Auger Observatory closes in on long-standing mystery,
links highest-energy cosmic rays with violent black holes
Scientists of the Pierre Auger Collaboration announced today (8 Nov. 2007) 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 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.
Jim Cronin, left, and Alan Watson
“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 ultra-high-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 kilometers (15 square miles) as they reach the
“This result heralds a new window to the nearby universe
and the beginning of cosmic-ray astronomy,” said
Watson, a 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.”
Members of the Pierre Auger Collaboration
The Pierre Auger Observatory records cosmic ray showers through an array of 1,600 particle detectors
placed 1.5 kilometers (about one mile) apart in a grid spread across 3,000 square kilometers (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 x 1019 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 318 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.
The celestial sphere in galactic coordinates (Aitoff projection)
showing the arrival directions of
the 27 highest energy cosmic rays detected by Auger. The energies are greater than 57 x 1018 eV
(57 EeV). These are shown as circles of radius 3.1°. The
positions of 472 AGN within 75 megaparsecs are shown as red *'s. The blue region defines the field of view
of Auger; deeper blue indicates larger exposure.
The solid curve
marks the boundary of the field of view, where the zenith angle equals 60°.
The closest AGN, Centaurus A, is marked as a white *. Two of the 27 cosmic rays
have arrival directions within 3° of this galaxy.
The supergalactic plane is indicated by the dashed curve. This plane delineates a region where large numbers of
nearby galaxies, including AGNs, are concentrated.
Click on the image for a better view.
Correo Argentino, S.A., has issued a 0.75 peso stamp
to honor the Pierre Auger Observatory.
“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
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.
An image of the peculiar galaxy Centaurus A (NGC 5128), taken by the Cerro Tololo
4-meter telescope. The center of the galaxy harbors the closest active galactic nucleus (AGN) to us, at a
distance of 11 million light years. Auger has found 2 high energy cosmic rays within 3° of this object.
Credit: National Optical Astronomy Observatories.
Scientists think that most galaxies have black holes at their centers, with masses ranging from a million to a
few billion times the mass of our sun. The black hole at the center 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 kilometer 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.
The Pierre Auger Observatory is a hybrid detector. On the hill is one of the 4
Fluorescence Detector buildings and communications tower. In the bottom foreground is one of the 1,600 Surface
“Our current results show the promising future of cosmic-ray astronomy,” said Auger cospokesperson 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.”
A cosmic ray event viewed by all four of the Fluorescence Detectors. Each
detector records the growth and decay of the extensive cosmic ray air shower comprised of billions
of secondary particles.
The Pierre Auger Observatory is being built by a team of more than 370 scientists and engineers
from 17 countries.
“The collaboration is a true international partnership
in which no country contributed more than 25 percent
of the US$54-million construction cost,” said Danilo Zavrtanik, of the University of Nova Gorica and chair of
the Auger Collaboration Board. The names of the funding agencies contributing to the Pierre Auger Observatory
are found here, and the names of the participating institutions are listed
Groundbreaking for the southern hemisphere site of the Pierre Auger Observatory took place on March 17, 1999,
in Argentina’s Mendoza Province. Following a period of detector deployment and testing, scientific data
collection began in January, 2004.
“Argentina is pleased to host and participate in this unique scientific endeavor,” said Alberto Etchegoyen,
of Comisión Nacional de Energía Atómica, and Southern Observatory spokesperson,
“and now, looking back into these years of efforts
and excitement, a feeling of gratitude and respect arises for all collaboration members who took care of every
single minor detail leading to today’s announcement.”
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