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High-energy X-rays detected at galaxy cluster

A distant galaxy cluster has turned into a giant particle accelerator, spinning electrons over vast distances at high speeds.
Image: Two galaxy clusters collide
Shockwaves travel through hot gas (in red) as two galaxy clusters collide and merge.Christophe Carreau
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A distant galaxy cluster has turned into a giant particle accelerator, spinning electrons over vast distances at high speeds.

Scientists discovered this phenomenon by observing highly energetic X-rays emanating from the Ophiuchus cluster of galaxies.

The European Space Agency's orbiting gamma-ray observatory Integral detected the X-rays, which are too energetic to originate from the inert gas in the cluster and must instead come from accelerated particles.

Previous observations have been able to detect only lower-energy radio waves released in other clusters-turned-particle accelerators.

"This is the first time we have detected significant high-energy X-ray radiation from a cluster," said Stephane Paltani, an astrophysicist at Geneva Observatory in Switzerland, who was involved in the finding. "Only now are we reaching the sensitivity that we need to detect this radiation."

The Ophiuchus cluster must have recently merged with a smaller galaxy cluster, Paltani said. The collision would have mixed the gases in each cluster, producing rippling shock waves. As electrons bounced back and forth in the chaotic merger, they likely picked up energy and accelerated.

This cosmic particle accelerator is 20 times more powerful than the largest man-made atom smasher, the Large Hadron Collider being constructed at CERN, the particle physics lab in Switzerland, Paltani said.

"Of course the Ophiuchus cluster is somewhat bigger," Paltani said. "While LHC is 27 kilometers [17 miles] across, the Ophiuchus galaxy cluster is over two million light-years in diameter."

The scientists don't know for sure why the sped-up electrons release X-rays, but there are two possibilities. Perhaps the electrons created synchrotron radiation, which is produced when charged particles fly though magnetic fields. Or maybe the electrons collided with the Cosmic Microwave Background radiation left over in the universe from the big bang. When the sped-up particles hit the radiation they would have given it an energy boost, pumping its frequency up to the X-ray range of the electromagnetic spectrum.

New observations will be needed to tell which scenario occurred, the scientists said.

"These findings will help us better understand the properties of these clusters," Paltani told LiveScience. "This has important consequences for the history of the cluster itself. We will be able to put constraints on when the particle acceleration takes place and understand better what happens when these clusters merge."