Cosmic Rays

A few weeks ago, the Auger Project published results (with a news story here) from searching for the properties of the very highest-energy Cosmic Rays, determining that they seem to have come from nearby “Active Galactic Nuclei”, which are believed to be Super-Massive Black Holes at the centers of galaxies, surrounded by discs of accreting matter which emit very high energy particles as the matter in the discs spirals into the Black Hole.

Cosmic Rays (CRs) are electrically-charged particles, moving at nearly the speed of light, constantly raining down on the earth. Their electric charge gives us a clue about why they’re moving so fast: a very strong magnetic field is one of the most efficient ways to accelerate charged particles (we use the same process in our terrestrial particle accelerators). But that same process makes understanding the origin of CRs even more challenging, since magnetic fields not only change the speed of a particle, but also its direction, and such fields permeate our galaxy. So we can’t just look back in the direction a given CR came from to see where it was created. On the other hand, the very highest-energy CRs are the least deflected by the magnetic fields: the highest-energy CRs should point close to their original directions. The Auger scientists have compared the distribution of their 27 most energetic CRs with those of the locations of known AGN. They’ve found that, on average, the directions line up, implying that these CRs come from AGN within about 75 Megaparsecs (about 250 million light years — just in our backyard).

The results appear to settle a controversy in the Cosmic Ray community. Cosmic rays are so energetic that even some of the smallest, least energetic particles in the Universe appear to them as (giant boulders). In particular, the photons that make up the ubiquitous cosmic microwave background, so dim that it was only first detected in the 1960s, can stop a cosmic ray in its tracks. In particular, Greisen, Zatsepin, and Kuzmin (GZK) realized in the 1960s that any cosmic rays with energies higher than about 1020 eV had to be coming from very nearby indeed. This has became known as the “GZK Cutoff” and it had been expected that we’d see a sharp cutoff in the cosmic ray spectrum above it. (This is a huge energy for a particle: about as much as a baseball pitched by a professional, but with 10-25 as much mass!) When we finally started seeing such CRs, they seemed to be too numerous to be coming from nearby. But if the Auger results hold up, we might be able to someday use details about CR origins to probe the details of the accreting Black Holes.

(However, I should point out that one of Auger’s competitors, the HiRes Fly’s Eye, hasn’t confirmed these results.)