In the high desert of western Utah, 507 scintillator detectors dot the sagebrush landscape while fluorescence telescopes watch from surrounding hills. The Telescope Array is the Northern Hemisphere's largest cosmic ray observatory — and in 2021, it detected something extraordinary: the Amaterasu particle, one of the most energetic particles ever observed.
Location and History
The Telescope Array (TA) sits about 200 km southwest of Salt Lake City, near the town of Delta, Utah. The site was chosen for many of the same reasons as Auger's Argentine location:
- Dark skies: Far from major light pollution sources
- Clear atmosphere: High desert with low humidity and minimal aerosols
- Flat terrain: The ancient Lake Bonneville bed provides ideal detector placement
- Altitude: ~1,400 m elevation, similar to Auger
- Northern Hemisphere: Complementary sky coverage to Southern observatories
TA began operations in 2008, building on the legacy of previous experiments at the site — HiRes (High Resolution Fly's Eye) and its predecessor Fly's Eye, which detected the famous "Oh-My-God" particle in 1991.
The project is primarily a collaboration between Japanese and American institutions, with the University of Tokyo and University of Utah as lead institutions.
The Surface Detector Array
Unlike Auger's water tanks, Telescope Array uses plastic scintillator detectors. Each surface detector (SD) station consists of:
- Two layers of 3 m² plastic scintillator, 1.2 cm thick
- Wavelength-shifting fibers to collect scintillation light
- Photomultiplier tubes at fiber ends
- Steel housing for protection
- Solar panel and battery for power
- Wireless communication system
- GPS for timing
The 507 detectors are arranged on a 1.2 km square grid covering approximately 700 km² — about one-quarter the size of Auger's array.
📊 Telescope Array Surface Detector
- Detectors: 507
- Spacing: 1.2 km (square grid)
- Total area: ~700 km²
- Scintillator area: 3 m² per station
- Detection technique: Plastic scintillator
- Duty cycle: >95%
Scintillator vs. Water Cherenkov
The choice of scintillator over water Cherenkov involves trade-offs:
Advantages of scintillator:
- Faster timing response (~nanoseconds)
- Simpler to deploy and maintain
- Lower cost per station
- No water to freeze or evaporate
Advantages of water Cherenkov:
- Larger sensitive volume
- Better muon/electromagnetic separation
- More uniform response to inclined showers
The different technologies have led to interesting comparisons — and initially some tension — between Auger and TA results, particularly regarding the energy spectrum.
Fluorescence Detectors
Three fluorescence detector (FD) stations surround the surface array at Black Rock Mesa, Long Ridge, and Middle Drum. Each station houses multiple telescopes viewing the atmosphere above the SD array.
Telescope Design
The fluorescence telescopes use a design evolved from the HiRes experiment:
- Mirror: Segmented spherical mirrors of ~5.2 m² total area
- Camera: 256 hexagonal PMTs per telescope
- Field of view: 18° × 16° per telescope
- Wavelength: UV filter selecting 300-400 nm
Black Rock Mesa and Long Ridge each have 12 telescopes; Middle Drum has 14 (repurposed from HiRes). Together they provide stereo coverage over most of the SD array.
🔭 Telescope Array Fluorescence Detectors
- Sites: 3 (Black Rock Mesa, Long Ridge, Middle Drum)
- Telescopes: 38 total
- PMTs per telescope: 256
- Duty cycle: ~10% (clear, moonless nights)
- Stereo capability: Yes
TALE: Extending to Lower Energies
The Telescope Array Low Energy extension (TALE) adds sensitivity down to ~3 × 10¹⁶ eV — bridging the gap between direct cosmic ray measurements and the ultra-high-energy regime.
TALE includes:
- 10 additional fluorescence telescopes viewing 31°-59° elevation
- A denser array of 103 scintillator detectors with 400 m spacing
This extension allows TA to study the transition region where the cosmic ray composition shifts and the "second knee" appears in the spectrum.
