Detection of Second-Highest-Energy Cosmic Ray Ever by Telescope Array

November 23, 2023

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In the year 1991, a record-breaking high-energy cosmic ray was identified by the University of Utah's Fly's Eye experiment. This ray, later referred to as the Oh-My-God particle, astounded the global astrophysics community. No known source in our galaxy could have produced it and the particle's energy exceeded the theoretical limits for cosmic rays journeying to Earth from external galaxies. This particle, by all logic, should not exist.

Since that time, the Telescope Array has recorded over 30 of these ultra-high-energy cosmic rays. However, none have matched the Oh-My-God particle's energy capabilities. Their origins and the means by which they travel to Earth remain undiscovered.

On May 27, 2021, a cosmic ray of extreme energy, second only to the previous record-holder, was identified by the Telescope Array experiment. The energy carried by this single subatomic particle, at 2.4 x 1020eV, is comparable to a brick landing on your toe from waist height. Led by the University of Utah and the University of Tokyo, the Telescope Array used for this experiment consists of 507 surface detector stations cemented in a grid formation over an area of 700km2, situated near Delta, Utah, in the state's West Desert.

This detection event resulted in 23 detectors in the north-west region of the Telescope Array being triggered, sprawling across an area of 48 km2. Its arrival seemed to originate from the Local Void, a vast empty expanse bordering our Milky Way galaxy. The extremely energetic particle, named the 'Amaterasu' particle, had its signal recorded and its event animated.

'These particles carry such powerful energy charges that they are unlikely to be affected by any magnetic fields, whether within or outside of our galaxy. Therefore, their source in the sky should be traceable,' shared John Matthews, a co-spokesperson for the Telescope Array at the University of Utah and also one of the study's co-authors. 'But the origins for both the Oh-My-God particle and the Amaterasu particle reveal no potential source capable of generating such high energy. This is the enigma—what exactly is happening?'

The international team of researchers, in their observation published in the Science journal, offers a comprehensive profile of the ultra-high-energy cosmic ray, and suggests that such unusual phenomenons could be related to as-yet-undiscovered particle physics concepts.

The 'Amaterasu' name derives from Japanese mythology's sun goddess. Both the Oh-My-God and Amaterasu particles were discovered utilizing various observation methodologies, affirming the authenticity of these rare, ultra-high-energy occurrences.

'These incidents seem to be originating from completely distinct places in the sky, rather than a single peculiar source,' revealed John Belz, a University of Utah professor and also a study co-author. 'Speculations range from defects in the structure of spacetime to colliding cosmic strings. I’m merely suggesting various outlandish theories that people are proposing as there is no conventional explanation available.'

Cosmic rays are essentially charged particles released by celestial incidents of extreme violence that have reduced matter to its subatomic components and then catapulted it almost at light speed across the universe. They are a spectrum of energies made up of protons, electrons, or full atomic nuclei that traverse through space and shower upon Earth almost uninterruptedly.

The collision of these cosmic rays against Earth's upper atmosphere disintegrates the nucleus of oxygen and nitrogen gases, creating numerous secondary particles. These secondary particles, after travelling a short distance into the atmosphere, repeat the disintegration process, resulting in a cascade of billions more particles that scatter to the surface. Detecting the massive 'footprint' left by this secondary particle shower necessitates an area as large as the Telescope Array. This array of surface detectors uses a varied set of instruments to inform researchers on each cosmic ray; the signal's timing gives the trajectory and the quantity of charged particles affecting each detector provides information on the original particle's energy.

Because particles have a charge, their flight path resembles a ball in a pinball machine as they zigzag against the electromagnetic fields through the cosmic microwave background. It's nearly impossible to trace the trajectory of most cosmic rays, which lie on the low- to middle-end of the energy spectrum. Even high-energy cosmic rays are distorted by the microwave background. Particles with Oh-My-God and Amaterasu energy blast through intergalactic space relatively unbent. Only the most powerful of celestial events can produce them.

'Things that people think of as energetic, like supernova, are nowhere near energetic enough for this. You need huge amounts of energy, really high magnetic fields to confine the particle while it gets accelerated,' said Matthews.

Ultra-high-energy cosmic rays must exceed 5 x 1019 eV. This means that a single subatomic particle carries the same kinetic energy as a major league pitcher's fastball and has tens of millions of times more energy than any human-made particle accelerator can achieve.

Astrophysicists calculated this theoretical limit, known as the Greisen–Zatsepin–Kuzmin (GZK) cutoff, as the maximum energy a proton can hold traveling over long distances before the effect of interactions of the microwave background radiation takes their energy.

Known source candidates, such as active galactic nuclei or black holes with accretion disks emitting particle jets, tend to be more than 160 million light years away from Earth. The new particle's 2.4 x 1020 eV and the Oh-My-God particle's 3.2 x 1020 eV easily surpass the cutoff.

Researchers also analyze cosmic ray composition for clues of its origins. A heavier particle, like iron nuclei, are heavier, have more charge and are more susceptible to bending in a magnetic field than a lighter particle made of protons from a hydrogen atom. The new particle is likely a proton. Particle physics dictates that a cosmic ray with energy beyond the GZK cutoff is too powerful for the microwave background to distort its path, but back-tracing its trajectory points towards empty space.

'Maybe magnetic fields are stronger than we thought, but that disagrees with other observations that show they're not strong enough to produce significant curvature at these 1020 electron volt energies,' said Belz. 'It's a real mystery.'

The Telescope Array is uniquely positioned to detect ultra-high-energy cosmic rays. It sits at about 1,200 m (4,000 ft), the elevation sweet spot that allows secondary particles maximum development, but before they start to decay. Its location in Utah's West Desert provides ideal atmospheric conditions in two ways: the dry air is crucial because humidity will absorb the ultraviolet light necessary for detection; and the region's dark skies are essential, as light pollution will create too much noise and obscure the cosmic rays.

Astrophysicists are still baffled by the mysterious phenomena. The Telescope Array is in the middle of an expansion that that they hope will help crack the case. Once completed, 500 new scintillator detectors will expand the Telescope Array will sample cosmic ray-induced particle showers across 2,900 km2 (1,100 mi2 ), an area nearly the size of Rhode Island. The larger footprint will hopefully capture more events that will shed light on what's going on.

Journal information: Science

Provided by University of Utah