Tracing the Origin of an Ultra-High-Energy Cosmic Ray
Astronomers are edging closer to understanding the source of one of the most energetic cosmic rays ever detected, a particle dubbed “Amaterasu” after the Japanese sun goddess. Detected in by the Telescope Array Project in Utah, the Amaterasu particle carries an energy of over 240 exa-electronvolts (EeV), making it the second-highest-energy cosmic ray ever observed. New analysis suggests its origin may lie not in the vast emptiness of space previously suspected, but within a nearby star-forming galaxy.
Cosmic rays, or astroparticles, are high-speed charged particles – primarily protons and atomic nuclei – that permeate the universe. They serve as messengers from distant and extreme cosmic environments, offering clues about the forces shaping the cosmos. Most are deflected by Earth’s magnetosphere, but some penetrate the atmosphere, allowing scientists to study them. Reconstructing the energy of these particles is a complex undertaking, and pinpointing their sources presents a significant statistical challenge.
The Mystery of the Local Void
Initial investigations pointed towards the Local Void, a relatively empty region of space bordering the Milky Way. This presented a puzzle, as the Local Void lacks the dense concentrations of matter and energetic phenomena typically associated with the acceleration of particles to such extreme energies. The Amaterasu particle’s apparent origin within this void defied conventional understanding.
Francesca Capel and Nadine Bourriche from the Max Planck Institute for Physics tackled this challenge by combining advanced simulations with modern statistical methods, specifically Approximate Bayesian Computation. This approach allowed them to create three-dimensional maps of cosmic-ray propagation and their interactions with magnetic fields within the Milky Way. Their work represents a milestone in the ongoing search for the sources of ultra-high-energy particles.
M82: A Potential Source
The researchers’ analysis suggests a more nuanced picture. Rather than originating in the sparsely populated Local Void, the Amaterasu particle is more likely to have been produced in a nearby cosmic environment, specifically the M82 Cigar Galaxy, located approximately 12 million light-years from Earth. M82 is a starburst galaxy, characterized by intense star formation activity, which is often associated with the acceleration of cosmic rays.
“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,” explained Bourriche. This finding doesn’t definitively solve the mystery, but it narrows the search and provides a concrete target for further investigation.
Understanding Ultra-High-Energy Cosmic Rays
The Amaterasu particle’s energy is approximately 40 million times greater than that of particles colliding within the Large Hadron Collider (LHC). Determining whether the original particle was a proton, a light atomic nucleus, or a heavier nucleus like iron remains an open question. Regardless of its composition, the particle’s immense energy provides a unique window into the extreme physics occurring in its source environment.
As Capel, who leads the “Astrophysical Messengers” group at the Max Planck Institute for Physics, stated, “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.”
A New Analytical Approach
The methodology employed by Capel and Bourriche is significant not only for its potential to unravel the origin of the Amaterasu particle but also for its broader implications for the field of cosmic ray research. By combining physics-based simulations with observational data, they’ve created a powerful new analytical approach. This method complements existing efforts by bridging the gap between theoretical models and empirical observations.
The findings, published in in The Astrophysical Journal, in a paper titled “Beyond the Local Void: A Data-driven Search for the Origins of the Amaterasu Particle“, represent a crucial step forward in understanding the most energetic phenomena in the universe. The ongoing investigation into the Amaterasu particle promises to reveal further insights into the extreme environments capable of accelerating particles to such astonishing energies.
