When it comes to the highest energy cosmic rays—subatomic particles raining in from space—the sky is lopsided: More come from one direction than the other, according to a new study. And because most come from a direction that points away from our galaxy, the observation bolsters the idea that the cosmic rays originate far beyond the Milky Way. However, the result falls short of astrophysicists’ goal of pinpointing the ultimate sources of such cosmic rays.
The new result comes from the Pierre Auger Observatory, a huge cosmic ray detector array in Argentina, and it receives mixed reviews from some leading researchers. The finding gives scientists their first solid evidence that the rays don't just come from all over, says Glennys Farrar, a theorist at New York University in New York City. "I think this is going to be a shot in the arm to the field," she says. However, Pierre Sokolsky, an experimental physicist at the University of Utah in Salt Lake City and a founder of Telescope Array, a smaller cosmic ray array, argues that other evidence already points to the rays’ extragalactic origin. "It's kind of a not a bang, but a whimper result," he says.
Cosmic rays are protons and heavier atomic nuclei zinging through space. The highest energy rays are millions of times more energetic than particles from human-made accelerators. In extreme cases, a single atomic nucleus can possess as much energy as a large hail stone falling on your head. However, physicists still don't know what sorts of astrophysical objects accelerate the particles to such tremendous energies.
Scientists built Auger to find out. When a high energy cosmic ray strikes the atmosphere, it produces a so-called air shower—an avalanche of lower energy particles. Detectors on the ground can count those particles and clock their arrival times, enabling astrophysicists to deduce the energy and direction of the cosmic ray. The particle avalanche also causes nitrogen molecules in the air to fluoresce, and on dark nights special telescopes can measure that light. The dual measurements allow a rough determination of the type of particle that produced the shower—for example, whether it was a proton or an iron nucleus.
To spot enough of the extremely rare highest energy cosmic rays, a detector array has to be huge, however. Auger consist of 1660 particle detectors covering 3000 square kilometers, an area nearly the size of Rhode Island, in the Pampa Amarilla in Argentina. Each detector is a tank holding 12,000 liters of ultrapure water that produces a flash of light when struck by particles. In addition, four stations of telescopes overlook the ground detectors.
Spotting the sources of the most energetic rays was always going to be tough. Because they are electrically charged, cosmic rays swirl in the galaxy’s magnetic field. To point back toward their sources, they have to be so energetic that their paths do not curve too much. More common lower-energy cosmic rays—thought to emerge in the aftermath of supernova explosions in the Milky Way—curve so much in the galaxy’s magnetic field that they appear to come from all over the sky.
In spite of the difficulties, at first it seemed that Auger would find the sources of the higher-energy rays. It started taking data in 2004, and in 2007 Auger researchers announced that cosmic rays with energies above about 60 exa-electron volts (EeV) appeared to come from the fiery hearts of galaxies thought to contain supermassive black holes feeding on in-falling debris, so-called “active galactic nuclei.” However, that correlation has not held up as more data has come in. Moreover, Auger researchers had expected the highest energy cosmic rays to be light-weight protons, which bend less in magnetic fields. Instead, they have found that many of the rays consist of heavier nuclei, which curve more—making the job of figuring out their origin tougher.
Now, however, Auger researchers have performed a simpler directional analysis. Instead of trying to correlate the directions of the incoming cosmic rays with objects in the sky, they have simply looked for an imbalance in the rays coming from opposite directions. Using 30,000 rays with energies above 8 EeV, they find that by a margin of about 12%, more cosmic rays come from one side of the sky than the other. And neither "pole" in this lopsided distribution aligns with the Milky Way’s galactic center, says the study, published today in Science.
Auger and Telescope Array have both reported potential hot spots for the highest energy cosmic rays before. But the evidence for an overall imbalance of the directions of cosmic rays in this new study is stronger, surpassing the so-called five sigma threshold for statistical significance, says Antonio Bueno, an astroparticle physicist at the University of Granada in Spain and co-spokesperson for the 500-member Auger team. “For many, many, many years we have wondered where the highest energy cosmic rays come from,” he says, “and now we have the statistical power to say that they do not come from within the galaxy.”
Farrar agrees that compared to previous results, the observation is more robust. “This is the first thing that you can sink your teeth into,” she says. However, Sokolsky says that because there was no theoretical reason to expect such an imbalance in the sky, he’s not sure what to make of the result. As for the evidence that the rays come from beyond the galaxy, other observations already support that position, including theorists' inability to come up with an accelerator within the Milky Way that could account for the rays' incredibly high energies. “They make a big deal about proving the flux is extragalactic,” he says. “I thought we’d settled that.”
The search for sources isn’t over. Telescope Array, which consists of 507 ground detectors covering 730 square kilometers in Utah, is doubling the number of its detectors and increasing the area of its array fourfold. And Auger researchers are adding additional detectors to their tanks that should better determine the type of cosmic ray that produced each air shower. By sifting out just the highest energy protons, Bueno says, astrophysicists may finally be able to spot those rays pointing back to their sources.