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arxiv: 2606.27270 · v1 · pith:UNPY7EZFnew · submitted 2026-06-25 · 🌌 astro-ph.HE

Origins of Cosmic Rays in the Galactic-extragalactic Transition Energy Range

A. Corstanje , S. Saha , S. Bouma , J. Bray , S. Buitink , V. de Henau , E. Dickinson , B. Hare
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A. Haungs H. He J. H\"orandel T. Huege C. James P. Laub X. Li H-J. Mathes K. Mulrey A. Nelles F. Schl\"uter O. Scholten R. Spencer C. Sterpka K. Terveer S. Thoudam G. Trinh P. Turekova D. Veberic K. Watanabe C. Zhang P. Zhang Y. Zhang
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classification 🌌 astro-ph.HE
keywords cosmic raysmass compositiongalactic-extragalactic transitionSKA-Lowradio detectionair showersparticle detectors
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The pith

SKA-Low augmented with particle detectors can measure cosmic ray mass composition between 10^16 and 10^18 eV to identify the galactic-extragalactic transition.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

Cosmic rays in the 10^16 to 10^18 eV range mark the point where galactic sources reach their highest energies before extragalactic sources dominate. Mass composition measurements distinguish source abundances and set limits on maximum acceleration energy, which scales with particle charge. A decade of LOFAR radio observations of air showers has validated techniques for extracting mass-sensitive parameters at these energies. The paper states that SKA-Low, combined with an array of small particle detectors for hybrid data, is positioned to extend these measurements and supply the needed composition data across the transition. This data would enable direct tests of galactic source and propagation models.

Core claim

The paper claims that SKA-Low, augmented with an array of small particle detectors, is well suited to measure the mass composition of cosmic rays in the 10^16 to 10^18 eV range, building on LOFAR experience, thereby providing essential information for comparing to source and propagation models from both source abundances and the charge-dependent maximum energy.

What carries the argument

Hybrid radio detection of cosmic-ray air showers at SKA-Low frequencies combined with ground particle detectors to extract mass-sensitive observables such as shower depth and muon content.

If this is right

  • Tighter bounds on the maximum energy galactic accelerators can impart to particles of given charge.
  • Clearer separation of galactic and extragalactic flux contributions around the transition energy.
  • Direct constraints on the elemental abundances at galactic sources from measured composition.
  • Improved input for propagation models that predict how composition evolves with energy.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • The same hybrid setup could test whether specific candidate galactic sources, such as young supernova remnants, match the observed composition cutoff.
  • Extending the energy reach slightly higher might reveal the onset of the extragalactic component without relying solely on spectral features.
  • Calibration against LOFAR data could serve as a benchmark for applying similar methods at even higher energies with future arrays.

Load-bearing premise

Radio detection techniques validated at LOFAR will scale directly to SKA-Low without introducing major new systematic limitations in the 10^16-10^18 eV range.

What would settle it

SKA-Low hybrid data yielding mass composition results that deviate systematically from independent measurements by other observatories in any overlapping energy bins.

Figures

Figures reproduced from arXiv: 2606.27270 by A. Corstanje, A. Haungs, A. Nelles, B. Hare, C. James, C. Sterpka, C. Zhang, D. Veberic, E. Dickinson, F. Schl\"uter, G. Trinh, H. He, H-J. Mathes, J. Bray, J. H\"orandel, K. Mulrey, K. Terveer, K. Watanabe, O. Scholten, P. Laub, P. Turekova, P. Zhang, R. Spencer, S. Bouma, S. Buitink, S. Saha, S. Thoudam, T. Huege, V. de Henau, X. Li, Y. Zhang.

