The Pierre Auger Observatory has published a landmark measurement of the ultra-high-energy cosmic ray energy spectrum across the full range of declinations accessible to the Southern Hemisphere array: from −90° to +44.8°. With 310,000 events accumulated over an exposure of 104,900 km² sr yr — the largest such dataset ever assembled — the Auger Collaboration has answered two long-standing questions at once.
Finding 1: The Spectrum Is the Same Everywhere
Auger divided its sky coverage into multiple declination bands and measured the energy spectrum independently in each. The result: no significant variation. Whether you look north or south within the Auger field of view, cosmic rays with energies above 2.5 EeV follow the same spectral shape.
This was tested both with and without corrections for the known large-scale dipole anisotropy in arrival directions — the established 6.8σ excess of events pointing roughly toward the direction of motion of our galaxy through the cosmic web. Even accounting for that directional asymmetry, the spectrum itself does not vary.
What this rules out: Any model in which the spectral features (particularly the instep — see below) originate from a small number of distinctive nearby sources. If the spectrum varied with declination, it would point toward a handful of sources concentrated in one part of the sky. It does not.
The authors conclude that the quasi-uniformity of the spectrum across declinations is consistent with the instep and the suppression arising from a population of sources distributed broadly across the sky, with a narrow range of maximum rigidity — meaning sources of similar type contributing roughly equally from many directions.
Finding 2: The Instep Is Now a Confirmed Discovery
The cosmic ray energy spectrum is not a smooth power law. It has named features — the knee at ~3 PeV, the ankle at ~7 EeV, and a softer hardening called the instep at approximately 10 EeV, first identified by Auger in earlier measurements. Until now, the instep had been reported but not established at discovery significance.
This paper changes that. By drawing mock samples from a reference model without the instep and comparing them to the data, the Auger Collaboration establishes the instep at greater than 5σ significance — the conventional threshold for a discovery in physics. The instep is real.
The prevailing interpretation is compositional: the instep reflects a transition in the mass composition of cosmic rays, specifically the transition from a helium-dominated component to a heavier carbon-nitrogen-oxygen group. This is consistent with the narrow maximum-rigidity distribution inferred from the composition measurements above 10 EeV. The instep, under this interpretation, is a consequence of different nuclear species reaching their acceleration ceiling at different absolute energies.
What This Means for UHECR Source Models
The combined result — isotropic spectrum, composition-driven instep — paints a consistent picture: UHECRs at these energies come from a population of sources distributed similarly to the large-scale structure of the universe, with no single dominant nearby contributor. The sources are many, broadly distributed, and of similar type.
This is a significant constraint. Models that rely on a small number of powerful nearby sources — certain gamma-ray burst models, some active galactic nucleus models — are disfavored by the lack of declination dependence. A distributed population of similar sources is required.
Binary black hole mergers, distributed by the cosmic merger rate across the observable universe, constitute exactly such a population — broadly isotropic, statistically similar in character, contributing from all sky directions.
About the Measurement
The analysis combines two detection techniques available at the Auger Observatory. The standard surface detector array (SD-1500, with 1500 m station spacing) detects air showers from cosmic rays arriving at zenith angles below 60°. The inclined array configuration extends coverage to zenith angles between 60° and 80°, reaching the northern edge of the Auger field of view. By combining both, the Collaboration achieves continuous coverage from the south celestial pole to +44.8° declination for the first time.
The paper has been accepted in Physical Review Letters as an Editor's Suggestion — a designation given to fewer than 15% of published papers, indicating exceptional interest to the broader physics community.
Consistency with the STF Framework
The Selective Transient Field (STF) framework predicts that ultra-high-energy cosmic rays are produced during the inspiral phase of binary black hole mergers, when the rate of spacetime curvature change exceeds a critical threshold. Binary mergers are distributed across the universe in proportion to the cosmic star formation rate — a population that is statistically isotropic on large scales, with no concentration toward any particular sky direction.
The Auger finding that the UHECR energy spectrum shows no dependence on declination is consistent with this prediction. A source population tied to the binary merger rate would produce exactly this kind of uniform spectrum across the sky. The result does not confirm STF, but it is consistent with the source distribution the framework requires.
For more on the STF framework and the pre-merger UHECR correlation, see the Observational Manuscript and the STF Theory paper.
The Energy Spectrum of Ultra-High Energy Cosmic Rays across Declinations −90° to +44.8° as measured at the Pierre Auger Observatory
Phys. Rev. Lett. 135, 241002 (2025) — Editor's Suggestion
arXiv:2506.11688