Ultra-high-energy cosmic rays have extragalactic origin

This discovery is described in an article published in the journal Science by the collaboration, which comprises some 500 scientists from 17 member countries, including 30 researchers from Brazil.

The participation of researchers affiliated with universities and research institutions in São Paulo State is supported by FAPESP. The participation of scientists from other states is supported by other state and federal research funding agencies (read more about Brazil’s participation in the Pierre Auger Collaboration at agencia.fapesp.br/20997).

“The probability that this conclusion is due to chance is two in 100 million,” said Carola Dobrigkeit Chinellato, a professor at the University of Campinas’s Gleb Wataghin Physics Institute (IFGW-UNICAMP) and chair of the Auger Collaboration’s Brazilian committee, in an interview with Victoria Flório of Pesquisa FAPESP magazine.

Using data recorded by the Observatory between January 2004 and August 2016, the researchers found that larger numbers of ultra-high-energy cosmic rays – with energies exceeding 8 x 1018 electron-volts (eV) – arrive here from one side of the sky.

The region from which most of the ultra-high-energy cosmic rays come coincides with the location of a large proportion of the galaxies in the vicinity of the Milky Way, i.e., within a radius of 700,000 light-years. “This is strong evidence that high-energy cosmic rays come from outside the Milky Way,” Chinellato said.

According to the authors of the article, this discovery contributes to an understanding of not only the origin of these particles but also the cosmic mechanisms capable of giving so much energy to these tiny subatomic entities, which travel trillions of kilometers through space and arrive at the Earth with extreme energy levels.

Cosmic rays are charged particles, mainly the atomic nuclei of hydrogen, helium, and other heavier elements such as iron, and are constantly bombarding Earth. Ultra-high-energy cosmic rays were first detected approximately 50 years ago, but their sources and production mechanisms have remained a mystery because the number of rays reaching the Earth decreases dramatically as the rays become more energetic. On average, only one ultra-high-energy cosmic ray (<1018 eV) falls on one square kilometer (km2) of the Earth’s surface each year.

The researchers of the Auger Collaboration have studied the distribution of the directions from which these ultra-high-energy particles arrive at the Earth in order to identify their origins.

When ultra-high-energy cosmic rays reach our atmosphere at altitudes ranging from 10 km-20 km, they collide with the atomic nuclei of elements in the air, such as nitrogen and oxygen. Each collision creates hundreds or thousands of other particles, which plunge toward Earth’s surface at almost the speed of light (approximately 300,000 km per second), forming cascades of particles that scientists call “extended atmospheric showers.”

The charged particles in these showers excite nitrogen particles in the air, producing a faint blue glow that is captured by Auger’s fluorescence telescopes on clear nights. The particles are also registered at 1,660 surface detectors spread in a grid around the observatory, which is named for the French physicist Pierre Auger (1899-1992).

Covering an area of 3,000 km2 of a vast plain overlooked by the Andes, the detectors are polyethylene tanks filled with 12,000 liters of purified water each and are equipped with photomultiplier tubes.

When the particles in an air shower pass through the water in the tank, they emit light called Cherenkov radiation, which can be measured by the photomultipliers. Based on their analyses of these two kinds of light and other data, scientists can extract a wealth of information about the primary cosmic rays that trigger particle cascades at the top of the atmosphere.