryllium, boron, iron and oxygen in stars of different ages, Fields and Olive derived new scaling factors that strongly support the traditional view of cosmic ray and light-element production, without requiring additional sources or mechanisms. Their research indicates that the standard picture of cosmic ray origin may be correct after all - cosmic ray particles originate in interstellar gas, not directly from supernovae.^43b

Cosmic rays were first discovered in 1912. As we know by now, they are jets from colliding stars that can produce lethal amounts of muons in the earth's atmosphere, destroy the ozone layer and radioactivate the environment.^44a Roughly once a second, a subatomic particle enters the earth's atmosphere carrying as much energy as a well-thrown rock. Somewhere in the universe, that fact implies, there are forces that can impart to a single proton 100 million times the energy achievable by the most powerful earthbound accelerators.

The interstellar medium contains atomic nuclei of every element in the periodic table, all moving under the influence of electrical and magnetic fields. The most well-known sources of charged particles--such as the sun, with its solar wind--have an energy limit. Cosmic rays appear at energy levels as high as astrophysicists can measure. The data run out at levels around 300 billion times the rest-mass energy of a proton because there is at present no detector large enough to sample the very low number of incoming particles predicted.

On October 15, 1991, for example, a cosmic-ray observatory in the Utah desert registered a shower of secondary particles from a 50-joule (3 x 10²⁰ electron volts) cosmic ray. The identity of high-energy cosmic rays is all but lost when they interact with atoms in the earth's atmosphere and form this type of shower of secondary particles. To ascertain the true nuclear composition, measurements must be made before the cosmic rays reach dense atmosphere. Unfortunately, to collect 100 cosmic rays of energies near 10¹⁴ eV, a 10-square-meter detector would have to be in orbit for three years. At present, typical exposures are more like the equivalent of one square meter for three days. Although the cosmic-ray flux decreases with higher energy, this decline levels off somewhat above about 10¹⁶ eV, suggesting that the mechanisms responsible for ultrahigh-energy cosmic rays are different from those for rays of more moderate energy.

In 1960 Bernard Peters of the Tata Institute in Bombay suggested that lower-energy cosmic rays are produced predominantly inside our own galaxy, whereas those of higher energy come from more distant sources. One reason to think this is that a cosmic-ray proton carrying more than 10¹⁹ eV, for example, would not be deflected significantly by any of the magnetic fields typically generated by a galaxy, so it would travel more or less straight.

If such particles came from inside our galaxy, we might expect to see different numbers coming from various directions because the galaxy is not arranged symmetrically around us. It has been found, however, that the distribution is essentially isotropic, as is that of the lower-energy rays, whose directions are scattered. Such tenuous inferences reveal how little is known for certain about the origin of cosmic rays.

Supernova Pumps

Astronomers have long speculated that the bulk of galactic cosmic rays--those with energies below about 10¹⁶ eV--originate with supernovae - the power required to maintain the observed supply of cosmic-ray nuclei in our Milky Way galaxy is only slightly less than the average kinetic energy delivered to the galactic medium by the three supernova explosions that occur every century. There are few, if any, other sources of this amount of power in our galaxy.

When a massive star collapses, the outer parts of the star explode at speeds of up to 10,000 kilometers per second and more. A similar amount of energy is released when a white dwarf star undergoes complete disintegration in a thermonuclear detonation. Ejected matter expands at supersonic velocities, sending a strong shock into the surrounding medium. It is thought that they then accelerate nuclei from the material they pass through, turning them into cosmic rays, which then follow complicated paths through interstellar magnetic fields. As a result, their directions as observed from the earth yield no information about the location of their original source.

However, by looking at the synchrotron radiation sometimes associated with supernova remnants, researchers have found more direct evidence that supernovae can act as accelerators. Synchrotron radiation is one characteristic of high-energy electrons moving in an intense magnetic field of the kind that might act as a cosmic-ray accelerator. The presence of synchrotron x-rays in some supernova remnants suggests particularly high energies. In earthbound devices, synchrotron emission limits a particle's energy because the emission rate increases as a particle goes faster; at some point, the radiation bleeds energy out of an accelerating particle as fast as it can be pumped in.

