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Last updated on April 23, 2014 at 16:59 EDT

Scientists Deduce Contents Of Cosmic Mystery Box

January 1, 2013
Image Caption: Cosmic rays are thought to originate in nests of massive stars called OB associations, like this one in the Tarantula Nebula in the Large Magellanic Cloud, a satellite galaxy of the Milky Way. But how did scientists figure that out? Credit: NASA, ESA, & F. Paresce (INAF-IASF), R. O’Connell (U. Virginia), & the HST WFC3 Science Oversight Committee

April Flowers for redOrbit.com – Your Universe Online

Science teachers in grade school sometimes hand out “mystery boxes,” which contain ramps, barriers and a loose marble. Rotating the marble and feeling it hang up or drop, the students begin to deduce the contents of the box.

Scientists who are trying to understand why tiny particles rain down from space face a similar dilemma, but on a much grander scale. Their mystery box is a hundred thousand light years across, and the only clues they have are the particles themselves. To add a wrinkle, the particles do not travel in straight lines through the galaxy. Rather, they follow tortuous paths that provide no clue to their point of origin.

Though it has taken nearly a century of work to partially solve the cosmic ray mystery, given all the challenges, scientists now feel they are finally close to a complete solution. Around the turn of the century, cosmic rays became an object of curiosity. The scientific world was agog with discoveries of varied types of invisible radiation, including electrons, X rays, and emanations from radioactive elements.

Working with new forms of radiation, scientists discovered that some high-energy radiation was able to reach detectors protected by lead shielding, but they didn’t know where this penetrating radiation originated. A theory caught hold that it was emitted by radioactive material in Earth’s crust.

Austrian scientist Victor Hess overturned this view. He demonstrated, during 10 high altitude balloon flights, that the higher one went, the more penetrating radiation one encountered, revealing that the source had to be extraterrestrial rather than earthly.

Initially, the radiation was thought to be gamma rays, which are high-energy electromagnetic radiation, leading to the misnomer of cosmic “rays.” Evidence accumulated, however, that the rays were affected by Earth’s magnetic field, suggesting that the radiation consisted of charged particles instead.

Robert Milikan and Arthur Holly Compton, both giants of science and Nobel laureates in physics, debated the issue at length. Milikan supported the view that cosmic rays were radiation, and Compton argued that they were corpuscular.

Compton has a special association with the Washington University in St. Louis, where the research team behind this new study is from. Compton was the chair of the department of physics at WUSTL from 1920 — 23. He was awarded the Nobel Prize in Physics in 1927 based on X-ray scattering work he did in the basement of Eads Hall on the university campus. Compton left WUSTL in 1923, but returned to serve as chancellor from 1945 — 53.

Like with Hess before, high-altitude balloons came to the rescue demonstrating first that the incoming radiation consisted of protons and then that there were stripped down nuclei of heavier elements among them.

Today, scientists understand that 90 percent of cosmic rays are hydrogen nuclei (the protons), 9 percent are helium nuclei, and the remaining 1 percent are the nuclei of heavier elements.

Most nuclei have energies between 108 and 1010 electron-volts, high enough that the particles zip through the galaxy at two-thirds the speed of light or faster. The number of particles decreases sharply at higher energies.

A handful of particles with energies above 1020 electron-volts have also been spotted since 1962. Although very little is known about them, scientists think these extra-energetic particles must be powered by an extragalactic source, such as a supermassive black hole at the center of nearby galaxy because their energies are so much higher than those of most cosmic rays.

Once the composition of cosmic rays was understood, scientists turned to the mystery of their origin. Martin Israel, PhD, professor of physics at Washington University and co-investigator on Super-TIGER says there are two parts to that question.

The first part of the question is where does all the energy come from?

“We know roughly the density of cosmic rays in the galaxy and, thanks to ℠clock´ nuclei among the cosmic rays, we know how long the particles wander through the galaxy before they leak out,” Israel says. “Together those two measurements tell us how much energy per unit time – how much power – goes into creating cosmic rays.”

“So we ask what in the galaxy is generating enough power to accelerate them, and almost certainly the only candidate is supernova explosions that mark the violent deaths of massive stars.”

“We know how much energy is released in this type of supernova explosion and roughly how frequent the explosions are,” Israel says. “And it turns out that something like 10 percent of the energy that´s released in supernova explosions probably goes to accelerating cosmic rays.”

The second part of the question, according to Israel, is where is all the energy loaded onto the particles?

The abundance of different elements in the cosmic rays gave the first clue. Those abundances, for the most part, match the abundances of elements in the giant molecular cloud from which the solar system condensed. This is called the “the solar system background.”

This is not always the case, however. Data obtained from ACE – a spacecraft designed to study cosmic rays and solar wind particles – in 1977 revealed that some isotopes are much more abundant in the cosmic rays than in the solar system background.

“The one that´s far and away the outlier is the ratio of neon 22 to neon 20,” says W. Robert Binns, PhD, research professor of physics and Super-TIGER principal investigator. “That turned out to be five times higher in the cosmic rays than in the solar system background, and that´s a huge difference.”

Wolf-Rayet stars, a spectacular stage in the evolution of stars born with a mass greater than about 30 solar masses produce copious amounts of neon 22, as shown by nucleosynthesis calculations and astronomical observations.

The star shines so brightly during the Wolf-Rayet stages that the force of the light pushing outward sets up fierce stellar winds that scour the surface of the star, carrying off an Earth´s-worth of material in as little as a year.

