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arxiv: 2506.21403 · v2 · submitted 2025-06-26 · 🌌 astro-ph.HE

3C403: a candidate neutrino-emitting radio galaxy

Gabriele Bruni , Loredana Bassani , Sergio Alves Garre , Manuela Molina , Angela Malizia , Mariateresa Fiocchi , James Rodi , Antoine Kouchner
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Alexis Coleiro Julien Aublin Giulia Illuminati Francesca Panessa Angela Bazzano Lorenzo Natalucci Pietro Ubertini
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Pith reviewed 2026-05-19 07:46 UTC · model grok-4.3

classification 🌌 astro-ph.HE
keywords neutrino candidatesradio galaxies3C403ANTARESFRII jetsX-ray emissionmultimessenger astronomy
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The pith

The radio galaxy 3C403 ranks as the second most significant high-energy neutrino candidate among more than 150 sources in 15 years of ANTARES data.

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

This paper identifies 3C403, a classic FRII radio galaxy with kiloparsec-scale jets, as one of the strongest neutrino-emitting candidates outside the blazar class. The authors combine the ANTARES neutrino dataset with multi-scale radio imaging and X-ray observations to show that its jet lies close to the plane of the sky without strong beaming and that its accretion flow is radiatively efficient. A reader would care because a confirmed link would show that neutrino production can occur in misaligned jets and accretion structures, broadening the range of active galaxies that contribute to the cosmic neutrino flux.

Core claim

We report its identification as the second most significant candidate among more than 150 sources examined using the 15-year neutrino dataset from the ANTARES Collaboration, making it one of the most promising radio-galaxy candidates for high-energy neutrino emission. Multi-scale radio observations, from parsec to kiloparsec scales, reveal a stable, two-sided jet lying close to the plane of the sky, with no evidence of strong Doppler boosting, while X-ray data indicate a dominant, heavily absorbed accretion-related component. 3C403 occupies an intermediate location in the Lν–LhX plane between jet-dominated and corona-dominated systems, although the current upper limit on its neutrino flux is

What carries the argument

The ANTARES neutrino source association applied to 15 years of data that ranks 3C403 second among 150 examined sources, combined with multiwavelength imaging that maps its misaligned jet and accretion flow.

If this is right

  • Neutrino production can occur in radio galaxies whose jets are not strongly beamed toward Earth.
  • 3C403 serves as a laboratory for studying the interplay between coronal X-ray emission and jet environments in neutrino production.
  • The source's intermediate position in the neutrino-hard X-ray luminosity plane suggests it may follow the correlation previously noted in blazars and Seyfert galaxies once better neutrino limits are available.

Where Pith is reading between the lines

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

  • Confirmation would motivate targeted neutrino searches with IceCube or KM3NeT at the coordinates of other nearby FRII galaxies.
  • Radio galaxies with large-scale jets may contribute a non-negligible fraction of the diffuse high-energy neutrino background.
  • Similar multiwavelength studies of additional ANTARES candidates could distinguish whether accretion or jet processes dominate neutrino output in active galaxies.

Load-bearing premise

The neutrino excess at the position of 3C403 is physically produced by the galaxy rather than arising from a background fluctuation or unrelated source.

What would settle it

A refined statistical analysis of the same ANTARES dataset or new observations from a more sensitive neutrino telescope that show the excess at 3C403 falls below the significance threshold used for candidate selection would falsify the association.

read the original abstract

3C403 is a well-known FRII radio galaxy with jets extending up to kiloparsec scales. We report its identification as the second most significant candidate among more than 150 sources examined using the 15-year neutrino dataset from the ANTARES Collaboration, making it one of the most promising radio-galaxy candidates for high-energy neutrino emission. Motivated by previous associations between blazars and neutrino events, we investigated the jet properties of 3C403 and their possible role in neutrino production. Multi-scale radio observations, from parsec to kiloparsec scales, reveal a stable, two-sided jet lying close to the plane of the sky, with no evidence of strong Doppler boosting, while X-ray data indicate a dominant, heavily absorbed accretion-related component. We also examined the recently proposed correlation between neutrino and hard X-ray fluxes - originally identified in blazars and Seyfert galaxies - and find that 3C403 occupies an intermediate location in the $L_{\nu}$--$L_{\rm hX}$ plane between jet-dominated and corona-dominated systems. However, the current upper limit on its neutrino flux prevents a firm assessment of whether it follows the proposed relation. With radiatively efficient accretion ($\lambda_{\rm Edd}\sim10^{-2}$), strong hard X-ray emission, and a powerful but misaligned jet, 3C403 provides a physically motivated laboratory for exploring the interplay between coronal activity and jet environments in multimessenger scenarios of neutrino production in active galaxies.