Detectors

Recently the Japanese x-ray satellite ASCA made images of the shell of Supernova 1006, which exploded 990 years ago. Unlike the radiation from the interior of the remnant, the X-Rays from the shell have the features characteristic of synchrotron radiation. Astrophysicists have deduced that electrons are being accelerated there at up to 10¹⁴ eV (100 TeV).

The EGRET detector on the Compton Gamma Ray Observatory has also been used to study point sources of gamma rays identified with supernova remnants. The observed intensities and spectra (up to a billion electron volts) are consistent with an origin from the decay of particles called neutral pions, that are produced by cosmic rays from the exploding star's remnants colliding with nearby interstellar gas.

Searches made by the ground-based Whipple Observatory for gamma rays of much higher energies from some of the same remnants have not seen signals at the levels that would be expected if the supernovae were accelerating particles to 10¹⁴ eV or more.

Testing

A complementary method for testing the association of high-energy cosmic rays with supernovae involves the elemental composition of cosmic-ray nuclei. The size of the orbit of a charged particle in a magnetic field is proportional to its total momentum per unit charge - heavier nuclei have greater total energy for a given orbit size. Any process that limits the particle acceleration on the basis of orbit size will thus lead to an excess of heavier nuclei at high energies.

The supernova that is the result of a white dwarf detonation would accelerate whatever nuclei populate the local interstellar medium. A supernova that followed the collapse of a massive star, in contrast, would accelerate the surrounding stellar wind, giving it the characteristics of the outer layers of the progenitor star at earlier stages of its evolution. In some cases, the wind could include an increased fraction of helium, carbon or even heavier nuclei.

Novel Government Research

National Aeronautics and Space Administration has developed techniques to loft large payloads (about three tons) with high-altitude balloons for many days. These experiments cost a tiny fraction of what an equivalent satellite detector would. The most successful flights of this type to date have taken place in Antarctica, where the upper atmosphere winds blow in an almost constant circle around the South Pole - some balloons have circled the continent for 10 days. These winds are so predictable that a payload launched at McMurdo Sound on the coast of Antarctica will travel at a nearly constant radius from the Pole and return eventually to near the launch site.

One of the authors of the foundation article (Swordy) is collaborating with Dietrich Müller and Peter Meyer of the University of Chicago on a 10-square-meter detector. This instrument could measure heavy cosmic rays of up to 10¹⁵ eV on such a flight. There are now efforts to extend the exposure times to roughly 100 days, and include similar flights nearer the equator.

The CASA-MIA-DICE experiment in Utah, in which two of the authors of the reference article (Cronin and Swordy) are involved, measures the distributions of electrons and muons at ground level. It also detects Cerenkov light generated by the shower particles at various levels in the atmosphere. These data enable scientists to reconstruct the shape of the shower more reliably and thus take a better guess at the energy and identity of the cosmic ray that initiated it.

AMANDA detects energetic muons produced in the same showers by observing Cerenkov radiation produced deep in the ice cap, and even now scientists are working with an array that measures showers reaching the surface at the South Pole. The primary goal of AMANDA is to catch traces of neutrinos produced in cosmic accelerators which may generate upward-streaming showers after passing through the earth.

The AGASA array in Japan currently has an effective area of 200 square kilometers.

Fly's Eye HiRes experiment in Utah will cover about 1,000 square kilometers.

The Auger Project^43b is the iniative to gather an even larger sample of ultrahigh-energy cosmic rays. The project is named after Pierre Auger, the French scientist who first investigated the phenomenon of correlated showers of particles from cosmic rays. The plan is to provide detectors with areas of 9,000 square kilometers that are capable of measuring hundreds of high-energy events a year. This development is a detector field that would consist of many stations on a 1.5-kilometer grid, where a single event might trigger dozens of stations. Auger Project design workshop held at the Fermi National Accelerator Laboratory in 1995. Construction of the Argentina site will begin in 1999, and construction of the Utah site will begin in 2000 or 2001 if all goes well at the Argentine site. The entire experiment is projected to run for 20 years.