“In the winds from Wolf-Rayet Stars there´s lots of neon 22,” says Binns. “So once it was understood that neon 22 was overabundant in cosmic rays, it seemly likely the Wolf-Rayet stars were contributing to them.”

“We couldn´t account for our data,” says Binns, “unless we assumed cosmic rays had two sources. One component is ordinary solar system background material, just the everyday dust and gas that´s lying around out there. The other component is material from the Wolf-Rayet stars.”

The physicists found they had to add two parts Wolf-Rayet material to eight parts interstellar medium to make the numbers work, allowing the bigger picture to begin emerging.

“The thing about Wolf-Rayet stars,” Binns says, “is they´re almost all found in loosely organized groups of hot, massive stars called OB associations.”

OB associations are clusters of stars that form a single interstellar cloud as they drift together through space. There are three types of stellar associations distinguished by the properties of the stars they harbor and 90% of all stars are thought to form in them.

OB associations consisting of 10 to a few hundred stars of spectral types O and B — both massive blue stars — are where most massive stars are born. Enormous cavities in the interstellar medium, from which the remaining stars shine boldly forth, are blown by high-velocity winds from the stars and supernovae explosions as they reach the ends of their short lives.

The interstellar medium in OB associations are seeded with exotic isotopes by winds from the Wolf-Rayet stars. These exotic isotopes are then swept up and accelerated, together with regular solar system material, by volleys of supernovae explosions.

Binns and Israel both bring up another small bit of the puzzle as a neat example of the methods of cosmic ray science.

Scientists wanted to know if cosmic rays are stuff that were made and expelled in a supernova explosion. Or, is the stuff of a cosmic ray made in a supernova, spewn out into the interstellar medium, and then accelerated a million years later by the shock wave from another supernova?

Perhaps more importantly, how could you possibly tell which scenario is final?

Nickel isotopes provided the answer. In nature, nickel is found in two stable isotopes, nickel 58 and nickel 60. Nickel 59, on the other hand, is unstable and radioactive. The decay of nickel 59 occurs when it captures one of its own electrons, turning a proton into a neutron. This converts nickel 59 to cobalt 59.

Cosmic rays are nuclei stripped of their electrons, so once nickel has been accelerated its stable nucleus will last indefinitely. The test is whether the team will be able to show the presence of nickel 59 in cosmic rays. It should show up if nickel 59 is made in supernova explosions and accelerated promptly to cosmic ray energies. On the other hand, if it is made in a supernova explosion and then lies around in the interstellar medium for a few million years, it shouldn´t’ be found in the cosmic rays.

The isotopes are being measured by the CRIS instrument on the ACE spacecraft as neither TIGER nor Super-TIGER can identify isotopes.

Looking at the ACE histogram of nickel isotopes, there’s a nice peak at nickel 58 and again at nickel 60 with nothing between them. The histogram of cobalt isotopes, however, has the missing nickel 59 reappearing as cobalt 59.

“The math all works,” says Binns. “Nickel 59 has a half-life of about 76,000 years and supernova go off in OB associations roughly once every million years, so there´s ample time for the nickel to decay before being accelerated.”

This begs the question, if scientists know where cosmic rays originate, why is there a WUSTL-led team in Antarctica lofting a two-ton instrument the size of a pool table into the polar vortex. What is the Super-TIGER looking for?

Scientists would order for the abundances of all the naturally occurring elements and all of their isotopes if they were able to order such things from a catalog. But when they capture cosmic rays in the field, this is not what they get. Only one out of every 100 rays intercepted will be the nucleus of an element heavier than helium. Most of the information currently available on cosmic-ray origins has been obtained from this one percent, however, making this the most desirable quarry.

TIGER flew for a record-breaking 31.5 days in 2001; however, its detector was struck by only about 300 particles of the elements between zinc and zirconium.

This didn´t give the scientists a very good measure of the relative abundance since it is only about 1o particles per element.

If it can stay up nearly as along, Super-TIGER, which is much bigger than TIGER, should catch nearly 8 times the cosmic rays, giving the scientists much better statistics.

“You´ll also notice,” says Israel, “that when I talk about supernova blasts accelerating the particles I´m waving my arms a lot.”

Scientists are almost positive that certain supernova blasts are the acceleration engine, although no one is sure exactly how this works. Binns and Israel, however, think a clue is about to start emerging from the cosmic ray data. The clue has to do with gas and dust. Although there isn’t much in space, it isn’t empty. About one gas atom per cubic centimeter and tiny grains of “dust” – like sand or ice – inhabit space.

When the interstellar medium is accelerated, the dust somehow gets the jump on the gas, the data suggests. A possible acceleration mechanism has been proposed, predicting that heavier volatiles should have higher cosmic-ray/solar system ratios than lighter ones. In this model, the refractories (dust) would not display mass dependence.

“However, our TIGER data indicate similar (but not identical) mass dependence for both the volatiles and refractories,” says Israel. “So one of the main things we are looking for with Super-TIGER is improved statistics for the heaviest elements, so that we can pin down the refractory mass-dependence.”

Hess would have understood this. He was repeating an earlier experiment of Theordor Wulf in 1919 when he went up in his balloons to measure cosmic rays. Wulf, a German scientist, carried a detector to the top of the Eiffel Tower in Paris, finding that radiation decreased rather than increased as he went up.

Hess climbed more than 17,000 feet, more than 30 times higher than the Eiffel tower, suspecting that better statistics were needed. He was right.


Source: April Flowers for redOrbit.com - Your Universe Online