For decades, physicists have sought the sources of the most energetic subatomic particles in the universe—cosmic rays that strike the atmosphere with as much energy as well-thrown baseballs. Now, a team working with the Telescope Array, a collection of 507 particle detectors covering 700 square kilometers of desert in Utah, has observed a broad "hotspot" in the sky in which such cosmic rays seem to originate. Although not definitive, the observation suggests the cosmic rays emanate from a distinct source near our galaxy and not from sources spread all over the universe.
Physicists have been down a similar road before. In 2007, researchers with the Pierre Auger Observatory, an even bigger array in Argentina, reported that ultra–high-energy cosmic rays appeared to spring from the fiery hearts of certain galaxies—only to see that correlation weaken with more data. However, the Telescope Array team has taken a more catholic approach, looking only for evidence that cosmic rays do not arrive in equal numbers in all directions. "It's simpler and more direct, and therefore more robust," says Glennys Farrar, a theorist at New York University (NYU) in New York City, who was not involved in the work.
Nobody knows how ultra–high-energy cosmic rays—mainly protons or heavier atomic nuclei—acquire energies millions of times higher than have been achieved with humanmade particle accelerators. (Physicists dubbed one of the first ones observed the “Oh-My-God particle.”) Lower energy cosmic rays are thought to spring from the lingering remnants of stellar explosions called supernovas. But such clouds are far too small to produce the highest energy cosmic rays. Instead, theorists generally expect that the most energetic cosmic rays rev up over millions of years in unidentified accelerators the size of galaxies.
The Telescope Array aims to help solve that mystery. When a high-energy cosmic array strikes the atmosphere, it disappears in an avalanche of lower energy particles. Those particles trigger the detectors in the array, enabling researchers to deduce the direction and energy of the original cosmic ray. From 2008 to 2013, researchers spotted 72 cosmic rays with energies above 57 exaelectron volts—15 million times the highest energy achieved with a particle accelerator. And 19 of them appear to cluster in a hotspot in the sky about 20° in radius, as Hiroyuki Sagawa, a co-representative for the Telescope Array team from the University of Tokyo, reported today in a press conference at the university.
The signal isn't strong enough for scientists to claim a discovery, cautions Pierre Sokolsky of the University of Utah in Salt Lake City, one of 125 members of the Telescope Array team. "It's an enhancement at a statistical level that raises eyebrows," he says. By running millions of simulations in which they dot the sky with 72 random points, researchers estimate the chances that random cosmic rays could produce such a hotspot at one in 2700, the Telescope Array team explains in a paper in press at The Astrophysical Journal Letters.
But analyzing such sparse data is tricky, says Karl-Heinz Kampert, a physicist at the University of Wuppertal in Germany and spokesman for the 500-member Auger team. The Telescope Array researchers have no reason to expect that a hotspot would be 20° in radius, he says, so they can't be sure they haven't adjusted that width to inadvertently emphasize a random clustering. Still, he says, it's plausible that the team is seeing a real signal, all the more so because Auger has long seen similar clustering in the direction of active galaxy Centaurus A. "We have what you might you might call a warm spot," he says.
But if the Telescope Array is starting to see a source of ultra–high-energy cosmic rays, then its identity remains unclear. Physicists think that the highest energy cosmic rays cannot come from more than 500 million light-years away, as interactions with lingering radiation from the big bang ought to snuff out cosmic rays from more distant sources. But no obvious candidate for a nearby cosmic accelerator lies directly in line with the hotspot, Sokolsky says. He notes, however, that in that region a filament of galaxies kinks toward Earth and speculates that magnetic fields in that string might help rev up particles.
That's not surprising, NYU's Farrar says. Over the past decade, physicists have developed much more detailed maps of the magnetic field within the galaxy, which can deflect charged particles such as protons and nuclei. Those fields can deflect the path of a high-energy proton by tens of degrees—even more for heavy atomic nuclei. Still, Farrar says, "I wouldn't be surprised if within 5 years we could predict the deflection to within a few degrees"—at least for protons.
In the meantime, the Telescope Array team hopes to prove whether the hotspot is real. The $25 million array was built primarily with funding from the Japanese government and is run mainly by the U.S. National Science Foundation. Sokolsky and colleagues hope to expand the array—doubling the number of detectors and quadrupling its area—in a proposed $6.4 million upgrade. The upgrade would enable them to collect five times more data in a few years and really settle the matter.
