The Hottest Spot for Cosmic Rays
Most of astronomy is based on light—whether it’s low-energy radio waves, high-energy gamma rays, or the narrow range of colors our eyes can see. However, there’s another whole area of research using other particles to study the universe: astroparticle physics. It includes the study of cosmic rays, which, despite their name, are actually electrically charged particles such as protons, accelerated to nearly light-speed.
Cosmic ray observations are more challenging than many other forms of astronomy. Few of the particles actually get through Earth’s atmosphere. Instead, much of what researchers measure on the surface are secondary particles created from collisions between the cosmic ray protons and molecules in the upper atmosphere. As a result, it’s often hard to tell where cosmic rays are coming from because we can’t easily backtrack them to their source.
In particular, nobody knows yet where the very highest energy cosmic rays originate, but a new study has narrowed down the search to a particular patch on the sky. Researchers at the creatively named Telescope Array in Utah showed that a significant fraction of the most energetic protons came from the same region—a “hotspot” about 80 times the width of the full Moon.
That’s not exactly a pinpoint, but it’s still a significant find. The hotspot is about six percent of the total sky, so if the sources of cosmic rays were evenly distributed throughout the universe, you’d expect roughly six percent of the highest energy particles to come from that region. Instead, the researchers found about 25 percent came from that region, indicating they may have a common source.
Also, the hotspot doesn’t occur in the dense parts of the Milky Way, so these cosmic rays probably come from beyond the galaxy. (Cue the dramatic music.) However, the cosmic ray source might be part of the Virgo Supercluster, a huge association of galaxies that dominates the local universe.
As with most types of light, the Sun is the biggest source of cosmic rays we observe, mainly because it’s close to Earth. However, solar particles are relatively low in energy. Other cosmic rays originate in deep space, driven by supernova explosions or their remnants, magnetic fields near neutron stars, and other powerful events. Due to their varying origins, they carry a variety of energies, which is called the cosmic ray spectrum.
According to theory, the spectrum we observe should cut off sharply at very high energies. That’s not because those particles don’t exist, but because they collide with the bath of photons (particles of light) left over from the early Universe known as the cosmic microwave background. Any cosmic ray more energetic than that cut-off is special: it probably also originates fairly close to the Milky Way, since it hasn’t slowed down from collisions.
Researchers refer to these as ultra high-energy cosmic rays, or UHECRs; the most famous and energetic of these gained an ironic nickname: the “oh my god particle”, in reference to the universally hated appellation for the Higgs boson. Lower-energy cosmic rays can be steered by magnetic fields, meaning they might enter the atmosphere in a very different direction from where they started. By contrast UHECRs move so rapidly that almost nothing can deflect them in their travels: if researchers see one coming from a particular spot in the sky, that’s probably close to where it originated.
However, that’s still easier said than done because UHECRs don’t pierce the atmosphere. For that reason, the Telescope Array looks for the faint flashes of light produced when a high-energy proton hits a nitrogen molecule high above the ground. The array itself consists of 523 small detectors distributed over 700 square kilometers (300 square miles) in the western Utah desert. The detectors don’t look like much—they’re basically desk-sized plastic rectangles wired up with electronic sensors—but by comparing the signals between multiple detectors, researchers can work backward to determine the energy and direction of the original cosmic ray.
While this method isn’t as precise as optical telescopes, the Telescope Array and its southern hemisphere counterpart, the Pierre Auger Observatory, are gradually improving our ability to find where UHECRs originate. Currently, the hope is to double the number of detectors in the array, which in combination with the larger area of coverage would make the Telescope Array five times more sensitive.
Knowing that many UHECRs come from the same general patch of the sky, astronomers may be able to identify possible sources. The high energy might be the result of something relatively mundane, like strong magnetic fields in intergalactic space. But everyone loves a good mystery—even astronomers—so no doubt a few researchers are holding out hopes of finding something entirely new lurking in the cosmic ray hotspot.