When Japanese scientist Toshihiro Fujii discovered a very high energy cosmic-ray event in May 2021, he christened it ‘Amaterasu’. It turned out to be the second-highest-energy cosmic ray to be discovered – so it was apt that he had named it after the sun goddess in Japanese mythology.
According to a paper published in the journal Science in November, Dr. Fujii, an astronomer at Japan’s Osaka Metropolitan University, discovered the cosmic ray when analysing data collected between May 2008 and November 2021 by the Telescope Array Project in the U.S.
Cosmic rays are streams of energetic particles and clusters of particles coming from outer space and the sun. They include protons and alpha particles (nuclei of helium atoms). Only low-intensity cosmic rays reach the earth’s surface. Their energy is mostly lost in the atmosphere itself, as they smash into atoms of the atmospheric gases and produce a shower of other particles. Otherwise life wouldn’t have been possible on the earth.
From the 1930s, studies of cosmic rays led scientists to discover many then-unknown subatomic particles. Yet the sources of cosmic rays and the reason they’re so energetic remain a mystery even 86 years after their discovery.
How much energy did Amaterasu have?
Data collected by the Telescope Array Project indicated the Amaterasu cosmic ray had an energy of 240 exa-electron-volt (EeV). The electron-volt (eV) is a unit of energy, like joules, used to measure the energy of subatomic particles. The energy of 1 eV is approximately 1.6 × 10-19 joules. One joule is the energy required to light a one-watt bulb for one second. It is easy to see how small this amount is when we realise a lamp we use at night uses about 15 J per second, or about 0.004 J/s.
The light-particles in sunlight have an energy of about 1.6-3.1 eV, for example. When one deuterium nucleus and one tritium nucleus undergo fusion, they release one helium atom, one neutron, and 17.6 million eV of energy. The mass-energy of a single Higgs boson particle, which is considered ‘heavy’, is 125.1 billion eV.
Cosmic rays typically range in energy from about one billion eV to about 100 billion billion eV. The Amaterasu cosmic ray had an energy of 240 EeV – or 240 billion billion eV. This is extremely high.
In fact, it’s about 40-million-times higher than the energy imbued in protons by the Large Hadron Collider (LHC), the world’s most powerful particle-smasher, located in Europe. So cosmic rays are our only source of very high energy particles, in spite of our best efforts.
The discovery of the Amaterasu cosmic ray could thus boost efforts to spot more such events as well as help make sense of their properties.
What do cosmic-ray energies tell us?
Ultra-high-energy cosmic rays (UHECRs) are subatomic particles from extragalactic sources with energies greater than 1 EeV. Scientists have observed UHECRs more energetic than 100 EeV. But typically, cosmic rays with more energy than around 60 EeV don’t ‘survive’ beyond a certain distance in space. This is because of the cosmic microwave background (CMB) – radiation in the microwave frequency leftover from the Big Bang and which today pervades the universe. This background radiation, as Dr. Fujii & co. wrote in their paper, “suppresses the flux of UHECRs above 60 EeV”.
The longer a UHECR passes through the CMB, the greater the suppression is. As a result, any UHECRs we spot on the earth should have come from a distance across which this suppression wouldn’t have been complete. Scientists have estimated this to be 50-100 megaparsec, or 1,500-3,000 billion billion km.
Moving near the speed of light, a cosmic ray will require 3-10 million years to travel this distance.
Where did Amaterasu come from?
An amazing feature of the Amaterasu particle is that if you look along the direction it came, towards its point of origin, there is nothing to be seen – meaning it appears to have come from an empty part of the universe.
“I thought there was a mistake because this particle had an unprecedented energy that hasn’t been seen in the last three decades,” Dr. Fujii told Cosmos. “No promising astronomical object has been identified that matches the direction from which the cosmic rays came, suggesting the possibility of unknown astronomical phenomena and new physical origins beyond the Standard Model.”
The Standard Model is the theory most physicists currently use to explain the universe’s subatomic building blocks.
Nonetheless, Dr. Fujii and his colleagues proposed three possible explanations for the particle’s origin: (i) it could be from a source we haven’t yet identified; (ii) it may have interacted with a magnetic field stronger than current models account for, changing its direction; or (iii) scientists may have to rethink their understanding of high-energy particle physics.
In 1991, another high-energy cosmic ray with an energy of 320 EeV was detected at the Dugway Proving Ground in Utah. It remains the most energetic cosmic ray ever recorded. Scientists have called it the “Oh My God” particle.
How can Amaterasu help?
Cosmic rays can be divided into two types: those originating from beyond our solar system, called galactic cosmic rays (GCR), and high-energy particles emitted by the sun, called solar cosmic rays, that are mainly protons.
Solar cosmic rays originate primarily in solar flares. In modernity, the particles in these rays have come to be called solar energetic particles. By tracking these cosmic rays, scientists have found that the mass ratio of helium to hydrogen nuclei – that is, the ratio of the total masses of hydrogen and helium present – is about 28:100, meaning there are about 28 grams of alpha particles for every 100 grams of protons in cosmic rays. This ratio is similar to the abundance of helium and hydrogen in the early universe.
GCRs are slowly changing streams of high-energy particles that constantly strike the earth. They are thought to originate outside the solar system in events such as supernovae. (A supernova is an explosion that occurs when a massive star nears the end of its life after running out of matter that it can fuse.)
Although some 89% of GCRs is hydrogen, the remainder includes the nuclei of all elements, down to and including trace amounts of uranium. These nuclei are also fully ionized, meaning all of their electrons have been stripped away. As a result, these particles interact with and are affected by magnetic fields. This is why the sun’s strong magnetic fields alter the energy levels of GCRs reaching the earth.
When cosmic ray particles reach the earth’s atmosphere, they ionise air molecules that are at least about 3 km above the surface. Beyond that, they will have lost most of their energy.
Against this background, we can see how high the energy of the recently discovered cosmic ray was, and how that energy helps us select theories that better fit the data.
V. Sasi Kumar is a scientist formerly at the Centre for Earth Science Studies, Thiruvananthapuram.
