Exploring Cosmic Rays Through the Shadows

At this week’s American Physical Society Meeting in Washington, DC, researchers from an observatory in Mexico unveiled unique images featuring a kind of shadow of the moon and sun. The images don’t contain a lot of new information about the sun and moon, but are a way of studying charged particles known as cosmic rays that move at really high speeds—their properties, interactions with magnetic fields, and even a bit about where they come from.

First, let’s talk shadows. If you place your hand between a lamp and a wall, you create a shadow on the wall. The hand-shaped shadow is just a dark area that the light can’t reach because your hand is in the way.

Now imagine replacing your hand with the moon and replacing the lamp with the high-energy charged particles that stream through space. Just like your hand stops visible light, the moon absorbs these rays. If you replace the wall with a set of particle detectors, you can capture the cosmic ray shadow of the moon. You can also do this for the sun.

This is the approach taken by Mehr Un Nisa (University of Rochester) and Zig Hampel (University of Wisconsin – Madison), two researchers that work with data from the High-Altitude Water Cherenkov Gamma-Ray (HAWC) Observatory. HAWC sits inside of one of Mexico’s national parks, on a plateau between Mexico’s highest peak and a large volcano in Sierra Negra.

Unlike a more traditional observatory with telescopes and retracting domes, this observatory consists of 300 giant tanks of purified water. It sounds kind of strange, but this is a tried-and-true method for detecting charged particles created in air showers by gamma rays and cosmic rays originating outside of the solar system.

The HAWC Observatory, August 2014.
Image Credit: Jordanagoodman (CC BY-SA 4.0).

When a gamma ray or cosmic ray hits the Earth’s atmosphere, it may collide with the nucleus of an atom. This causes a cascading shower of charged particles, like a rack of pool balls scattering toward one end of the table after being struck by the fast-moving cue ball. These showers reach the ground all the time—they aren’t dangerous or noticeable. However, when charged particles travel through purified water, they emit a particular kind of radiation. Detectors in the HAWC tanks pick up that radiation. By strategically placing tanks over a large area, scientists can identify showers and work backwards to determine the energy and properties of the inciting ray.

The new images are the result of about 17 months of data collection. Although they might not look as visually interesting as some images of the sun and moon taken in visible light, the images contain a lot of information useful for validating and improving cosmic ray models.

This is well illustrated in a series of images that show the moon’s shadow in the light of cosmic rays of different energies. The actual position of the moon is at the center of each image, but you’ll see that the shadow appears far to the right in the lowest-energy image (on the left). As the energy increases, the moon’s shadow moves toward the center of the grid.

This series of images shows the moon’s cosmic ray shadow as it appears in cosmic rays of increasing energy. If the cosmic rays were not deflected by the Earth’s maganetic field, the shadow would be directly in the center of each image. The colors, ranging from red (high) to blue (low), illustrate the number of cosmic rays detected from that location.
Image Credit: Mehr Un Nisa and Zig Hampel.

Cosmic rays don’t take a straight path on their way to Earth, even when the moon isn’t in the way. Cosmic rays are charged particles, which means that they respond to magnetic fields. In many cases, the Earth’s magnetic field changes the path of a cosmic ray. The size of the deflection depends on the energy of the cosmic ray. How does this relate to the moon images? The off-center shadows occur because the cosmic rays at those energies are deflected. The farther off-center, the more deflection.

As these images show, high-energy cosmic rays are not influenced by the Earth’s magnetic field as strongly as lower-energy cosmic rays. These observations match well with the results of simulations, validating the process the HAWC team uses to process and analyze their data.

The shadow is offset to the right, not the left, because the cosmic rays that reach Earth’s surface are primarily positively charged particles (protons). Magnetic fields deflect particles with different charges in opposite directions. If cosmic rays were primarily negatively charged, the shadow would be offset to the left in these images. There are negatively charged cosmic rays such as anti-protons (antimatter particles as heavy as protons but with a negative charge), but not nearly as many.

As they refine the process of separating the background from the data, the researchers are optimistic that an anti-proton shadow will appear on the left side. Although antimatter is ordinarily annihilated when it comes in contact with matter (like the air molecules in our atmosphere), some cosmic rays move fast enough that they can make it all the way to the planet’s surface without this happening. If the anti-proton shadow shows up, it could provide insight on the ratio of protons to anti-protons in cosmic rays—a useful piece of information for dark matter research and better understanding how cosmic rays travel through magnetic fields.

Usually we use light to illuminate objects, to see things as they really are and to discover the unknown. But as this work demonstrates, in some cases a shadow can be even more illuminating.

Kendra Redmond

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