People make a big deal about the energies reached by the Large Hadron Collider, to the extent that they filed suit to block its operation over fears it would destroy the Universe. But as the physicists running the accelerator noted in their response, when it comes to high energies, nature got there first. While the LHC will eventually reach energies of 14 Tera-electronVolts, a cosmic ray called the Oh-My-God particle struck Earth with an energy of 300 Exa-electronVolts—over 10,000,000 times more energetic.

Such insanely energetic particles, while uncommon, aren't exactly rare. Over the last five years, an observatory in Utah built to study cosmic rays has identified 72 particles that struck the Earth with energies above 50 Exa-electronVolts. By roughly mapping their origin, the observatory found that many seem to originate in a cosmic hotspot located in the northern hemisphere sky. But the source (sources?) that raises particles to these energies remains unidentified.

Any particle physicist hoping to accelerate something to these energies better be incredibly patient. "To accelerate particles up to the ultrahigh-energy region," the authors of the paper describing the hotspot write, "particles must be confined to the accelerator site for more than a million years by a magnetic field and/or a large-scale confinement volume." Once released, however, they have a finite lifetime. Over time, they will interact with the cosmic microwave background in a way that will gradually slow them down.

Because of this process, we know that these particles must have been generated relatively nearby, at least in cosmic terms: within 100 Megaparsecs. That still covers a lot of ground, given that the nearest galaxy, Andromeda, is less than one Megaparsec away—it's enough to encompass some local galaxy clusters.

To understand what's out there that could unleash particles at such insane energies, we have to know where they're coming from. That's somewhat challenging given that these particles are charged, and thus their paths are bent by both the Milky Way's magnetic field, as well as the intergalactic magnetic fields. Still, if the source is reasonably nearby, these deflections should only twist the particles' paths by a few degrees. Meaning that we could still identify the rough area where they're originating (assuming they're not generated by a process that goes on all over the place).

That's been the goal of a long-running survey called (creatively) the Telescope Array. The array consists of more than 500 hunks of plastic, each three meters square, scattered across the landscape in Utah. When the plastic is hit by energetic particles, it emits a characteristic glow. Nearby optical telescopes also scan the skies for energetic nitrogen molecules. Those energetic particles are generally produced by the interaction between a cosmic ray and our atmosphere, which creates a downward spray of debris. By working out when and where that debris hits the array, researchers can figure out the energy carried by the initial particle, as well as the rough direction that it arrived from.

As noted above, the team behind the Telescope Array has been accumulating data on these collisions since 2008, and they've recently published their latest progress report. Over the period in question, over a million individual events triggered the detectors' data recording system. Of these, 72 were clearly resolved and had an energy of over 57 Exa-electronVolts. (That energy was chosen to limit the degree to which the particles would have their origins obscured by interactions with magnetic fields.)

To search for hotspots, they moved a circle that covered 20 degrees of the sky around and summed up the number of particles contained within the circle. The results showed a clear area in the northern sky (the Array doesn't image the southern sky) that had a maximum of 19 events in a single circular area. To test its significance, the authors generated a million random datasets and determined how often a hotspot with this strength occurred; based on this, they rate the significance of their finding at 3.4 sigma.

If you look for known objects in that area of the sky, you see... nothing in particular. It's a bit off what's called the "supergalactic plane," an area where some of the local clusters of galaxies (Virgo, Ursa Major, etc.) tend to line up. That makes it somewhat less likely that the hotspot is driven by interactions among galaxies, but it certainly doesn't rule it out.

As the Telescope Array continues to take data, the significance of this finding should continue to go up, and it's possible that the team could narrow down the direction a bit. A better estimate of the strength and orientation of the intergalactic magnetic field would also help identify the location of the source. But all this will still undoubtedly leave a large area of the sky to examine for events that can drive particles to such extreme energies. There's a good chance the object(s) will remain a mystery for a while yet.

The Astrophysical Journal Letters, 2014. DOI: 10.1088/2041-8205/790/2/L21 (About DOIs).