The input material pertains to a significant event in high-energy astrophysics—the detection and subsequent explanation of the ultra-high-energy cosmic ray (UHECR) known as the Amaterasu particle.
Suggested Reviewer Group: Theoretical and Experimental High-Energy Astrophysicists.
Abstract:
The Amaterasu particle, an Ultra-High-Energy Cosmic Ray (UHECR) detected in 2021 by the Telescope Array in Utah, presented an astrophysical paradox due to its inferred energy ($\sim 244$ EeV) and apparent trajectory origin from the "local void," a low-density region lacking known UHECR sources. This challenge to the standard model of physics prompted a re-evaluation of the particle’s composition and propagation. A recent study (2026) utilized Approximate Bayesian Computation (ABC) and 3D magnetic field simulations to model potential curved trajectories. The findings suggest that if the UHECR was a heavier nucleus (e.g., iron), its path would be significantly deflected by galactic magnetic fields, permitting its actual origin to be traced back to highly active starburst galaxies, such as Messier 82, thereby resolving the conflict with known physics mechanisms.
The Amaterasu Particle: Compositional Inference and Magnetic Field Deflection as Resolution to the Origin Paradox
0:00 Initial Detection and Energy: The Amaterasu particle, detected in 2021, is classified as one of the most energetic particles ever observed, comparable to the famous 1991 Oh-My-God (OMG) particle, possessing energy levels reaching approximately $2 \times 10^{20}$ electron volts (EeV). This energy level is 40 million times higher than anything produced by terrestrial accelerators.
1:21 Nature of the Particle: These events are mass particles, traveling at velocities approaching the speed of light (e.g., 99.9999999% of $c$). The Amaterasu event was measured at $244$ EeV.
2:11 Origin Paradox: When researchers traced the particle's arrival trajectory, it appeared to originate from the "local void," a vast, low-density region near the Milky Way containing only six known galaxies. The void lacks the requisite violent astrophysical sources (e.g., quasars or massive black holes) capable of producing such UHECRs.
3:04 Detection Mechanism: The particle was detected by the Telescope Array project in Utah, where 23 of 500 ground detectors simultaneously registered an "air shower" on May 27, 2021, following the particle’s collision with the upper atmosphere.
5:00 Composition Hypothesis: Researchers Nadine Burish and Franchesca Cable focused on the particle's unknown composition. If the particle were a proton (stripped hydrogen atom), it would travel in a straight line, confirming the void origin. However, if it were a heavier nucleus, such as iron, its higher electric charge would cause its trajectory to be significantly deflected by intervening galactic magnetic fields.
6:30 Simulation Methodology: The 2026 study employed Approximate Bayesian Computation (ABC) combined with 3D simulations of the universe’s magnetic fields to model millions of potential curved paths the particle might have taken.
7:04 Resolved Origin: The simulations indicated a high probability that the particle originated from outside the local void. The deflected path suggests an origin in nearby, highly active starburst galaxies known for intense star formation and powerful magnetic fields.
7:29 Top Candidates: The study identified Messier 82 (M82), located 12 million light-years distant, as the top candidate source, with NGC 6946 (Fireworks Galaxy) and NGC 2403 (Caldwell 7) also listed as plausible origins.
8:46 Astrophysical Significance: UHECRs act as messengers from the universe's most violent events (e.g., powerful supernovae, tidal disruption events). Accurate source tracing is essential for defining the limits of natural particle acceleration.
9:34 Future Outlook: Cosmic ray detection infrastructure is undergoing upgrades, including the expansion of the Telescope Array to four times its current size. Additionally, the proposed space-based PMA project, expected to launch post-2029, is designed to detect UHECRs directly from space using dedicated orbiting satellites.
10:12 Conclusion: The new modeling suggests the Amaterasu particle paradox is resolved by magnetic field deflection acting on a heavy nucleus, allowing the source to be traced back to known, powerful starburst galaxies, rather than requiring new, unexplained physics.