Write your abstract here.Andes
Observatory Links Cosmic Rays to Cores of Nearby Galaxies.
The
sprawling Auger
Cosmic Ray
Observatory -a detection area the size of
Rhode Island in the vast plains of western Argentina- has traced the
rain of high-energy cosmic rays that continually pelts the Earth to the
cores of nearby galaxies, which emit awesome quantities of energy.
"This is a fundamental discovery," said Nobel laureate James Cronin,
the University Professor Emeritus in Physics at the University of
Chicago.
Scientists
of the Pierre Auger Collaboration announced hat 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. Active Galactic Nuclei (AGN) 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 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.
Until now, the history of astronomical discovery has been dominated
by the detection of light. "We are doing astronomy with protons-charged
particles," said Joao de Mello Neto, a Visiting Scholar from the
University of Rio de Janeiro in Brazil. "We are opening a new window in
astronomy."
Cosmic rays-mostly protons-fly through the universe at nearly the
speed of light. The most powerful cosmic rays contain more than one
hundred million times more energy than the particles produced in the
world''s most powerful particle accelerator. Fortunately, Earth''s
atmosphere provides protection against their potentially harmful
effects on humans.
Since 1938, when French physicist Pierre Auger discovered cosmic
rays, their origin has been a mystery. Now the Auger collaboration has
tracked them to Active Galactic Nuclei. Likely powered by
supermassive black holes, AGN shine far brighter than regular galaxies
as a byproduct of their gravitationally destructive force.
"After decades of negative results from past experiments, Auger
physicists finally find that cosmic rays do not come equally from every
direction in space," Olinto said.
Scientists have long considered AGN to be possible sources of
high-energy cosmic rays. And while they have now found a strong
correlation between the two, exactly what accelerates cosmic rays to
such extreme energies remains unknown.
"They are really spectacular objects," said Maximo Ave, a Research
Associate at the Kavli Institute for Cosmological Physics at Chicago.
"They most likely can be produced only in a place where some very
extreme physical process is happening." One such extreme process might
be gamma-ray bursts, the possible result of collapsing or colliding
stars.
The numbers are relatively meager, considering that only one
high-energy cosmic ray will strike a given square kilometer (less than
half a square mile) of Earth approximately once each century.
The Auger collaboration has increased the odds of detection by
building an array of detectors that cover 1,200 square miles of the
Pampa Amarilla. When complete, the
array will consist of 1,600 detectors spaced at one-mile intervals.
Ninety percent of the array is now operational. The observy was named after the French physicist Pierre Victor
Auger.
Today, more than 200 physicists from 55 institutions
around the world are collaborating to build the southern site. The 15
participating countries are sharing the $50 million construction
budget, each providing a minor part of the total cost.
The observatory will consist of northern and southern sites. The
southern site is in the final phase of construction and is currently
operating and collecting data. The northern site will be located in southeastern Colorado
Each detector consists of a plastic water tank measuring 5- feet
tall and 12 feet in diameter. When a cosmic ray collides with an air
molecule in Earth''s atmosphere, it triggers a shower that multiplies
into billions of secondary particles before reaching the ground. When
these particles cross from air into water, the speed changes, producing
a shock. The shock creates a flash of light that is detected in the
dark chamber of the water tank.
Complementing the ground detectors are 24 telescopes that monitor
the sky for signs of cosmic rays on clear, moonless nights. The
telescopes detect the time emission of fluorescent light that results
from the interaction of cosmic rays with nitrogen molecules in the
atmosphere.
"With this we can estimate the energy, and we can estimate the
direction it comes from, which are the two parameters that are
important for this analysis," de Mello Neto said. When correlated with a catalog of objects in the sky, their direction
of travel matched AGN locations in galaxies no more than 180 million
light years distant from Earth and its galaxy, the Milky Way.
"We have taken a big step forward in solving the
mystery of the nature and origin of the highest-energy cosmic rays," said Cronin.
Posted by Casey Kazan adapted from a University of Chicago release.