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PHYSICS

The origin of cosmic rays

Study indicates with greater precision that these ultra-high-energy particles come from outside the Milky Way

A. CHANTELAUZE/S.STAFFI/L.BRET Illustration of particles from cosmic rays falling into a detection tank at the Pierre Auger Observatory in ArgentinaA. CHANTELAUZE/S.STAFFI/L.BRET

In nature, the particles with the highest energy levels are observed in cosmic rays. Some are millions of times more energetic than particles created in accelerators like the Large Hadron Collider (LHC). Cosmic rays are atomic nuclei composed of protons and neutrons which travel through space at near-light speeds. These nuclei can be light, like hydrogen (the most abundant element in the Universe), or heavy like iron. Clues about the origin of ultra-high-energy cosmic rays can be found by analyzing the directions from which they arrive to earth. When they enter the planet’s atmosphere, cosmic rays collide with the nuclei of nitrogen and oxygen in the air; avalanches of particles resulting from these collisions fall to the earth’s surface in a phenomenon known as an air shower. Over a 13-year period, researchers at the Pierre Auger Observatory in the Argentine province of Mendoza studied the particles from these showers to trace the origin of more than thirty thousand cosmic rays with energy exceeding 8 exa-electron volts (8×1018 electron volts). The article resulting from this collaboration was published on September 22 in the journal Science, and indicates that most of these rays originate in galaxies other than the Milky Way.

“This result strongly indicates the extra-galactic nature of ultra-high-energy cosmic rays,” says physicist Carola Dobrigkeit Chinellato of the University of Campinas (UNICAMP). Dobrigkeit leads the Brazilian contingent at the Pierre Auger Observatory, which includes just over 400 researchers from 18 countries. “The chances that this conclusion is just coincidence are 2 in 100 million, the equivalent of a person hitting all six lottery numbers,” she says. In a note published with the article, the British physicist Alan Watson of the University of Leeds, UK, who is spokesman emeritus of the observatory, said that the result of the study is “one of the most exciting we have had, and solves a problem which has been pursued since the observatory was designed by Jim Cronin [American winner of the 1980 Nobel Prize in Physics, deceased in 2016] and I more than 25 years ago.”

The Pierre Auger Observatory was planned and began to be built in the 1990s to detect high-energy cosmic rays, the existence of which was proven in the 1960s. It began operations in 2004, and construction was completed in 2008 at a cost of US$54 million (2008 value). The rain of cosmic rays is recorded by 1,660 surface detectors known as Cherenkov tanks, which operate continuously and are spread across a flat area alongside the Argentine Andes spanning 3,000 square kilometers, twice the size of the city of São Paulo. The sensors detect ultraviolet light which is emitted in the water when the energetic particles that create the showers pass through the tanks. On clear, moonless nights, the measurements are supplemented with data obtained by 27 fluorescence telescopes, which record the ultraviolet light nitrogen molecules in the upper atmosphere emit when they are excited by a shower of particles.

Rare particles
Unlike lower-energy cosmic rays, which are abundant in the earth’s atmosphere (one of these particles per square meter reaches the top of the atmosphere each second), high-energy particles are much more rare. Only one cosmic ray hits the atmosphere per square kilometer each year. For the physicist Ronald Cintra Shellard, director of the Brazilian Center for Physics Research (CBPF), one of 30 Brazilians participating in the international collaboration, the main merit of this work lies in the accuracy with which it determined the origin of these ultra-high-energy cosmic rays. “They do not travel great distances on a cosmic scale. So they must come from neighboring galaxies, at the most 200 megaparsecs away, roughly 250 times as far as Andromeda, the galaxy closest to ours,” explains Shellard. Initially, the researchers at Auger began to consider that high-energy cosmic rays came from near the center of the Milky Way, where there are potential sources for this type of phenomenon. The Sagittarius A region, for example, is home to one of these candidates, a supermassive black hole. “But our results show that the ultra-high-energy cosmic rays come from a direction quite far from the center of the Milky Way, they come from other galaxies,” explains Dobrigkeit. The researchers measured the anisotropy pattern of the cosmic rays, in other words, how their incidence varies in different regions of space, and found that the phenomenon is more likely to occur in an area with a high concentration of galaxies (see image).

Despite the evidence that the phenomenon originated outside the Milky Way, many enigmas remain. It is not yet possible to state which galaxies cosmic rays emerge from, and the nature of these particles and the phenomena that produce them are not clearly understood. In a 2007 study based on observations of only 27 cosmic rays with even higher energy levels than those analyzed in the current study, researchers at the Auger Observatory suggested that this phenomenon originates in the cores of active galaxies neighboring the Milky Way. But this observation was not confirmed as work advanced at the observatory.

By the end of 2018, an international partnership is planned to make improvements to the observatory’s system of surface detectors. The Auger costs US$1.9 million per year to run; Brazil contributes US$120,000 per year, which is funded by FAPESP, the Ministry of Science, Technology, Innovation and Communications (MCTIC), and the Brazilian Funding Authority for Studies and Projects (FINEP). The observatory is scheduled to continue collecting data until at least 2025.

Project
Study of the highest-energy cosmic rays with the Pierre Auger Observatory (No. 10/07359-6); Grant Mechanism Thematic Project; Principal Investigator Carola Dobrigkeit Chinellato (UNICAMP); Investment R$5,122,504.57.

Scientific article
The Pierre Auger collaboration. Observation of a large-scale anisotropy in the arrival directions of cosmic rays above 8×1018 eV. Science. Sept. 22, 2017.

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