One of the strongest cosmic rays in history was detected in May of 2021 in Utah. Amaterasu hit Earth’s atmosphere and caused a big shower of secondary particles. By 2023, scientists had determined that it was the second strongest particle after the famous Oh-My-God cosmic ray that was observed back in 1991.What made the observation especially interesting is that despite measuring its effect on Earth, researchers could not determine where it originated. That way, this raised numerous questions for future studies.How was Amaterasu detected?Amaterasu was detected by the Telescope Array, a vast observatory based in the Utah desert. As per the University of Utah, it is the largest cosmic-ray detector in the Northern Hemisphere and is created to study ultra-high-energy particles. Scientists did not detect the particle itself but the air shower it caused. When an outer space cosmic particle strikes the Earth’s atmosphere, it hits other particles, triggering a chain reaction of particles spreading out in different directions and being detected through various ground-based equipment.The energy and direction of this primary particle can be determined in this process. However, there will not be an actual image of the cosmic ray’s source. This particularity became the focal point of the Amaterasu study.Why is finding the source of the cosmic ray difficultOne of the main concerns when analysing the event was the source of the cosmic ray itself. In one of the studies published in Nature, the source of the particle was stated to come from a region of outer space with no known sources of high-energy radiation, described as “void-like”. Cosmic rays usually appear from highly energetic places in outer space. Such a place may be an active galaxy nucleus, remnant of a supernova, or jets from a black hole.None of the above was apparent in the Amaterasu event. Therefore, what remained was a challenge to scientists. How could it be that this immensely strong particle came from an area that seemed devoid of any matter?
Scientists observing the sky recorded a massive energy burst. Image credit – Gemini
The complexity of finding Amaterasu’s originsSince cosmic rays have been known to interact with magnetic fields, finding out the origins of these particles can be quite difficult. Charged particles travelling through space will be influenced by magnetic fields, which affect their path and trajectory.Various literature studies found that high-energy cosmic rays can be bent before entering our atmosphere. Their path, thus, does not always point to their origin. Hence, even a good detection will not solve the problem, since while it will accurately indicate the energy of the rays, determining their path is tricky.Origin of Amaterasu becomes a bit clearerHowever, things may now be a little clearer after the Amaterasu. In 2025, it became apparent that one possibility could exist. A 2025 Nature Research Highlight suggested that the blazar PKS 1717+177 might be a possible source.Blazars are considered active galaxies in which jets of high-energy particles are emitted towards Earth. Such jets are fueled by supermassive black holes that can emit radiation and other high-energy particles.According to this study, protons emitted from the blazar’s jet may collide with ambient photons, producing these high-energy particles. Some of those particles would be able to travel through space and eventually reach Earth in the form of cosmic rays.This model provides an explanation rather than an ultimate solution since it is consistent with what we know about such celestial objects. At the same time, it helps to connect the Amaterasu event with a certain kind of source of cosmic events.Role of neutrinos in identifying the source of cosmic raysScientists use neutrinos to investigate sources of cosmic rays. Neutrinos are subatomic particles with no mass that move in straight paths unaffected by magnetic fields. This describes that high-energy neutrinos often appear together with cosmic rays.In multi-messenger astronomy, scientists try to analyse several types of particles simultaneously. This makes the process much easier for the researchers.Significance of the Amaterasu cosmic ray The discovery represents a great success in terms of the achievements of mankind. At the same time, it indicates certain limits of our capabilities. Scientists detected and quantified one of the most powerful cosmic rays, but found it hard to trace back its origin.This pattern is not uncommon for research in astrophysics, and even the most advanced detectors are unable to establish the origins of cosmic rays. However, each new discovery brings science one step closer to the solution of mysteries.From mystery to an explanationAt this point, the scientific world continues to unravel the secret of the mysterious cosmic ray called Amaterasu. This became another interesting cosmic event registered by Earth-based observatories.For a while, it seemed to have no origin at all. Nowadays, researchers are trying to trace the signal back to its real origin. Regardless of whether the blazar theory is valid or not, this phenomenon has already helped cosmic rays research advance significantly.This proves that there are still many mysteries that need to be solved. Just one particle could pave the way to discovery.
