Extract from ABC News
It's been a mystery of astrophysics for decades — where do the most energetic particles that bombard the Earth come from?
Now, for the first time, an international team of scientists using data from the Pierre Auger Observatory in Argentina have confirmed they are extragalactic visitors from beyond our Milky Way.
The discovery, reported in the journal Science, will help scientists understand more about the origin of the Universe and how black holes and galaxies formed.
"These results will make people think again about how we understand things we thought we understood," study co-author Roger Clay, of the University of Adelaide, said.
High-energy cosmic rays are subatomic particles — predominantly the nucleus of common elements such as hydrogen and iron — travel close to the speed of light.
But it's very rare for cosmic rays above 2 joules to reach Earth — there's only about one per square kilometre per year — which means you need an observatory that covers a lot of ground to pick up a significant number of rays.
The Pierre Auger Observatory is just that. The 3,000 square kilometre observatory, which sits at the base of the Andes in Argentina, has detected more than 30,000 cosmic particles over 12 years.
The Pierre Auger Observatory has an array of 1,600 detectors, a handful of which will pick up a single cosmic ray strike.
By determining when different detectors pick up the cosmic ray, scientists can calculate the direction from which it came and its energy.
Data captured by the observatory indicated the particles were not hitting the Earth uniformly.
Instead they were coming from an area of sky about 90 degrees away from the direction of the centre of our Milky Way.
This area is about 100–200 million light years away and is dense with galaxies, said study co-author Bruce Dawson, also of the University of Adelaide.
"There must be something more powerful in these other galaxies … accelerating cosmic rays to these energies."
Professor Dawson said lower-energy cosmic rays come from our Sun, or when stars explode in a supernovae.
But the energy generated by those sources is not enough to accelerate a cosmic ray to the levels of energy detected by the Pierre Auger Observatory.
Whatever's accelerating the rays must be an even more violent force, he said.
"Supermassive black holes in other active galaxies are spewing out magnetic fields and particles in big jets, and we think in those jets there may be particle acceleration up to these enormous energies," Professor Dawson said.
"Other possibilities include gamma ray bursts in other galaxies, which may accelerate cosmic rays."
While scientists weren't sure on the source of cosmic rays yet — or precisely which galaxies they come from — Professor Dawson said understanding the direction was a key piece of the puzzle.
"We haven't solved that part of the mystery — we haven't identified individual galaxies or sources at this stage, but this is a big step," he said.
The next step is honing in more precisely on those sources.
That'll be made possible by upgrades to the Pierre Auger Observatory happening now and which will become operational in 2018.
"We'll be able to be much more sensitive in being able to determine the source directions — and hopefully hone in on some particular source types," Professor Dawson said.
Now, for the first time, an international team of scientists using data from the Pierre Auger Observatory in Argentina have confirmed they are extragalactic visitors from beyond our Milky Way.
The discovery, reported in the journal Science, will help scientists understand more about the origin of the Universe and how black holes and galaxies formed.
"These results will make people think again about how we understand things we thought we understood," study co-author Roger Clay, of the University of Adelaide, said.
High-energy cosmic rays are subatomic particles — predominantly the nucleus of common elements such as hydrogen and iron — travel close to the speed of light.
But it's very rare for cosmic rays above 2 joules to reach Earth — there's only about one per square kilometre per year — which means you need an observatory that covers a lot of ground to pick up a significant number of rays.
The Pierre Auger Observatory is just that. The 3,000 square kilometre observatory, which sits at the base of the Andes in Argentina, has detected more than 30,000 cosmic particles over 12 years.
How do we know when a cosmic ray hits Earth?
We can tell when a cosmic ray hits the Earth because it creates a shower of electrons and photons as it strikes the atmosphere.The Pierre Auger Observatory has an array of 1,600 detectors, a handful of which will pick up a single cosmic ray strike.
By determining when different detectors pick up the cosmic ray, scientists can calculate the direction from which it came and its energy.
Data captured by the observatory indicated the particles were not hitting the Earth uniformly.
Instead they were coming from an area of sky about 90 degrees away from the direction of the centre of our Milky Way.
This area is about 100–200 million light years away and is dense with galaxies, said study co-author Bruce Dawson, also of the University of Adelaide.
"There must be something more powerful in these other galaxies … accelerating cosmic rays to these energies."
What creates cosmic rays?
While the scientists say they've shown the direction high-energy cosmic rays come from what's still unclear is what creates them.Professor Dawson said lower-energy cosmic rays come from our Sun, or when stars explode in a supernovae.
But the energy generated by those sources is not enough to accelerate a cosmic ray to the levels of energy detected by the Pierre Auger Observatory.
Whatever's accelerating the rays must be an even more violent force, he said.
"Supermassive black holes in other active galaxies are spewing out magnetic fields and particles in big jets, and we think in those jets there may be particle acceleration up to these enormous energies," Professor Dawson said.
"Other possibilities include gamma ray bursts in other galaxies, which may accelerate cosmic rays."
While scientists weren't sure on the source of cosmic rays yet — or precisely which galaxies they come from — Professor Dawson said understanding the direction was a key piece of the puzzle.
"We haven't solved that part of the mystery — we haven't identified individual galaxies or sources at this stage, but this is a big step," he said.
The next step is honing in more precisely on those sources.
That'll be made possible by upgrades to the Pierre Auger Observatory happening now and which will become operational in 2018.
"We'll be able to be much more sensitive in being able to determine the source directions — and hopefully hone in on some particular source types," Professor Dawson said.
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