NASA is planning two missions to further investigate gamma- ray bursts - the High Energy Transient Experiment II (HETE II), scheduled to launch in the fall of 1999 (now sometime in the year 2000). HETE II will be able to precisely locate gamma-ray bursts in near real- time and quickly transmit their locations to ground-based observatories, permitting rapid follow-up studies. The Gamma Ray Large Area Space Telescope (GLAST), scheduled to launch in 2005, will detect only those gamma-ray bursts that emit the highest energy gamma rays, and will be able to locate them with sufficient precision to permit coordinated observations from the ground.

Across Intergalactic Space

Higher-energy cosmic rays--those produced by sources as yet unknown--requires large ground-based detectors, but they still have to overcome the problem of low flux by watching enormous effective areas for months or years. Astronomers in Britain and Russia say that at least some are coming from the remains of a star, within a few hundred light years of Earth, that died in a supernova explosion^45a. It's the first direct evidence that supernova remnants can accelerate nuclei to such extraordinary energies," says Arnold Wolfendale of Durham University^45a.

The information, however, must be extracted from cascades of secondary particles--electrons, muons and gamma rays, which are created high in the atmosphere by an incoming cosmic-ray nucleus. Such indirect methods can only suggest general features of the composition of a cosmic ray on a statistical basis, but are unable to identify the atomic number of each incoming nucleus. The millions of secondary particles unleashed by one cosmic ray are spread over a radius of hundreds of meters - detectors typically sample these air showers at a few hundred or so discrete locations, although technical improvements have enabled such devices to collect increasingly sophisticated data sets

Rare Cosmic Rays

Cosmic rays with energies above 10²⁰ eV strike the earth's atmosphere at a rate of only about one per square kilometer a year, and studying them requires an air-shower detector of truly gigantic proportions. However, it has been shown that there are exceptions to this rule. Data analyzed by scientists from 7 of the 9 detector arrays around the world indicated a pair of bumps. These bumps signal an excess of showers from cosmic rays with energies of 3 x 10¹⁵ and 1.2 x 10¹⁶ electronvolts^45a. The result can be explained if cosmic rays are being accelerated by the "Fermi mechanism." What happens is:

• a) Nuclei are swept up by the expanding supernova remnant and bounce repeatedly off turbulent supersonic shock waves in the gas.
• b) the maximum energy which the Fermi mechanism can give a nucleus goes up with its charge
• c) The energies at the two bumps correspond to the maximum energies of oxygen and iron nuclei

Using all available information, it was determined that the bumps were due to oxygen and iron from a supernova remnant. .

* In addition to the 1991 event in Utah, particles with energies above 10²⁰ eV have been seen by groups elsewhere in the U.S., in Akeno, Japan, in Haverah Park, U.K., and in Yakutsk, Siberia. Two of the conclusions arrived at by the scientists from these stations are:

• (a) they are likely to come from outside our galaxy because no known acceleration mechanism could produce them and because they approach from all directions even though a galactic magnetic field is insufficient to bend their path.
• (b) their source cannot be more than about 30 million light-years away, because the particles would otherwise lose energy by interaction with the universal microwave background.

In the relativistic universe that the highest-energy cosmic rays inhabit, even a single radio-frequency photon packs enough punch to rob a particle of much of its energy. Bearing this in mind, if the sources of such high-energy particles were distributed uniformly throughout the cosmos, wouldn't their interaction with the microwave background cause a sharp cutoff in the number of particles with energy above 5 x 10¹⁹ eV ? As of this time scientist think that:

• There are as yet too few events above this nominal threshold for us to know for certain
• These rays are essentially undeflected by the weak intergalactic magnetic fields
• Measuring the direction of travel of a large enough sample should yield unambiguous clues to the locations of their sources.

Hypothesis One: Galactic black-hole accretion disks - Astrophysicists have predicted that black holes of a billion solar masses or more, accreting matter in the nuclei of active galaxies, are needed to drive relativistic jets of matter far into intergalactic space at speeds approaching that of light. Such jets have been mapped with radio telescopes. Hot spots seen in these radio lobes are thought to be shock fronts that accelerate cosmic rays to ultrahigh energy.

