Researchers One Step Closer to Solving the Mystery of Cosmic Rays

The Pierre Auger Collaboration has been able to definitively answer the question of whether cosmic particles come from outside the Milky Way Galaxy in an article published today in the journal Science.

The article, titled “Observation of a large-scale anisotropy in the arrival directions of cosmic
rays above 8 × 1018 eV”, notes that examining the distribution of the cosmic ray arrival directions is the primary step in establishing where extragalactic particles originate.

The collaborating Researchers were able to do their recordings using the largest cosmic ray observatory ever constructed, the Pierre Auger Observatory in Argentina. Included in this partnership are David Nitz and Brian Fick, Professors of Physics at Michigan Technological University.

"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,” says Karl-Heinz Kampert, a Professor at the University of Wuppertal in Germany and spokesperson for the Auger Collaboration, which involves over 400 Scientists from 18 countries.

Cosmic rays are the nuclei of elements from iron to hydrogen. Analyzing them gives Researchers a way to examine matter from outside Earth’s solar system—and currently, outside Earth’s galaxy. Cosmic rays help us comprehend the composition of galaxies and the processes that happen to quicken the nuclei to virtually the speed of light. By analyzing the cosmic rays, Researchers may learn about what mechanisms make the nuclei.

The nitrogen in our DNA, the calcium in our teeth, the iron in our blood, the carbon in our apple pies were made in the interiors of collapsing stars. We are made of starstuff.

Carl Sagan, American Astronomer

Simply put, understanding cosmic rays and where they originate can help Researchers answer major questions about the origins of the universe, Earth’s galaxy and ourselves.

Incredibly Energetic and Far-Traveling

It is very rare for cosmic rays with energy greater than two joules to enter Earth; the rate of their arrival at the top of the atmosphere is just about one per square kilometer yearly, the equivalent to one cosmic ray reaching an area the size of a soccer field about once per century.

Energy is measured in joule; one joule is equal to one 3,600th of a watt-hour. When a single cosmic ray particle reaches the Earth’s atmosphere, that energy is deposited within a few millionths of a second.

Such rare particles are detectable because they produce showers of photons, electrons and muons through consecutive interactions with the nuclei in the atmosphere. These showers pan out, sweeping through the atmosphere at the speed of light in a disc-like structure, like a massive dinner-plate, several kilometers in diameter. They contain over 10 billion particles.

At the Pierre Auger Observatory, cosmic rays are detected by measuring the Cherenkov light—electromagnetic radiation discharged by charged particles passing through a medium, such as water, at more than the phase velocity of light in that medium. The team measures the Cherenkov light formed in a detector, which is a large plastic structure that holds 12 tons of water. They pick up a signal in a few detectors within a group of 1,600 detectors.

The detectors are span across 3,000 km² near the town of Malargüe in western Argentina, an area measuring the same size as Rhode Island. The arrival times of the particles at the detectors, measured with GPS receivers, are used to establish the direction from which the particles came within nearly one degree.

By examining the distribution of the arrival directions of over 30,000 cosmic particles, the Pierre Auger Collaboration has found an anisotropy, which is the variance in the rate of cosmic ray arrivals based upon which direction one looks. This means the cosmic rays do not come consistently from all directions; there is a direction from which the rate is greater.

The anisotropy is important at 5.2 standard deviations (a chance of approximately two in ten million) in a direction where the distribution of galaxies is comparatively high. Although this finding clearly specifies an extragalactic origin for the particles, the specific sources of the cosmic rays are still unidentified.

The direction points to a wide area of sky instead of to specific sources because even such energetic particles are deflected by a few tens of degrees in the magnetic field of Earth’s galaxy.

There have been cosmic rays noticed with even higher energy those used in the Pierre Auger Collaboration research, some even with the kinetic energy of a well-struck tennis ball. As the deflections of such particles are anticipated to be smaller because of their higher energy, the arrival directions should point closer to their origins. Such cosmic rays are even rarer and additional studies are ongoing to pin point which extragalactic objects are the sources.

Knowledge of the nature of the particles will help this identification, and ongoing work on this issue is targeted in the upgrade of the Pierre Auger Observatory to be finalized in 2018.

It Takes A (Global) Village

Conducting this caliber of science is not the responsibility of one person. Over 400 Researchers have added to the research. At Michigan Tech, David Nitz, Professor of Physics, was the one who added the electronics that record the signals in the water tanks. He has written the code that is programmed into the circuits, which transforms the Cherenkov light in the water tank detectors into digital signals. This enables the hardware to make very rapid decisions about the signals recorded in the tanks and whether they are worth additional analysis.

I really enjoy this kind of science. But I’m a hands-on guy. I visualize how we go from concept to actually building an instrument so we can address that science. That’s what I’ve been doing all my scientific career: answering how do we address making those measurements.

David Nitz, Professor of Physics, Michigan Tech

Part of the upgrade to the Pierre Auger Observatory is to change older circuit boards with newer ones that have better capability to process signals quicker and more accurately, and add the signals from extra detectors. These extra detectors include a scintillator detector above each surface detector, and incorporating a fourth photomultiplier tube to each detector.