鈥淚T WAS as if they went out to catch butterflies and caught an F-111 aircraft.鈥� This often-quoted line captures just how surprised researchers were when an astonishingly energetic particle from space was caught in an experiment in 1991.
Since then they have puzzled over the origin of such rare bullets, called ultra-high energy cosmic rays. While lower energy cosmic rays are known to come from the sun and other sources within our galaxy, such as the remnants of supernova explosions, the highest energy versions are detected so rarely that no one has been able to pin down their origin.
Last week, a team of researchers announced the answer: the fastest particles come from enormous black holes at the hearts of energetic galaxies millions of light years away. As well as solving a long-standing mystery, high-energy cosmic rays might provide a novel way to image the sky. 鈥淭he result heralds a new window to the nearby universe and the beginning of cosmic-ray astronomy,鈥� says Alan Watson from the University of Leeds, UK, a spokesperson for the team.
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Until last week, there had been a range of theories to explain where high-energy cosmic rays came from. Active galactic nuclei (AGNs) 鈥� energetic galaxies powered by matter swirling onto a supermassive black hole 鈥� were strong contenders, but researchers also suspected gamma-ray bursts, violent explosions sometimes thought to mark the collapse of a massive star into a black hole. Another, bizarre theory was that they signalled the decay of heavy particles trapped since the big bang inside weird knots in space-time.
To catch enough of the cosmic rays to pinpoint their origin meant building the largest cosmic-ray detector on the planet 鈥� the Pierre Auger Observatory in Argentina, which began operating in 2004 (see Illustration). The Auger researchers analysed 27 of the most energetic cosmic rays, detected up to August this year, and found that almost all appeared to come from AGNs less than 250 million light years away. The chances of the pattern being a coincidence are just 1 in 100 (Science, vol 318, p 938).
鈥淚鈥檓 very pleased because this is what I鈥檝e argued on the basis of detailed physics for 20 years,鈥� says team member Peter Biermann from the Max Planck Institute for Radio Astronomy in Bonn, Germany. However, he cautions that astronomers can鈥檛 yet rule out the possibility that some high-energy cosmic rays come from gamma-ray bursts. This is likely to be ironed out as the Auger array and similar observatories collect more data.
Perhaps just as important as providing the first hard evidence of an origin, the result is the first in the new field of cosmic-ray astronomy, says Watson. Big arrays like Auger are capable of capturing large quantities of cosmic rays, so astronomers will be able to image various phenomena not visible with light. For example, Auger could help track magnetic fields in our galaxy, because these fields bend charged high-energy cosmic rays slightly off a straight path. The deviations should reveal the hand of the Milky Way鈥檚 magnetic fields.
Auger observations will also allow researchers to probe the physics of particle collisions, because when they hit the atmosphere cosmic rays can produce collisions 30 times more energetic than the world鈥檚 most powerful particle accelerator, the Large Hadron Collider, due to open next year in Switzerland.
鈥淲e鈥檙e really just starting 鈥� we鈥檝e got a fantastic 10 years ahead,鈥� says Watson.
