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 the universe.

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 energy threshold.

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 square kilometers). That gives a rate of 30 extraordinary cosmic rays per year.