A New Window to the Universe
Scientists of the Pierre Auger Observatory have found that the arrival directions of the
highest energy cosmic rays are well correlated with the
distribution of Active Galactic Nuclei (AGN). These are
supermassive black holes, residing at the centers of galaxies, which
spew out enormous amounts of radiation as they consume gas and dust from the
inner parts of their host galaxies. Their activity is powered by a spin-down of the
rotating black hole as well as by direct accretion.
This result heralds a new window to the universe. The Auger
Observatory will be able to study individual AGN sources as it
observes more and more of these extraordinarily energetic cosmic rays. Astronomers
have so far learned about the universe using the electromagnetic
spectrum of radiation, from radio waves up to very high energy gamma
rays. The messengers that carry electromagnetic energy are photons, having zero mass and
zero electric charge. Cosmic rays, however, represent a different kind of messenger:
they are atomic nuclei, which have both mass and electric charge. Most common is the hydrogen nucleus, which is
simply a proton.
This new type of astronomy differs not only in the type of messenger
but also in the energy of the messengers. Each of the particles observed by Auger
has an energy that is more than a million times greater
than any previously detected messenger from any identified astrophysical source.
Thus, the Auger cosmic rays naturally probe the highest energy processes observed in
Cosmic ray astronomy is challenging in part because charged particles
are deflected by magnetic fields. Imagine looking at the night sky
through thick lenses that blur individual point sources into wide smudges on the
sky. You cannot hope to resolve individual sources if they occur
close together. The highest energy particles, which the Auger Observatory
has found correlate with AGNs, are the least susceptible to magnetic field deflections.
Therefore, the amount of blurring is only a few degrees.
Cosmic ray astronomy is also feasible at extremely high energy because
only relatively nearby sources can send messengers to us. The clutter of
particles from distant sources are filtered out by an effect discovered by theoretical physicists
Greisen, Zatsepin, and Kuzmin. They found that
above a certain high energy threshold (~ 6 x 1019 eV), cosmic rays lose energy by colliding
with photons of the cosmic microwave background (CMB) radiation, the remnant of the Big Bang.
This phenomenon, now called the GZK effect, limits the highest energy cosmic rays to travel
distances not greater than a few hundred million light years. At
lower energies, there are no interactions with the CMB, allowing a background of cosmic rays to travel for
distances up to billions of light years. Like looking for stars in a
bright city, a large background makes it hard to see point
sources. The Auger Observatory has succeeded in finding the
correlation of cosmic ray arrival directions with the nearby distribution of AGN
sources because there are no background cosmic rays above the GZK
Another striking difference between cosmic ray astronomy and
electromagnetic astronomy is the flux of messengers. A photograph
normally represents billions of photon messengers. The high energy
cosmic ray messengers, however, are extremely rare. The rate is only
one every century over a square kilometer. The Auger Observatory is
therefore designed to measure each energetic cosmic ray arriving
anywhere in an area the size of the US state of Rhode Island (1200 square miles, or 3000
That gives a rate of 30 extraordinary cosmic rays per year.