Cosmic rays are a fascinating and potentially hazardous phenomenon. These high-energy particles typically consist of protons that have been stripped of their electrons and accelerated to nearly the speed of light. When these rays collide with Earth’s atmosphere, an enormous amount of secondary particles known as an “air shower” results. Ordinarily, these showers are a source of frustration for astronomers since they leave “tracks” on telescope images that obscure the celestial objects (asteroids, stars, galaxies, exoplanets, etc.) being observed.
However, a research team from the National Astronomical Observatory of Japan (NAOJ) and Osaka Metropolitan University has found a new application for these energetic particles. Using a novel method, they could observe these extensive cosmic-ray air showers with unprecedented precision. The key to their method is the Subaru Prime Focus Camera (Suprime-Cam) mounted on the Subaru Telescope atop the Mauna Kea volcano in Hawaii. This method and the team’s findings could provide a new method for studying the Universe’s most energetic particles.
The team was led by Associate Professor Toshihiro Fujii from the Nambu Yoichiro Institute of Theoretical and Experimental Physics at Osaka Metropolitan University and graduate student Fraser Bradfield of the OMU Graduate School of Science. They were joined by researchers from the NAOJ, Hosei University, The Graduate University for Advanced Studies (SOKENDAI), the Berkeley Center for Theoretical Physics, Lawrence Berkeley National Laboratory, the Kavli Institute for the Physics and Mathematics of the Universe (WPI), and the Planetary Exploration Research Center (PERC) and the Center for Frontier Science (CFS) at the Chiba Institute of Technology.
Located within the NOAJ on Mauna Kei, the 8.2-meter (~27-ft) Subaru Telescope was designed for visible light (optical) astronomy. During normal astronomical data processing, the “tracks” caused by cosmic rays are dismissed as interference (aka. “noise”) that is separated from the images. For the sake of their study, the team led by Toshihiro and Bradfield focused on this very noise by analyzing thousands of images captured by Subura’s Suprime-Cam. This 80-megapixel optical camera can efficiently image a wide field of view while also capturing very faint objects with high resolution.
The team examined approximately 17,000 images captured by Suprime-Cam between 2014 and 2020. For this sample, they isolated 13 images that contained extensive air showers, indicated by the much larger-than-normal particle tracks. Their data and the method used to acquire it could have significant implications for astronomy, particularly where the study of exotic particles is concerned. As Professor Toshihiro explained in a recent OMU press release:
“With conventional observation methods, it is challenging to distinguish between the types of particles that constitute extensive air showers. Our method, on the other hand, has the potential to determine the nature of individual particles. Furthermore, by integrating our method with conventional approaches, we hope to advance our understanding of extensive air showers. This technique may allow us to search for dark matter or other exotic particles, offering additional insights into the transition of the Universe into a matter-dominated era.”
Their results were published in Scientific Reports (a Nature journal) on October 12th, 2023.