'The age of cosmic ray astronomy has arrived'

International scientists within the Pierre Auger Collaboration have revealed that galactic nuclei are the most likely source of highest-energy cosmic rays, these energetic particles that
occasionally hit the Earth are travelling the universe at close to the speed of light. Although they were first identified nearly 70 years ago, their source has remained a mystery.

The research, on which an article has been published in the current issue of Science magazine, shows that sources of highest-energy particles – protons and atomic nuclei – are not distributed
uniformly across the sky, but are linked to the locations of nearby galaxies that have active nuclei in their centres. Active galactic nuclei (AGN) are thought to be powered by supermassive
black holes. However, while most galaxies have black holes at their centre, few have AGNs. Scientists think that galaxies that have an AGN will probably have collided with another galaxy or
have experienced some other massive disruption in the last few hundred million years.

‘We have taken a big step forward in solving the mystery of the nature and origin of the highest-energy cosmic rays,’ says Nobel Prize winner Professor James Cronin of the University of
Chicago, who conceived the Pierre Auger Cosmic Ray Observatory, together with Professor Alan Watson of the University of Leeds. ‘We find the southern hemisphere sky as observed in
ultra-high-energy cosmic rays is non-uniform. This is a fundamental discovery. The age of cosmic ray astronomy has arrived. In the next few years our data will permit us to identify the exact
sources of the cosmic rays and how they accelerate these particles.’

‘This result heralds a new window to the nearby universe and the beginning of cosmic ray astronomy,’ adds Professor Watson. ‘As we collect more and more data, we may look at individual galaxies
in a detailed and completely new way. As we had anticipated, our observatory is producing a new image of the universe based on cosmic rays.’

The Pierre Auger Cosmic Ray Observatory, named after the French physicist who first identified cosmic rays in 1938, is situated on the Pampa Amarilla, a vast plain in Western Argentina. The
observatory – although still under construction – has been operating since 2004 and has recorded almost a million cosmic ray showers so far, of which only 77 showed ultra-high energy (above
4×1019 electron volts).

‘Low-energy cosmic rays are abundant and come from all directions, mostly within our own Milky Way Galaxy,’ Dr Paul Mantsch, project manager of the observatory, explains. ‘Until now the only
source of cosmic ray particles known with certainty has been the Sun. Cosmic rays from other likely sources such as exploding stars take meandering paths through space, so that when they reach
Earth it is impossible to determine their origins.’ The reason for this is that particles with lower energetic profiles will be deflected by galactic or intergalactic magnetic fields.

Highly energetic particles, on the other hand, take an almost straight path from their point of origin. ‘When you look at the highest-energy cosmic rays from the most violent sources, they
point back to their sources,’ Dr Mantsch says. ‘The challenge is now to record enough of these cosmic bullets to understand the processes that hurl them into space.’

As highest-energy cosmic rays are so rare – one event per square kilometre per century – an observatory designed for capturing such events has to be exceptionally large: The Auger Observatory
is composed of an array of 1,600 particle detectors, of which 1,400 have already been set up, placed 1.5 kilometres apart in a grid spread across 3,000 square kilometres. 24 specially designed
telescopes record the emission of fluorescent light from the air shower created when the particles hit the Earth’s upper atmosphere.

While the Auger Collaboration is already developing plans for a second, even larger site further North in the US-state of Colorado, more than 370 scientists and engineers from 17 countries
around the world are still working on the completion of the Argentinean site. The participating countries and organisations, including 10 EU Member States and the Latin American
Formación Académia-European Community (ALFA-EC) network, are sharing the USD 54 million (?37 million) construction budget.

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