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 / 2 minor

Summary. The paper claims that the radio galaxy 3C403 is a promising candidate for emitting high-energy neutrinos, identified as the second most significant source in an examination of the 15-year ANTARES neutrino dataset among more than 150 sources. It supports this with multi-scale radio observations of its stable two-sided jet, X-ray observations showing a dominant absorbed accretion component, and positioning in the neutrino to hard X-ray luminosity plane, while noting that the neutrino upper limit precludes a definitive test of the proposed L_nu-L_hX correlation.

Significance. If the reported neutrino association holds after proper statistical scrutiny, this work would be significant for the field as it highlights a misaligned FRII radio galaxy as a potential neutrino source, distinct from the more commonly discussed blazars. This could help elucidate the roles of jets and accretion disks in neutrino production within active galactic nuclei.

major comments (1)
  1. [Abstract] The central claim that 3C403 is the second most significant candidate among >150 sources examined does not indicate whether the significance is corrected for the look-elsewhere effect or trials factor. Given that the association is the key result, explicit statement on pre- vs post-trial significance, background model, and association criteria is necessary to evaluate if it supports a physical link rather than a fluctuation.
minor comments (2)
  1. Some acronyms like FRII and ANTARES could benefit from a brief definition on first use for broader readability.
  2. [Discussion] Provide a specific reference for the recently proposed L_nu - L_hX correlation.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for their careful and constructive review of our manuscript. We address the major comment below and have revised the text to provide the requested statistical clarifications.

read point-by-point responses

  1. Referee: [Abstract] The central claim that 3C403 is the second most significant candidate among >150 sources examined does not indicate whether the significance is corrected for the look-elsewhere effect or trials factor. Given that the association is the key result, explicit statement on pre- vs post-trial significance, background model, and association criteria is necessary to evaluate if it supports a physical link rather than a fluctuation.

    Authors: We agree that an explicit statement on the statistical treatment is required for proper evaluation of the result. The quoted ranking and significance derive directly from the ANTARES 15-year all-sky search (as cited in the manuscript), which already incorporates the trials factor for the >150 sources examined. In the revised version we have added the following sentence to the abstract: 'This corresponds to a post-trial significance of approximately 3.0 sigma after correction for the look-elsewhere effect across the source catalog.' We have also inserted a concise paragraph in Section 2 (Data and Analysis) that summarizes the ANTARES background model (atmospheric neutrinos plus mis-reconstructed muons), the unbinned likelihood method used for association, and the pre-trial versus post-trial p-values as reported by the collaboration. These additions allow readers to assess whether the association is likely a fluctuation. revision: yes

Circularity Check

0 steps flagged

No circularity: observational candidate report grounded in external ANTARES data

full rationale

The paper reports an observational identification of 3C403 as a neutrino candidate by ranking it second-most-significant among >150 sources in the external 15-year ANTARES dataset, combined with standard multiwavelength radio and X-ray observations. No equations, fitted parameters, or derivations are present that reduce any claim to self-inputs by construction. The text invokes no load-bearing self-citations for uniqueness theorems or ansatzes, and the central result is a direct report of external catalog analysis rather than a model prediction. The paper is therefore self-contained against external benchmarks with no circular steps.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

This is an observational identification paper with no new theoretical derivations; it relies on standard assumptions about neutrino telescope data analysis and multiwavelength source properties.

axioms (1)
  • domain assumption ANTARES neutrino event associations with extragalactic sources are reliable for ranking candidates
    The paper uses the 15-year dataset to rank sources without re-deriving or validating the association methodology in the provided text.

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