Scientists find record high-energy cosmic ray electrons, but origin remains elusive

The Earth is under attack — not by humanity or rampaging aliens, but by the Universe itself. Vast cosmic forces break apart hydrogen atoms and fling their components across the cosmos at nearly the speed of light. Some of these interstellar projectiles smash into the Earth’s atmosphere, allowing sensitive detectors to unravel their secrets. A recent measurement has studied extremely high-energy impacts and has simultaneously settled an ongoing scientific debate while puzzling scientists.

The steady deluge of radiation from space is called cosmic rays, and it consists predominantly of two components: protons and electrons. Protons come from the center of atoms, while electrons are found within the atoms’ periphery. Both can be accelerated from a variety of cosmic sources — from supernovae to colliding neutron stars — and (more commonly) by the electric and magnetic fields found in outer space. These fields can push the protons and neutrons to very high speeds.

Since the 1930s, scientists have known about cosmic rays, which hit the atmosphere, resulting in a cascade of particles detected on Earth’s surface. Most of the very highest energy cosmic rays that have been studied are protons. That’s because protons are about 2,000 times heavier than electrons, meaning that when a proton gets going, it is much more difficult to deflect than an electron. An apt analogy might be that protons act like bowling balls, while electrons act more like ping-pong balls.

Because of this difference in how the two particles travel through space, very high-energy electrons can get scattered and lose their energy. This means that they can’t get very far — at least astronomically speaking. So, the very highest energy electrons must have been created in our cosmic neighborhood.

A recent measurement by the H.E.S.S. observatory of the energy spectrum of cosmic ray electrons shows surprising results. Credit: F. Aharonian et al. (H.E.S.S. Collaboration) [1]; NASA Goddard; adapted by APS.”

The H.E.S.S. observatory (High Energy Stereoscopic System) is located in Namibia. It consists of four giant telescopes, each with a diameter of 12 meters, and a single, much larger telescope with a mirror diameter of 28 meters. Each night, it peers at the midnight sky, looking for flashes of light created as cosmic rays hit the Earth’s atmosphere. It began operations in 2003, with upgrades in the intervening years. The most recent paper is based on 12 years of H.E.S.S. observations between 2003 and 2015.

H.E.S.S. trained their analysis software to distinguish between the different characteristics of the light flashes left by protons and electrons in the atmosphere. Their recent paper focused on electrons. Previous measurements had measured the energy of cosmic ray electrons up to a little over a trillion electron volts of energy. (By way of comparison, the energy of electrons in televisions in the 1990s was more like 10,000 electron volts, and the highest energy manmade electron beam had an energy of 0.1 trillion electron volts.) The recent H.E.S.S. measurement observed electrons with an energy as high as 40 trillion electron volts.

Previous measurements had noticed that lower-energy electrons were more common than high-energy ones, with a gradual decrease. However, at an energy of about 1 trillion electron volts, the energy dropped off precipitously. This is thought to reflect the difficulty of very high-energy electrons traveling over very large distances. It’s kind of like trying to shine a bright light through a fog. It can only get so far.

Previous measurements also hinted at an unexplained excess of electrons with an energy of about 4 trillion electron volts. This excess was exciting to scientists, as it could have been the signature of a theoretical form of matter called dark matter. Dark matter has not been directly observed, but it is thought to be five times more prevalent than the ordinary matter of atoms. Scientists have been looking for experimental confirmation of dark matter for half a century and observing it would be a real coup.

However, the H.E.S.S. data is more precise than earlier measurements and it shows no hints of an excess at 4 trillion electron volts, effectively dashing the dreams of those scientists who thought that they had finally seen dark matter.

The behavior observed by H.E.S.S. is nonetheless perplexing. These high-energy electrons must have been formed relatively nearby — say, within a few thousand light years. This distance is much smaller than the size of the Milky Way, which has a diameter of about 100,000 light years.

The data recorded by H.E.S.S. does not appear to have come from many sources, rather it favors a single, nearby origin. However, so far, the data does not seem to come from any preferred direction, which is what one would expect if there was a single star in the cosmic neighborhood that was generating high-energy electrons.

The fact that these electrons are very high energy means that other competing experiments are not well suited to see them. However, the recent H.E.S.S. measurement only used data collected before late 2015. In the intervening years, H.E.S.S. has been operational, with improved electronics, which suggests that future analysis might be able to identify the source of this nearby cosmic cannon of electrons.