Where Do Cosmic Rays Come From?


PureInsight | November 11, 2007

Auger Observatory closes in on long-standing mystery, links highest-energy cosmic rays with violent black holes

MALARGUE, Argentina -- Scientists of the Pierre Auger Collaboration
announced today (Nov. 8) 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
will 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.

"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 Earth's

"The highest-energy cosmic rays must come from some of the most violent
processes in the universe. Up until now we have known very little about
their source," said Dennis Kovar, acting associate director of the
Office of Science for High Energy Physics at the Department of Energy.
"These results represent an important step towards learning about the
origin of these particles."

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.

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.

"Hundreds of scientists and dozens of funding agencies have contributed
to the construction of the Pierre Auger Observatory," said Joe Dehmer,
director of the Physics Division at the National Science Foundation.
"Now all these efforts are paying off. We already have seen Auger
results on the cosmic-ray spectrum and composition, and today's new
result is the most dramatic achievement yet."

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 77 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.

"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."

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.

"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 Fermilab
scientist 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

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.

"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."

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. 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 Comision Nacional de Energi­a
Ao³mica 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 atmosphere.



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