Cosmic rays—high energy particles that rain down on Earth from deep space—are something of a mystery: What are they made of? Where do they come from? And how do they gain such enormous energies—far above those achieved with the world's best particle accelerators? Now, a radio telescope originally designed to study the early universe may help answer some of those questions, or it might just deepen the mystery.
Cosmic rays are typically protons or atomic nuclei of elements such as helium, carbon, or iron. The most energetic have energies more than 10 million times greater than those in the world’s most powerful atom smasher, the Large Hadron Collider. Physicists aren't sure what astrophysical process could accelerate particles to such energies. Possible culprits include the lingering remains of supernovae, the explosions that occur when massive stars run out of fuel and die; and active galactic nuclei, superheated galaxies with supermassive black holes at their centers that spew out energy at prodigious rates.
Studying cosmic rays is difficult, however. On their journey through space they are deflected this way and that by magnetic fields, making it difficult to figure out where they’ve come from. The high-energy ones are also very rare, and none of them get very far once they reach Earth’s atmosphere; they’re instantly destroyed in collisions with the air at high altitude. To study the highest energy cosmic rays, physicists use vast arrays of particle detectors on the ground to pick up the "air shower" of debris created by the high altitude collisions or telescopes to spot the flash of light caused by the debris particles as they slow down in the atmosphere.
But now there is a new way to detect cosmic rays. A team has made use of a collection of radio telescopes known as the Low Frequency Array (LOFAR), which is centered in the Netherlands but has outposts in several other northern European countries. LOFAR does not have large steerable dishes like other radio telescopes, but is instead made up of many thousands of simple wire antennas staked out on the ground in dozens of “stations.” The antennas essentially pick up everything coming from space, and it is then up to a superfast processor cluster to sift through the data and focus on a particular phenomenon or part of the sky.
The main aim of LOFAR is to study the era in the early universe when the very first stars and galaxies were forming and ionizing all the interstellar gas around them. But the cosmic ray team is able to piggyback on normal astronomical observations in their search for air showers. As the debris particles from the cosmic ray collision cascade down through the atmosphere, their interactions with each other and Earth’s magnetic field produce a radio signal that is detectable by LOFAR’s antennas. The team can’t scour through all the data 24/7, there’s just too much. So the researchers installed particle detectors on the ground that can alert the system that an air shower has just happened. When a particle detector trips the alarm, LOFAR's cosmic ray system grabs the previous 5 seconds of data that is held in the system's buffer, knowing that the signal from an air shower is somewhere in it.
The team had earlier modeled what these radio signals would look like and when they started observing with LOFAR they struck gold very quickly. “A half of all observations agreed absolutely [with the models]. They fit perfectly, which is a rare experience in experimental physics,” says team member Heino Falcke, an astrophysicist at Radboud University in Nijmegen, the Netherlands. Using this technique, the researchers were able to measure how far down into the atmosphere the cascade of particles went before it reached its maximum size. That depth could tell them what sort of particle the original cosmic ray was—proton, helium nucleus, or something heavier.
As the scientists report today in Nature, about 80% of the more than 120 events analyzed turned out to be light cosmic rays—protons or helium nuclei. That’s not entirely unexpected. The LOFAR team probed particles with a range of energies between 1017 and 1017.5 electron-volts (eV). “A terra incognita,” Falcke says, which is hard to reach with other techniques. “The compositional data is very sparse.” This range occupies a middle ground between lower energy cosmic rays expected from sources in our galaxy and higher energy cosmic rays from much more distant galaxies. Current theory suggests that the highest energy cosmic rays are mostly protons rather than heavier nuclei. But the researchers say that you wouldn’t expect as big a fraction as 80% at energies below 1017.5 eV.
Also, Falcke says, there are hints in the distribution of particles across the range of energy they studied, that some of those light cosmic rays may have come from sources in our galaxy. That would be surprising because it is not thought that local accelerators, such as supernova remnants, can achieve energies for protons higher than 1015 eV. Falcke acknowledges that this interpretation “remains speculative. It’s a first step,” he says. If it holds up, however, it only deepens the mystery of cosmic rays, because it implies that there is some object or mechanism within our galaxy—as yet unknown —that is able to boost particles to these supercharged speeds.
But such a conclusion would be “challenging” to current astrophysical explanations, says Andrew Taylor, an astrophysicist at the Dublin Institute for Advanced Studies. He thinks the evidence is not yet there that they come from a source in our galaxy rather than an extragalactic one. “It’s too early to make definitive statements,” he says, but “both would be interesting.”
Taylor does think the new technique holds great promise for the future, because of its ability to identify types of particle and the fact that it can operate day or night in any weather (optical techniques of observing air showers only work on clear moonless nights). “Radio can gather data much faster,” he says. “It will provide a whole new set of opportunities to explore.”