Scientists of the Pierre Auger Collaboration, which includes New York University Physics Professor Glennys R. Farrar, have concluded that active galactic nuclei are the most likely candidate for the source of the highest-energy cosmic rays that hit Earth. Using the Pierre Auger Observatory in Argentina, the largest cosmic-ray observatory in the world, a team of scientists from 17 countries found that the sources of the highest-energy particles are not distributed uniformly across the sky. Instead, the Auger results link the origins of these mysterious particles to the locations of nearby galaxies that have active nuclei in their centers. The results appear in the Nov. 9 issue of the journal Science.
Active Galactic Nuclei (AGN) are thought to be powered by super-massive 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 center, 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.
The Pierre Auger Collaboration is comprised of more than 200 physicists from 55 institutions around the world; the 17 participating countries shared the $50 million construction cost. Other NYU members of the collaboration are NYU graduate student Jeff Allen, senior research scientist Aaron Chou, and postdoctoral fellow Ingyin Zaw.
The Pierre Auger Observatory's Southern Hemisphere detector in Malargue, Argentina, consists of an array of particle detectors on the ground, plus four air-fluorescence telescopes which operate at night allowing simultaneous measurement of the longitudinal development of about 10 percent of the showers. By studying hybrid events--those that are simultaneously detected in the surface and air florescence detectors--the Auger Observatory is able to cross calibrate the energy measurement of the independent detection methods. Previously, scientists could only roughly estimate the energy created by such collisions. By combining the detection methods researchers will be able to more accurately measure the energy and elucidate the identity of the highest energy particles.
Auger's greatest strength is perhaps its enormous size. It has already recorded more events than all previous detectors combined and can accumulate ultrahigh energy cosmic ray events at a rapid rate. Understanding these high-energy particles demands knowledge of fundamental physics at untested energies and an extension of theoretical models of astrophysical objects into an unprecedented domain.
Current work at NYU within Auger involves using the SENECA air shower simulation developed at NYU to test the validity of the underlying particle physics interaction models as well as the approximations made in air shower simulations. The speed of SENECA allows the effects of the atmosphere on shower development to be explored and permits detailed simulation of hybrid events, to allow direct comparison between simulations and real data for individual events.
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