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Cosmic rays efficiently accelerated by exploded stars

Posted: June 26, 2009

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Using ESO’s Very Large Telescope and NASA’s Chandra X-ray Observatory, astronomers have shown that cosmic rays from the Milky Way are very efficiently accelerated in the remnants of exploded stars.

Cosmic rays are extremely energetic particles – mostly protons – moving at close to the speed of light. They originate from outside our Solar System and are constantly bombarding the Earth’s atmosphere at a rate of some 100,000 per square metre per second.

“It has long been thought that the super-accelerators that produce these cosmic rays in the Milky Way are the expanding envelopes created by exploded stars, but our observations reveal the smoking gun that proves it,” says Eveline Helder from the Astronomical Institute Utrecht of Utrecht University in the Netherlands.

Part of the stellar remnant from RCW 86. The shock wave visible in this area is very efficient at accelerating particles and the energy used in this process matches the number of cosmic rays observed on Earth. Image: ESO/E. Helder & NASA/Chandra.

Helder, along with colleague Jacco Vink and others, have come up with a measurement that indicates that stellar explosions – supernova events – are indeed able to produce enough accelerated particles to explain the number of cosmic rays that hit the Earth’s atmosphere. The measurement also reveals how much energy is removed from the shocked gas in the stellar explosion and used to accelerate particles.

“Ever since the thirties, people calculated that in order to maintain the cosmic ray energy density in the Galaxy, supernovae should transform about 10 percent of their total kinematic energy into cosmic rays, assuming three supernova explosions per century in the Galaxy,” Helder tells Astronomy Now. “The energy that is used for particle acceleration is at the expense of heating the gas, which is therefore much colder than theory predicts”.

The team studied supernova remnant RCW 86, the remains of a star that exploded in AD 185 and is located around 8,200 light years away towards the constellation of Circinus. “We measured that for the northeast rim of the supernova remnant RCW 85 a minimum of 50 percent of the energy of the shocked gas went into accelerating cosmic rays,” describes Helder.

wide-field image of the study area This wide-field image contains the area where a team of researchers confirmed that cosmic rays from our Galaxy are very efficiently accelerated in the remnants of exploded stars. The red is centred on the supernova explosion and the boxed area contains an insert of data from the VLT and Chandra. Image: ESO and Digitized Sky Survey 2 /Davide De Martin.

Using ESO’s Very Large Telescope (VLT) the team measured the temperature of the gas right behind the shock wave created by the stellar explosion to be 30 million degrees Celsius, and using Chandra the shock wave was found to be moving at between 10 and 30 million kilometres per hour, based on images taken over the last three years. The surprising outcome of this result is that the temperature is much lower than expected for the shock wave’s velocity.

Helder comments that if the cosmic ray particles were not being accelerated by the supernova event the gas should measure some 500 billion degrees centigrade. “The missing energy is what drives the cosmic rays”, concludes Vink. “You could even say that we have now confirmed the caliber of the gun used to accelerate cosmic rays to their tremendous energies.”

While Helder and colleagues are the first team to use proton temperature to calculate the amount of energy going into accelerating cosmic rays, other groups have found that in the Tycho supernova remnant and the supernova remnant of SN1006, there is a higher compression ration behind the shock front. “Normally, the density behind the shock front is four times the density just in front of the shock. A higher shock compression is an indication for cosmic ray acceleration,” says Helder.

The team plans to expand their study to other remnants, in order to investigate how the efficiency of cosmic ray acceleration depends on shock velocity, density of the local interstellar medium and other parameters.

A paper describing the results appears in today’s issue of the journal Science.

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