In a paper published today in Science, we report the detection of an "extreme energy cosmic ray" of energy 2.44×10 to the power of 20th electron volts = 244 EeV (equivalent to 40 Joules) which was detected on May 27, 2021, by the Telescope Array Experiment. The cosmic ray was named Amaterasu, the goddess of the sun in Japanese mythology, because the discoverer was Japanese, and it was detected at dawn local time.
Extreme energy cosmic rays are extremely rare! We only have a handful of events detected at energies above 200 EeV. So, any new detection is a confirmation that these particles do exist and provides a new clue as to their origin. The ‘Oh-My-God particle’ remains the wildest cosmic ray ever seen with 320 EeV of energy, detected in 1991 by Fly’s Eye. Combining the statistical and systematics errors, the energy of Amaterasu could be as low as ~140 EeV and as high as 324 EeV.
There are no astronomical candidates in the arrival direction.
Why is it important?
Because they are charged particles, cosmic rays are constantly deflected and scattered by magnetic fields in the Galaxy and extragalactic medium, until any trace of their original motion is lost. Heavier particles like oxygen and iron experience more bending in the magnetic field than protons. Since there are no astronomical candidates in the arrival direction of Amaterasu, this may indicate that the particle is likely not a proton, but a heavier mass nucleus.
Such "extreme" events are important to probe the origin of the highest energy cosmic rays. The cosmic microwave background radiation imposes a limit on how far extreme energy cosmic rays can travel. This limit is different for protons and heavy nuclei. For example, at energies above 150 EeV, this limit is only ~40 Mpc hence reducing the number of source candidates to only ~1,400 local galaxies (which includes 40 starburst galaxies, 11 jetted radio galaxies and 2 cluster accretion shocks).
Being able to measure the particle composition in the future will be crucial to understand their origin. At ∼300 EeV, only protons have a horizon of ~ 30 Mpc. If a single, medium mass, like nitrogen, is detected at ∼300 EeV, then it must have originated from a source in the Local Group. This would rule out radio galaxies or cluster accretion shocks and it will be a strong indication that sources are either transients (magnetars flares, relativistic jets associated with stellar-mass back holes), unknown sources or more exotic scenarios involving new physics.
In the future, we will continue to operate the Telescope Array experiment, and we will investigate these extremely energetic particles in more detail through the ongoing upgrade experiment, called TAx4, and improving our approaches to mass measurements to unveil the nature and origin of the most energetic particles in the Universe.
Noemie Globus (co-author on the paper), for the Telescope Array Collaboration
Title: An extremely energetic cosmic ray observed by a surface detector array
Author: Telescope Array Collaboration
Publication date: November 23, 2023, 14:00 (U.S. Eastern Time)