TAx4: Quadrupling the Size
The TAx4 upgrade, underway since 2019, will expand the surface array to nearly four times its original size:
- New detectors: ~500 additional SD stations
- New coverage: ~2,500 km² total
- New FD sites: 2 additional stations
- Timeline: Full operation expected by 2026
The expanded array will dramatically improve statistics at the highest energies and enable detailed studies of the "hotspot" — a suggestive excess of events from a particular direction in the northern sky.
Major Discoveries
The Hotspot (2014)
In 2014, Telescope Array reported an intriguing concentration of cosmic rays above 57 EeV from a 20° radius region centered near the constellation Ursa Major. About 19 events clustered in this "hotspot" — more than expected from random chance at the ~3.4σ level.
The hotspot direction doesn't obviously correspond to any single powerful source, but lies in a region containing several potential candidates, including the Ursa Major cluster of galaxies.
Whether the hotspot is a real anisotropy signal or a statistical fluctuation remains debated. TAx4's expanded statistics should resolve the question.
Spectrum Measurements
TA's energy spectrum measurements generally agree with Auger's but showed initial differences in the exact position of the GZK suppression. Joint working groups determined that most differences arose from different energy calibrations and systematic uncertainties.
When analyses use consistent assumptions, the two experiments agree within uncertainties — a reassuring cross-check for the field.
The Amaterasu Particle (2021)
On May 27, 2021, at 00:56:57 UTC, the Telescope Array detected a cosmic ray with energy 2.4 × 10²⁰ eV — the second-highest ever recorded, after the 1991 Oh-My-God particle detected by the predecessor Fly's Eye experiment at the same Utah site.
The event triggered 23 surface detectors across a 48 km² footprint. Named "Amaterasu" after the Japanese sun goddess, the particle's arrival direction pointed toward the Local Void — a region of space notably devoid of galaxies.
This presents a puzzle: where could such an energetic particle have originated if there are no obvious sources in that direction? Possibilities include extreme magnetic deflection, unknown sources within the void, or new physics.
Composition Studies
Like Auger, Telescope Array measures composition through the depth of shower maximum (Xmax). The results have been somewhat different:
- TA data tends to favor a lighter composition (more proton-like) than Auger
- The difference is most pronounced at the highest energies
- Both experiments have significant systematic uncertainties
Understanding this "composition discrepancy" is an active area of research. Possibilities include:
- Different sky regions having different source populations
- Systematic differences in reconstruction or hadronic models
- Statistical fluctuations in limited samples
Joint Auger-TA working groups continue to investigate.
The Utah Heritage
Telescope Array continues a tradition of cosmic ray physics in Utah spanning decades:
Fly's Eye (1981-1992): Two stations with 880 total PMTs detected the Oh-My-God particle in 1991 and pioneered the fluorescence technique.
HiRes (1997-2006): High Resolution Fly's Eye improved on Fly's Eye with 42 telescopes and detected the GZK suppression (simultaneously with Auger).
Telescope Array (2008-present): Combined fluorescence and surface detection, Japan-US collaboration.
The site's infrastructure, expertise, and dark skies make it a natural home for continued cosmic ray research.
Comparison with Pierre Auger
📊 Auger vs. Telescope Array
| Property | Pierre Auger | Telescope Array |
|---|---|---|
| Location | Argentina (35°S) | Utah (39°N) |
| SD Area | 3,000 km² | 700 km² (→2,500) |
| SD Technique | Water Cherenkov | Plastic scintillator |
| SD Count | 1,660 | 507 (→~1,000) |
| FD Sites | 4 + HEAT | 3 + TALE |
| Start | 2004 | 2008 |
| Sky Coverage | Southern sky | Northern sky |
Together, Auger and TA provide nearly full-sky coverage for UHECRs. Their different techniques and locations make them complementary — agreements strengthen confidence, while differences reveal systematic effects or genuine sky variations.
Future Outlook
With TAx4 expansion and continued operations, Telescope Array will remain the premier Northern Hemisphere UHECR observatory for the foreseeable future. Key goals include:
- Resolving whether the hotspot is a real anisotropy
- Detecting more extreme-energy events like Amaterasu
- Improving composition measurements at highest energies
- Joint analyses with Auger for full-sky studies
The Utah desert, where the first >10²⁰ eV particle was detected in 1991, continues to be humanity's window on the universe's most extreme accelerators.