Figure 1
Figure 1. Figure 1: The cosmic-ray energy spectrum. While low-energy cosmic rays are abundant, the flux drops steeply with increasing energy, at a power law of roughly 𝐸 −3 up to smaller features. At the highest energies, very large detector arrays are needed to collect significant data. 1 Introduction Cosmic rays arrive on Earth in an energy range spanning about 11 decades, from 109 to over 1020 eV. Therefore, at the high en… view at source ↗
Figure 2
Figure 2. Figure 2: The mass composition fractions for two different scenarios of Galactic cosmic ray production at the highest energies attainable. They are labeled GW for Galactic wind, and WR for Wolf-Rayet supernovae. A notable difference in hydrogen/helium ratios is visible. The flux cuts off around 109 GeV = 1018 eV, hence the abundance fractions are only reliable up to this energy. Beyond this, mainly extragalactic pro… view at source ↗
Figure 3
Figure 3. Figure 3: Left: a schematic picture showing how, to lowest order, the radio footprint becomes smaller for air showers that reach a maximum 𝑋max closer to the ground (higher values), allowing to infer 𝑋max from data. Right: probability densities of 𝑋max for four selected primary elements; nitrogen is usually chosen as proxy for C/N/O. lenges, such as the need for dedicated hardware that is able to reject millions of … view at source ↗
Figure 4
Figure 4. Figure 4: The antenna layout of the SKA inner core at the AA4 stage, with an example distribution of 100 particle detector boxes of 1 m2 (points not to scale). The final locations are to be optimized to on-site logistics. The earlier development milestone denoted AA* comprises about 90 % of this array. Figure taken from Corstanje et al. (2025). is read out and the data is processed offline. Reading out the buffer is… view at source ↗
Figure 5
Figure 5. Figure 5: A voltage trace at one of the antennas in [PITH_FULL_IMAGE:figures/full_fig_p009_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: An example of a cosmic-ray radio footprint from a 1017 eV proton. The color-code shows the energy fluence in the pulses at each antenna. A detection threshold was set at 5 sigma. Figure taken from Corstanje et al. (2025). 9 [PITH_FULL_IMAGE:figures/full_fig_p009_6.png] view at source ↗
Figure 7
Figure 7. Figure 7: An example of an 𝑋max reconstruction. Fitting each of the showers in the ensemble gives a reduced 𝜒 2 value, which follows a parabolic curve near its minimum value. The minimum of a parabola fitted to the lower envelope of points is taken as the 𝑋max estimate. The right panel shows a close-up. Figure taken from Corstanje et al. (2025). fields at the antenna locations. Applying the antenna model to the elec… view at source ↗
Figure 8
Figure 8. Figure 8: The precision in reconstructing 𝑋max, versus primary energy. Left panel: using single antennas. Right panel: using beamforming in groups of 4, 16, or 64 antennas, with higher numbers towards lower primary energy. This considerably lowers the reliable detection threshold. Figure taken from Corstanje et al. (2025). tight systematic uncertainty budget is required, as well as a sufficient number of measured sh… view at source ↗
Figure 9
Figure 9. Figure 9: Left: The expected number of measured air showers in bins of width 0.1 in log-energy, in one net observing year. Technical limitations are not yet known, but are expected around an order of magnitude indicated by the area shaded in red. Right: example uncertainties on the mass composition fractions in a mock dataset of 1000 showers, assuming the same systematic uncertainties as for the LOFAR analysis. Figu… view at source ↗
Figure 10
Figure 10. Figure 10: An example of a typical longitudinal distribution curve. Left: the effect of varying parameter 𝐿. Right: varying parameter 𝑅. 4 Towards measuring the full air shower evolution As shown above, the shower maximum 𝑋max is at present the most important observable used to estimate the mass composition of cosmic rays. But the full longitudinal shower evolution, of which 𝑋max is the maximum, contains more inform… view at source ↗
Figure 11
Figure 11. Figure 11: Measuring the sensitive parameter 𝑆 combining curve parameters 𝐿 and 𝑅 from Eq.6 using the same fitting procedure as for 𝑋max. A clearly detectable optimum near the true value is found, whereas the small variations in reduced chi-squared values indicate that a considerable number of antennas is needed to measure this accurately. Color-coding of the data points in the right panel is proportional to 𝐿, and … view at source ↗
Figure 12
Figure 12. Figure 12: The mean values of 𝐿 and 𝑋max for four different elements, at primary energies 1016 , 1017, and 1018 eV, and for three hadronic interaction models. Notably, hydrogen (noted as p, for protons) stands out from the other elements, which is helpful for mass composition analysis. Figure taken from Buitink et al. (2023). The mean values depend on energy and on the hadronic interaction model, which may put limit… view at source ↗
Figure 13
Figure 13. Figure 13: Histograms of 𝐿 for various elements, with raw counts on the vertical axis; the tail end becomes longer towards lighter elements, up to helium. Again, hydrogen stands out with somewhat shorter tails than hydrogen, which is another piece of information that helps distinguishing hydrogen from helium in a mass composition analysis. Figure taken from Buitink et al. (2023). methods utilizing information field … view at source ↗
Figure 14
Figure 14. Figure 14: An example of an interferometric reconstruction of a simulated SKA event. Left: The beam￾formed signal at varying atmospheric depths along the shower axis. Right: The peak fo the beamformed signal at different atmospheric depths along the shower axis. Antennas that are illuminated in the radio footprint see emission from the entirety of the air shower. On the Cherenkov cone, emission from all parts of the… view at source ↗
Figure 15
Figure 15. Figure 15: Interferometric reconstruction of air showers at SKA-Low. Left: simulated shower core positions within the SKA-Low core. Right: Interferometric reconstruction of 𝑋RIT, correlating with 𝑋max. of antennas that can be included in the interferometric reconstruction will allow us to enhance very small signals, thereby reconstructing low energy air showers (see the discussion of PeV gamma-ray event reconstructi… view at source ↗
read the original abstract

