Distant black holes may be source of high-energy cosmic
rays
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ARGONNE, Ill. (Nov. 9, 2007) Breakthrough astrophysics research may have
established the hitherto mysterious source of exceptionally high-energy cosmic
ray emissions, according to recently published research that culminates a
project developed by a scientist at the U.S. Department of Energy's (DOE) Argonne
National Laboratory.
This extraordinary result is a product of DOE's investment in high-energy
physics research, giving scientists the resources they need to explore the
interactions between matter, energy, time and space.
Argonne senior physicist Harold Spinka, in collaboration with more than 300
scientists from around the world affiliated with the Pierre
Auger Observatory in western Argentina, determined a correlation between emanations of sufficiently
energetic cosmic rays with a particular class of extrastellar objects, known
as active galactic nuclei (AGNs). Scientists believe that AGNs are massive
black holes in the center of distant galaxies that devour matter while ejecting
plasma streams composed of high-energy particles.
We have taken a big step forward in solving the mystery of the nature and
origin of the highest-energy cosmic rays, said Nobel Prize winner and University
of Chicago professor emeritus James Cronin, who founded the Pierre Auger Observatory
with Alan Watson of the University of Leeds. 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.
After observing and recording approximately two years' worth of cosmic rays
hitting the earth, the Pierre Auger team noticed that the cosmic rays a misnomer
for energetic atomic particles, mainly protons -- with energies in excess of
60 EeV (60 exa-electron volts, or 10 18 electron volts) tended to emanate from
locations near known AGNs.
Most cosmic rays that strike the Earth originate from within our own Milky
Way galaxy, where they emanate from supernovae, black holes or neutron stars.
However, these cosmic rays have a substantially lower energy than those under
investigation in the Pierre Auger study. Researchers knew that they could not
attribute the production of those rays to any phenomenon or body within our
own galaxy, and until now research to identify an extra-galactic source had
yielded little more than hypotheses.
Astronomers had difficulty pinpointing the sources of especially energetic
cosmic rays because they hit the Earth so infrequently, in contrast to the
lower-energy cosmic radiation that continually bombards the Earth. During more
than two years of observation, the Pierre Auger scientists detected only 28
cosmic rays that matched their stringent criteria. They excluded extragalactic
cosmic rays with energies lower than 40 to 60 EeV, because the trajectories
of these particles are so badly bent by deep-space magnetic fields that scientists
cannot determine their origin; they also did not look at cosmic rays that had
traveled more than 300 million light years due to concerns that interactions
with cosmic background radiation during such a long journey would have significantly
reduced their energy.
The concern is that if you look too far back in time and space, it becomes
harder to figure out a correlation, Spinka said.
Since 2004, the observatory, which contains a telescope array the size of
Rhode Island, has detected only 80 cosmic rays with energies greater than
40 EeV. Of the 28 of these that had energies greater than approximately 60
EeV and originated within about 250 million light-years of Earth, 20 were located
close to known AGNs. Six of the remaining eight cosmic rays come from directions
where the source may be obscured by other matter in our galaxy.
According to Spinka, astronomers have worked hard to complete the catalog
of all the AGNs in the observable universe, and he believes that cosmic rays
may offer clues as to where others might be. I think that many astronomers
will indeed go back and look at the areas of space to which we traced the cosmic
rays, because it's definitely possible we might have missed something, he
said.
Cosmic ray observations provide astronomers with another way of examining
celestial features outside of the Milky Way, Spinka said. Up until now there
has been no way of doing astronomy for objects outside our galaxy except by
using various wavelengths of light. This paper represents the first time that
we've been able to use charged particles to observe these faraway objects.
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 $54
million construction cost, said Danilo Zavrtanik of the University
of Nova Gorica and chair of the Auger Collaboration Board.
The paper, Correlation
of the Highest Energy Cosmic Rays with Nearby Extragalactic Objects, appears in the November 9 issue of Science. A press release
from the Auger Observatory can be found at http://www.auger.org/news/PRagn/AGN_correlation.html.
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For more information, please
contact Steve McGregor (630/252-5580 or media@anl.gov)
at Argonne.
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