Far-off galaxies are firing rare high-energy cosmic rays at us

Pierre Auger Observatory

Detecting cosmic rays takes patience

Steven Saffi

A long-standing mystery about the origin of the highest energy particles arriving from outer space has been solved. Ten years of readings from the world’s largest cosmic ray observatory show that these ultra-high-energy cosmic rays are coming from outside our galaxy.

Cosmic rays are charged particles, mainly atomic nuclei of hydrogen, helium and some other heavier elements, that are constantly bombarding Earth. Particles with energies greater than about 8 billion billion electronvolts were first detected about 50 years ago, but their sources have remained a mystery.

That is because as cosmic rays become ever more energetic, their numbers fall dramatically. On average, only one ultra-high-energy cosmic ray (UHECR) falls on one square kilometre of Earth’s surface per year. Telescopes couldn’t collect enough particles to determine their origins.

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That changed with the Pierre Auger Observatory, which is spread over 3000 square kilometres of Argentinian grassland. It studied the skies for a decade, detecting 30,000 UHECRs.

Some thought that the high-energy particles could have been produced at the centre of the Milky Way, which is known to harbour celestial objects that can accelerate particles to at least a million billion electronvolts.

The extreme universe

But the Pierre Auger collaboration’s analysis shows that the UHECRs are coming from outside our galaxy. After accounting for the deflection of the cosmic rays by the Milky Way’s magnetic field, the team found that the particles are travelling about 326 million light years, from a region of extragalactic space containing several potential sources such as active galactic nuclei and starburst galaxies.

“We can conclude that these are truly extragalactic sources, and it’s the basis of ongoing studies looking for the sources,” says Karl-Heinz Kampert, a spokesperson for the Pierre Auger Observatory.

To identify these precise sources, the observatory’s telescopes will now try to measure the exact atomic weights of the cosmic rays so their deflection by the Milky Way’s magnetic field can be worked out. For example, helium nuclei will be deflected twice as much by this field as hydrogen nuclei of the same energy. This can then be used to retrace each cosmic ray’s path back to its source.

Other telescopes have also been looking at the extreme universe of high-energy particles by searching for highly energetic neutrinos and gamma rays. “The energy density in the extreme universe observed in cosmic rays, in neutrinos and gamma rays turns out to be the same,” says Francis Halzen at the University of Wisconsin-Madison and the principal investigator of the IceCube Neutrino Observatory. “This may point at common sources and not be an accident.”

Neutrinos don’t get deflected by the Milky Way’s magnetic field, so if it can be established that these are from the same sources as the cosmic rays, the neutrinos can be used to better locate their shared origins.

Journal reference: Science, DOI: 10.1126/science.aan4338

Read more: Mystery cosmic radio blasts come with side of gamma rays

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