An international team of astronomers using the Pierre Auger Observatory has obtained the best evidence yet that ultrahigh-energy cosmic rays striking the Earth come from black holes lying at the heart of nearby galaxies. Using thousands of detectors spread over 3000 square kilometres in Argentina, the team has managed to identify 27 such cosmic rays -- with energies of over 1019 electron volts (eV) -- as having come from these active galactic nuclei. Having solved the mystery of where these cosmic rays come from, researchers now hope to get a better understanding of exactly how these charged particles are accelerated to such high energies (Science 318 938). .
First discovered in the early 1960s, ultrahigh-energy cosmic rays (UHECRs) are the most energetic charged particles in the Universe. They are also exceedingly rare – the Pierre Auger Observatory has only detected about 27 particles with energies above 5.7 *1019 eV since it started taking data in 2004.
Because they have such high energies, UHECRs are likely to be produced in extremely violent astrophysical events. The leading contenders are active galactic nuclei (AGN), which are believed to have black holes at their centre. Such black holes pull in vast quantities of matter, which forms an extremely hot plasma before disappearing into the abyss. It is thought that magnetic instabilities in this plasma produce electromagnetic shock waves that could travel some distance away from the black hole, where they could accelerate protons or heavier nuclei to ultrahigh energies. However, the exact nature of the acceleration process is not understood.
Such shock waves could also be produced by turbulence in intergalactic space, “fossil” (or invisible) black holes or supernovae. However, UHECRs lose energy very easily because they scatter off the cosmic microwave background, which means that they do not travel very far through space. This rapid fall in the number of such cosmic rays above a certain energy — known as the Greisen-Zatsepin-Kuzmin (GZK) effect — means that UHECRs are much more likely to come from nearby sources such as AGNs relatively close to Earth. Indeed, Venya Berezinsky of the Gran Sasso National Laboratory in Italy told physicsworld.com that UHECRs with energies above about 6*1019 eV cannot have travelled further than about 100 Mpc – or about 300 million light years.
New discovery
Now, however, a team of researchers has used the Pierre Auger Cosmic Ray Observatory in Argentina to find the first compelling evidence that UHECRs do indeed come from nearby AGNs. The Auger Observatory detects cosmic rays by observing the particle “showers” that are produced when cosmic rays interact with air molecules.
The observatory consists of an array of 1600 tanks of water that detect the Cerenkov light generated by particles in the air shower. The sky above the tanks, which are separated from one another by 1.5 km, is monitored by four atmospheric fluorescence detectors that can track the particle showers as they pass through the atmosphere on dark nights.
After taking data for over three and a half years, the observatory detected 27 cosmic rays with energies greater than about 5.7 * 1019 eV. Twenty of these could be traced back to within 3.1 degrees of AGNs closer than 75 Mpc from Earth. According to the team, it is highly unlikely that 20 UHECRs coming from random directions would appear to all come from known AGNs.
Energy spectrum
Auger spokesperson Alan Watson of the University of Leeds told physicsworld.com that the team has spotted five “pairs” of cosmic rays that each appear to originate from the same AGN. He hopes that by detecting a number of UHECRs from the same source, physicists will be able to build up an energy spectrum that could explain exactly how the cosmic rays are produced. “As we collect more and more data, we may look at individual galaxies in a detailed and completely new way” he says. “As we had anticipated, our observatory is producing a new image of the universe based on cosmic rays instead of light.”
The snag is that any decent spectrum would require the detection of hundreds of cosmic rays from the same AGN, which would take the Auger Observatory hundreds of years to obtain. Astrophysicists are planning to build a second observatory called Auger North in Colorado in the US. Although Auger North is intended to be 3.5 times bigger than its southern partner, Watson hopes that the team’s positive results will lead to a decision to make Auger North being 10 times larger than Auger South – something that he says is possible in principle, but could be very difficult and expensive to build and maintain.
Watson told physicsworld.com the knowing the precise source of UHECRs could help astrophysicists gain a better understanding of our own galaxy’s magnetic fields. This is because the charged particles are deflected slightly by magnetic fields, which could be studied as a function of a particle’s energy and origin. In addition, by studying the collisions between UHECRs and nuclei in the atmosphere, physicists could study matter at much higher energies than any current or planned particle accelerator. The Large Hadron Collider at CERN, for example will accelerate protons to energies of only 7 TeV (7 * 1012 eV)
