Monday, November 12, 2007

Highest-Energy Cosmic Rays

Active Galactic Nuclei (AGN) are found at the hearts of some galaxies and are thought to be powered by supermassive black holes that are devouring large amounts of matter.

They have long been considered sites where high-energy particle production might take place. They swallow gas, dust and other matter from their host galaxies and spew out particles and energy.

While most galaxies have black holes at their centre, only a fraction of all galaxies have an AGN. The exact mechanism of how AGNs can accelerate particles to energies 100 million times higher than the most powerful particle accelerator on Earth is still a mystery.
[+/-] Click here to expand

Cosmic rays are protons and atomic nuclei that travel across the universe at close to the speed of light. When these particles smash into the upper atmosphere of our planet, they create a cascade of secondary particles called an air shower that can spread across 40 or more square kilometres as they reach the Earth’s surface.

Professor Subir Sarkar of the Physics Department at Oxford University, a member of the Auger Collaboration, said: ‘The Auger data indicates that the sources of ultrahigh energy cosmic rays are associated with nearby 'active galaxies' which harbour supermassive black holes that are gobbling up stellar matter and ejecting huge jets of plasma. Our own galaxy too has such a black hole at its centre but, fortunately for us, it is not 'feeding' at the moment!’

The Pierre Auger Observatory records cosmic ray showers through an array of 1,600 particle detectors placed 1.5 kms apart in a grid spread across 3,000 square kms. Twenty-four specially designed telescopes record the emission of fluorescence light from the air shower. The combination of particle detectors and fluorescence telescopes provides an exceptionally powerful instrument for this research.

While the observatory has recorded almost a million cosmic-ray showers, only the rare, highest-energy cosmic rays can be linked to their sources with sufficient precision. Auger scientists so far have recorded 81 cosmic rays with energy above 4 x1019 electron volts, or 40 EeV. This is the largest number of cosmic rays with energy above 40 EeV recorded by any observatory.

At these ultra-high energies, the uncertainty in the direction from which the cosmic ray arrived is only a few degrees, allowing scientists to determine the location of the particle’s cosmic source.

The Auger collaboration discovered that the 27 highest-energy events, with energy above 57 EeV, do not come equally from all directions. Comparing the clustering of these events with the known locations of 381 Active Galactic Nuclei, the collaboration found that most of these events correlated well with the locations of AGNs in some nearby galaxies, such as Centaurus A.

Click Image to Enlarge: Centaurus A

Low-energy cosmic rays are abundant and come from all directions, mostly from within our own Milky Way galaxy. Until now the only source of cosmic ray particles known with certainty has been the sun. Cosmic rays from other likely sources such as exploding stars take meandering paths through space so that when they reach Earth it is impossible to determine their origins.

"But when you look at the highest-energy cosmic rays from the most violent sources, they point back to their sources. The challenge now is to record enough of these cosmic bullets to understand the processes that hurl them into space," said Paul Mantsch, project manager of the Pierre Auger Observatory.

Cosmic rays with energy higher than about 60 EeV lose energy in collisions with the cosmic microwave background, (radiation left over from the Big Bang that fills all of space). But cosmic rays from nearby sources are less likely to lose energy in collisions on their relatively short trip to Earth. Auger scientists found that most of the 27 events with energy above 57 EeV came from locations in the sky that include the nearest AGNs, within a few hundred million light years of Earth.

Scientists think that most galaxies have black holes at their centres, with masses ranging from a million to a few billion times the mass of our sun. The black hole at the centre of our Milky Way galaxy weighs about 3 million solar masses, but it is not an AGN. Galaxies that have an AGN seem to be those that suffered a collision with another galaxy or some other massive disruption in the last few hundred million years. The AGN swallows the mass coming its way while releasing prodigious amounts of radiation. The Auger result indicates that AGNs may also produce the universe's highest-energy particles.

Cosmic-ray astronomy is challenging, because low-energy cosmic rays provide no reliable information on the location of their sources: as they travel across the cosmos, they are deflected by galactic and intergalactic magnetic fields that lead to blurry images. In contrast, the most energetic particles come almost straight from their sources, as they are barely affected by the magnetic fields. Unfortunately, they hit Earth at a rate of only about one event per square kilometre per century, which demands a very large observatory.

Because of its size, the Auger Observatory can record about 30 ultra-high-energy events per year. The Auger collaboration is developing plans for a second, larger installation in Colorado to extend coverage to the entire sky while substantially increasing the number of high-energy events recorded.

"Our current results show the promising future of cosmic-ray astronomy," said Auger co-spokesperson Giorgio Matthiae, of the University of Rome. "So far we have installed 1400 of the 1600 particle detectors of the Auger Observatory in Argentina. A northern site would let us look at more galaxies and black holes, increasing the sensitivity of our observatory. There are even more nearby AGNs in the northern sky than in the southern sky."

Source: Auger Observatory closes in on long standing mystery, links highest-energy cosmic rays with violent black holes

Breakthrough in Cosmic Ray mystery from SciTech
AGN and Ultra High Energy Cosmic Rays from Space Daily
AUGER: millions of TeV cosmic rays from black holes from The Reference Frame
Ultra High Energy Cosmic Rays (UHECR) from Auger @ BackReaction

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