Monday, November 27, 2006

High energy cosmic rays

Astrophysicists have wondered for a long time where cosmic rays come from, especially the most energetic ones.

A couple of recent research reports identify two different sources. The first of these is our old friend, Cassiopeia A:

Chandra discovers relativistic pinball machine

New clues about the origins of cosmic rays, mysterious high-energy particles that bombard the Earth, have been revealed using NASA's Chandra X-ray Observatory. An extraordinarily detailed image of the remains of an exploded star provides crucial insight into the generation of cosmic rays.

For the first time, astronomers have mapped the rate of acceleration of cosmic ray electrons in a supernova remnant. The new map shows that the electrons are being accelerated at close to the theoretically maximum rate. This discovery provides compelling evidence that supernova remnants are key sites for energizing charged particles.

This situation is described as a "pinball machine", because the electrons making up the cosmic rays (in this case) are accelerated by being bounced back and forth between magnetic fields and the expanding shock wave generated by the supernova:
"The electrons pick up speed each time they bounce across the shock front, like they're in a relativistic pinball machine," said team member Glenn Allen of the Massachusetts Institute of Technology (MIT), Cambridge. "The magnetic fields are like the bumpers, and the shock is like a flipper."

There are other accounts of this research here, here, and here.

Meanwhile, in another part of the universe, another team has identified a completely different source:

'Big bang gas' in cosmic particle-accelerator shock
Giant shockwaves around a distant cluster of galaxies could be generating some of the mysterious cosmic rays that strike Earth. They could also give us a clue as to why the universe is threaded with magnetic fields.

The cluster, called Abell 3376, is a swarm of galaxies about 600 million light years away. On either side of this swarm are two huge arc-like structures, each about 3 million light years across, that are sending out radio waves.

However, interestingly enough, apparently it is still the combination of magnetic fields and shock waves that is responsible for the particle acceleration – even though Cassiopeia A is 10,000 light years distant from us, while Abell 3376 is 60,000 times further away. The latter is also several million light years across, while Cassiopeia A is only about 10 light years wide. So there remains a major mystery about what produced such huge shock waves in Abell 3376:
Then what created the shocks in the first place? There are two possibilities. It may be that roughly a billion years ago, two clusters crashed into one another to form Abell 3376. The collision could have sparked a shockwave that travelled out through the cluster gas, whose remnants we are now seeing.

But there is a more intriguing possibility. Primordial gas, untouched since the big bang, should be constantly pouring into all galaxy clusters. Computer simulations of the cosmos show that gravity tends to pull the gas into stringy structures called filaments.

Abell 3376 could be threaded on one such filament, and the two shockwaves could mark where this cool ancient gas smacks into the super-hot gas of the cluster.

Other accounts of this research: here, here.

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