Hypothesis Two: gamma-ray bursts - The speculation about gamma-ray bursts takes off from the theory that the bursts are created by relativistic explosions, perhaps resulting from the coalescence of neutron stars. Mario Vietri of the Astronomical Observatory of Rome and Eli Waxman of Princeton University independently noted a rough match between the energy available in such cataclysms and that needed to supply the observed flux of the highest-energy cosmic rays. They argue that the ultrahigh-speed shocks driven by these explosions act as cosmic accelerators.
Hypothesis Three: topological defects in the fabric of the universe - Perhaps most intriguing is the notion that ultrahigh-energy particles owe their existence to the decay of monopoles, strings, domain walls and other topological defects that might have formed in the early universe. These hypothetical objects are believed to harbor remnants of an earlier, more symmetrical phase of the fundamental fields in nature, when gravity, electromagnetism and the weak and strong nuclear forces were merged. They can be thought of, in a sense, as infinitesimal pockets that sustain parts of the universe as it existed in the fractional nanoseconds after the big bang. As these pockets collapse, and the symmetry of the forces within them breaks, and the energy stored in them is released in the form of supermassive particles that immediately decay into jets of particles with energies up to 100,000 times greater than those of the known ultrahigh-energy cosmic rays. The ultrahigh-energy cosmic rays we observe are the comparatively sluggish products of cosmological particle cascades. In addition, there are some indications that the directions of the highest-energy cosmic rays follow the distribution of radio galaxies in the sky to some extent.

Cosmic Rays from Neutron Stars

It is currently thought that neutron stars whose collisions create cosmic rays are the extremely massive and dense remnants of some supernova explosions. Some neutron stars come in pairs. These stars circle each other at closer and closer range until they eventually spiral into each other. Scientists have suggested that when they collide, a brilliant disk forms briefly and spews enormous jets of high-energy particles known as cosmic rays.

If the earth were in the path of a jet and less than 3,000 light years away, we on earth would experience an intense bombardment, up to a month long, of fast-moving muons which would flood the earth, penetrating hundreds of yards underwater or underground, destroying an organism's central nervous system and causing death within days. Chemical compounds that form would deplete the earth's ozone layer, increasing the amount of ultraviolet light that reaches the surface. That, combined with the radioactivity produced would be deadly to many plants, disrupting the food chain.

Astronomers have identified five pairs of neutron stars in our galaxy - two of which are within 3,000 light years of earth. However, they calculate that it will be several hundred million years before any of them collide.^44a In addition, scientists estimate that there are hundreds more pairs in the Milky Way - as these pairs are discovered, their orbits can be measured to determine when they can be expected to collide.

Cosmic rays are believed to be one of the main contributors to slight genetic damage, so they may well play a part in our evolution. Scientists at LSU have recently been funded to investigate cosmic-rays that are a trillion times more energetic than the run-of-the-mill events that bombard earth at the rate of one per square meter every second. The ones studfied are to be rare and fast - only 10 of them have been recorded in the past 30 years, and they are so fast they must be created by something bigger than a galaxy.

The energy a particle has is measured in electron volts - If you want to get a particle moving at one electron volt, you hook it up to a 1-volt battery. If you want it to move at 1,000 electron volts, which is three orders of magnitude higher, you accelerate it with a classroom generator. To get 1 million electron volts, you have to use a generator the size of those in the Grand Coulee dam. To go up to a trillion electron volts, you need something like Fermilab in Chicago, and to go up another three orders of magnitude (a thousand trillion electron volts) you need a supernova. It is possible to get higher energies from a black hole, or the heart of an active galaxy, but these particles are six orders of magnitude (one billion trillion) more energetic than even these can generate, so it's hard to conceive of what might be making them.^46a

These high-energy particles can't travel very far - they have to come from our galactic neighborhood - about 200 million light years away. That seems like a long way, but in cosmic terms, it's not. The laws of probability say that these high-speed cosmic rays are going to run into something by the time they've traveled 200 million light years - even "empty" space is filled with wandering bits of protons, atoms, photons, dust and background radiation..