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Cosmic rays are messengers from space that scientists use to study the Universe. Francesca Capel and Nadine Bourriche from the Max Planck Institute for Physics have investigated one of the most energetic cosmic rays ever observed: the Amaterasu particle, named after the Japanese sun goddess. Their work marks an important step toward solving the puzzle of its origin, providing a new analytical approach to tracing the possible sources of ultra-high-energy cosmic rays.
Cosmic rays are extremely fast, charged particles that travel through space at nearly the speed of light. The Amaterasu particle was detected in 2021 by the Telescope Array experiment in the USA. It is the second-highest-energy cosmic ray ever observed, carrying around 40 million times more energy than particles accelerated at the Large Hadron Collider. Such particles are exceedingly rare and thought to originate in some of the most extreme environments in the Universe.
The arrival direction of the Amaterasu particle appeared to point toward the Local Void, a region of space with few known galaxies or energetic objects capable of producing such particles - posing a puzzle for scientists.
Beyond an ‘Empty’ Region of Space
In their study, Francesca Capel and Nadine Bourriche show that the particle’s origin does not have to be confined to a single empty region of the Universe. Instead, it may lie within a broader range of nearby cosmic environments. “Our results suggest that, rather than originating in a low-density region of space like the Local Void, the Amaterasu particle is more likely to have been produced in a nearby star-forming galaxy such as M82,” says Nadine Bourriche.
These conclusions are based on a novel, data-driven method developed by the researchers to trace the particle’s possible path through space. Using detailed three-dimensional simulations of cosmic-ray propagation and their interaction with magnetic fields, the study applies a statistical technique known as Approximate Bayesian Computation. “This approach works by comparing the results of realistic, physics-based simulations with actual observational data to infer the most probable source locations,” Bourriche explains.
New Analytical Groundwork for Data-Driven Searches
By combining advanced simulations with modern statistical methods, the researchers were able to generate probability maps showing where the particle could realistically have originated. The framework developed in this study sets important milestones for future research. It provides a powerful tool to guide observations and sharpen the search for the cosmic sources capable of accelerating particles to such extreme energies. “Exploring ultra-high-energy cosmic rays helps us to better understand how the Universe can accelerate matter to such energies, and also to identify environments where we can study the behavior of matter in such extreme conditions”, Francesca Capel, leader of the group “Astrophysical Messengers” at the MPP says. “Our goal is to develop advanced statistical analysis methods to exploit the available data to its full potential and gain a deeper understanding of the possible sources of these energetic particles.” These new approaches focus on complementing existing efforts by enabling a closer connection between theory and data and combining information from different observations.
About the authors:
Francesca Capel grew up in Cardiff, UK, and studied physics at Imperial College London. Her academic path took her to École Polytechnique Fédérale de Lausanne in Switzerland for her Master’s thesis, followed by work at the European Space Agency in the Netherlands, a PhD at KTH Royal Institute of Technology in Sweden, and a Postdoc at the Technical University of Munich, Germany. She is now based at the Max Planck Institute for Physics, where she leads the research group “Astrophysical messengers” working at the intersection of data science and astroparticle physics.
Nadine Bourriche grew up in Bologna, Italy, in a family of Algerian origins. She studied physics at Université Grenoble Alpes in Grenoble, France and went on to complete a Master’s degree in Astroparticle Physics at the University of Innsbruck, Austria. She is currently a Ph.D. candidate in Francesca Capel’s group at the Max Planck Institute for Physics, where her research focuses on identifying the sources of the most extreme cosmic rays using novel statistical methods and machine learning.
Dr. Francesca Capel
francesca.capel@mpp.mpg.de
Nadine Bourriche
nadine.bourriche@mpp.mpg.de
Beyond the Local Void: A Data-driven Search for the Origins of the Amaterasu Particle
Nadine Bourriche, Francesca Capel
Astrophys.J. 997 (2026) 2, 264
DOI: 10.3847/1538-4357/ae2c89
https://www.mpp.mpg.de/en/news/news/cosmic-investigations-tracing-the-origin-of-...
Starburst galaxy Messier 82 (M82), located 12 million light-years away
Copyright: NASA, ESA, CSA, STScI, A. Bolatto (UMD)
The authors Dr. Francesca Capel (left) and Nadine Bourriche
Source: Jarred Green
Copyright: MPP
Criteria of this press release:
Journalists
Physics / astronomy
transregional, national
Research results
English
Starburst galaxy Messier 82 (M82), located 12 million light-years away
Copyright: NASA, ESA, CSA, STScI, A. Bolatto (UMD)
The authors Dr. Francesca Capel (left) and Nadine Bourriche
Source: Jarred Green
Copyright: MPP
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