5th November 2007

 

STFC99/07

 

Embargoed until Thursday, 8 November 2007, 19.00 GMT /14.00 EDT

 

 

Breakthrough in Cosmic Ray mystery – possible sources found for high energy particles that bombard the Earth

 

The Pierre Auger Collaboration has announced today (8th November) that high energy cosmic rays that rain down on the Earth’s atmosphere do not come from evenly across space. They have plotted the directions these particles originate from and found a pattern for the first time – most come from near a particular type of galaxy, known as Active Galactic Nuclei, which are thought to contain super-massive black holes. Their results are published in the journal Science and were obtained using the largest cosmic ray observatory in the world, the Pierre Auger Observatory in Argentina, which is used by a team of scientists from 17 nations, including the United Kingdom. The Auger Observatory is the first to be able to determine the direction of these energetic particles with sufficient accuracy and also the first to detect sufficient numbers of the high energy events to be able to study them meaningfully.

 

Professor Keith Mason, CEO of the Science and Technology Facilities Council which funds the Universities of Leeds and Oxford to work in Auger said “These results are a significant breakthrough in our understanding of cosmic rays. Prior to this, researchers could only theorise where these high energy particles were being produced. Now we have evidence for high energy cosmic rays coming from active galactic nuclei and a new way of probing these extreme environments.”

 

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.

 

“We have taken a big step forward in solving the mystery of the nature and origin of the highest-energy cosmic rays, first revealed by French physicist Pierre Auger in 1938,” said Nobel Prize winner James Cronin, of the University of Chicago, who conceived the Pierre Auger Observatory together with Alan Watson of the University of Leeds. “We find the southern hemisphere sky as observed in ultrahigh energy cosmic rays is non-uniform. This is a fundamental discovery. The age of cosmic ray astronomy has arrived. In the next few years our data will permit us to identify the exact sources of these cosmic rays and how they accelerate these particles.”

 

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 (15 square miles) as they reach the Earth’s surface.

 

“This result heralds a new window to the nearby universe and the beginning of cosmic-ray astronomy,” said Watson, the spokesperson of the Pierre Auger Collaboration. “As we collect more and more data, we may look at individual galaxies in a detailed and completely new way. As we had anticipated, our observatory is producing a new image of the universe based on cosmic rays instead of light.”

 

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 UK has contributed vital research to the development of the Auger project. The Auger Observatory consists of two types of detector, fluorescence detectors and water tanks – the latter build on development work done from the Haverah Park air shower array operated by the University of Leeds until 1987. UK scientists and engineers also designed and built the communication systems for Auger – linking up all the detectors, so that the vital information – the time events occur in each can be recorded. The expertise developed for this has led to a spin-off company being formed that is improving the safety and reliability of single track railway lines.

 

Prof Watson adds “Detecting cosmic rays is a huge challenge – they enter the Earth’s atmosphere with the kinetic energy of a 60 mile per hour tennis serve squeezed into a sub-atomic particle. This crashes into our atmosphere, triggering a shower of slower particles over a wide area. The Auger Observatory covers an area similar to that inside the M25 to spot and track these showers of particles!”

 

The Pierre Auger Observatory records cosmic ray showers through an array of 1,600 particle detectors placed 1.5 kilometres (about one mile) apart in a grid spread across 3,000 square kilometres (1,200 square miles). 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 381Active 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. 

 

“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.”

 

The Pierre Auger Observatory is being built by a team of more than 370 scientists and engineers from 17 countries.

 

Groundbreaking for the southern hemisphere site of the Pierre Auger Observatory took place on March 17, 1999, in Argentina’s Mendoza Providence. Following a period of detector deployment and testing, scientific data collection began in January, 2004.

 

The observatory is named for French scientist Pierre Victor Auger (1899-1993), who in 1938 was the first to observe the extensive air showers generated by the interaction of high-energy cosmic rays with the Earth’s atmosphere.

Notes for editors:

Images
http://www.auger.org/media/image_highlights.html   

An image of an example Active Galactic Nucleii, Centaurus A, can be downloaded at http://hubblesite.org/newscenter/archive/releases/1998/14/image/j/  
Credit: EJ Schreier/NASA

Contacts

Julia Maddock

Science and Technology Facilities Council

Julia.maddock@stfc.ac.uk

Tel +44 1793 442094

Mob +44 7901 514 975

 

Professor Alan Watson (travelling – so please use mobile number if unable to reach him)

University of Leeds

Email a.a.watson@leeds.ac.uk

Tel 0113 343 3888

Mobile 0787 010 9602

 

Professor Subir Sarkar (from Thursday onwards)

University of Oxford

Tel +44 (0)1865 273962  

Email sarkar@thphys.ox.ac.uk

 

Photos as well as press information from the countries
participating in the Auger Project may be found at:

http://www.auger.org/media

Auger is an international collaboration – a full list of participating agencies and institutions can be found at http://www.auger.org/collaboration/auger_institutions.html

Science and Technology Facilities Council

The Science and Technology Facilities Council ensures the UK retains its leading place on the world stage by delivering world-class science; accessing and hosting international facilities; developing innovative technologies; and increasing the socio-economic impact of its research through effective knowledge exchange partnerships.

 

The Council has a broad science portfolio including Astronomy, Particle Physics, Particle Astrophysics, Nuclear Physics, Space Science, Synchrotron Radiation, Neutron Sources and High Power Lasers. In addition the Council manages and operates three internationally renowned laboratories:

·         The Rutherford Appleton Laboratory, Oxfordshire

·         The Daresbury Laboratory, Cheshire

·         The UK Astronomy Technology Centre, Edinburgh

The Council gives researchers access to world-class facilities and funds the UK membership of international bodies such as the European Laboratory for Particle Physics (CERN), the Institute Laue Langevin (ILL), European Synchrotron Radiation Facility (ESRF), the European organisation for Astronomical Research in the Southern Hemisphere (ESO) and the European Space Agency (ESA). It also contributes money for the UK telescopes overseas on La Palma, Hawaii, Australia and in Chile, and the MERLIN/VLBI National Facility, which includes the Lovell Telescope at Jodrell Bank Observatory.

The Council distributes public money from the Government to support scientific research.  Between 2007 and 2008 we will invest approximately £678 million. <Ends>