Cosmic rays arrive at Earth with energies ranging from $10^9$ to over $10^{20}$ eV. One of the open questions in high-energy cosmic ray science concerns the origin of the highest-energy cosmic rays that can be accelerated by Galactic sources, and the transition energy beyond which only extragalactic sources can provide. Measuring the mass composition gives essential information for comparing measurements to source and propagation models, both from the abundances at the source and from the maximum attainable energy which is proportional to the particle charge (and hence its mass). The highest-energy cosmic rays from the Galaxy are found in a range of $10^{16}$ to $10^{18}$ eV which is well suited for radio detection. Building on a decade of experience in measuring cosmic rays at LOFAR, we show that SKA-Low, augmented with an array of small particle detectors, is well suited to advance the field by measuring the mass composition of cosmic rays across this energy range.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

1 major / 0 minor

Summary. The manuscript proposes that SKA-Low, augmented with an array of small particle detectors and building on a decade of LOFAR experience in radio detection of cosmic rays, is well suited to measure the mass composition of cosmic rays in the 10^{16} to 10^{18} eV range. This range is identified as critical for understanding the transition from Galactic to extragalactic cosmic ray sources, since mass composition constrains source abundances and maximum acceleration energies (proportional to charge).

Significance. If the suitability claim holds and the proposed hybrid measurements can be realized with the required precision, the work would advance high-energy cosmic ray astrophysics by supplying mass composition data in an energy window where existing instruments have limited reach, enabling tighter tests of source and propagation models.

major comments (1)
  1. [Abstract] Abstract: The assertion that LOFAR-validated radio techniques will transfer directly to SKA-Low (with particle-detector augmentation) to deliver accurate mass composition measurements rests on an unquantified assumption. No error budget, simulation results, or scaling tests are presented to bound potential new systematics arising from differences in frequency coverage, antenna response, baseline density, or hybrid trigger/reconstruction methods. This transferability is the load-bearing step for the central suitability claim.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for their constructive review and the recommendation for major revision. We address the single major comment below, focusing on the scope and limitations of the current manuscript as a conceptual proposal.

read point-by-point responses

  1. Referee: [Abstract] Abstract: The assertion that LOFAR-validated radio techniques will transfer directly to SKA-Low (with particle-detector augmentation) to deliver accurate mass composition measurements rests on an unquantified assumption. No error budget, simulation results, or scaling tests are presented to bound potential new systematics arising from differences in frequency coverage, antenna response, baseline density, or hybrid trigger/reconstruction methods. This transferability is the load-bearing step for the central suitability claim.

    Authors: We agree that the manuscript does not include new quantitative error budgets, dedicated simulations, or explicit scaling tests for SKA-Low-specific parameters. The central claim is framed as a proposal that builds directly on validated LOFAR techniques rather than asserting identical performance. In revision we will expand the discussion (likely in a new subsection) to qualitatively address the listed differences, citing existing LOFAR publications on frequency response, baseline effects, and hybrid reconstruction. We will explicitly note that a full error budget requires instrument-specific simulations that lie outside the present scope. This addition will clarify the evidential basis without changing the proposal nature of the work. revision: partial

Circularity Check

0 steps flagged

No circularity; proposal paper with no derivations or self-referential predictions

full rationale

The manuscript is a forward-looking proposal paper. Its central claim is that SKA-Low (augmented by particle detectors) is well suited to measure cosmic-ray mass composition in the 10^16–10^18 eV range, building on LOFAR experience. No equations, fits, predictions, or derivation chains appear in the provided abstract or framing. The text contains no self-definitional steps, fitted inputs renamed as predictions, load-bearing self-citations, uniqueness theorems, or ansatzes. The transferability assertion is presented as an assumption grounded in cited prior instrument work rather than a mathematical reduction to the paper's own inputs. This is the most common honest non-finding for proposal-style documents; the derivation chain is empty by construction.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The central claim rests on the domain assumption that radio detection of air showers yields reliable mass composition, transferred from LOFAR to SKA-Low without new validation shown here.

axioms (1)
  • domain assumption Radio detection of extensive air showers can determine primary cosmic ray mass composition
    Invoked via reference to a decade of LOFAR experience; no independent derivation or new evidence supplied in the abstract.

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