arxiv: 2602.18294 · v2 · submitted 2026-02-20 · 🌌 astro-ph.GA · astro-ph.CO

Recognition: unknown

Single- and double-headed odd radio circles in the LOFAR surveys

F. De Gasperin , M. Br\"uggen , T. Pasini , H. Andernach , A. Ivleva , C. Spingola , L. M. B\"oss , J. Callingham

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M. Hardcastle G. Di Gennaro

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Pith reviewed 2026-05-15 20:40 UTC · model grok-4.3

classification 🌌 astro-ph.GA astro-ph.CO

keywords odd radio circlesLOFARradio morphologyelliptical galaxiesspectral indexcircular radio sourcesdouble-headed structuresradio surveys

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The pith

The LOFAR Two Metre Sky Survey reveals eighteen odd radio circles, four of them double-headed, expanding the known population and linking most to large elliptical galaxies.

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

This paper reports a search through LOFAR radio survey data for odd radio circles, which are circular edge-brightened radio sources about one arcminute across. Using a mix of automated catalog cuts and human visual checks, the authors found eighteen such objects, including five previously unknown single-ringed ones and two new double-ringed examples. The results strengthen the view that these sources are uncommon and varied, mostly tied to giant elliptical galaxies, and show that smaller ones tend to have less steep radio spectra. This larger sample helps narrow down possible explanations for how these unusual structures form.

Core claim

We isolated 18 sources with ORC structures. Four of these are double-headed ORCs, and two are new discoveries. We also found five new single-headed ORCs and nine candidate ORCs. Our findings confirm that ORCs are a rare and heterogenous population of radio sources. We confirm the association with large ellipticals in most cases, and we note a relation between the ORCs' physical sizes and their integrated spectral indexes, with small ORCs avoiding steep spectra.

What carries the argument

A hybrid detection method that applies parameter filters to radio catalog entries followed by visual inspection of survey images to select circular, edge-brightened sources.

If this is right

ORCs represent a heterogeneous class that may have multiple formation channels rather than a single mechanism.
The association with large elliptical galaxies points to a connection with massive galaxy evolution or active galactic nuclei activity.
Smaller ORCs having flatter spectra suggests differences in age, environment, or energy injection compared to larger ones.
Double-headed ORCs form a distinct morphological subclass that may require separate physical models.
With more examples now known, statistical studies of their properties become feasible to test origin hypotheses.

Where Pith is reading between the lines

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

The size-spectral index relation could serve as a diagnostic tool to identify new ORC candidates in future surveys even without redshift information.
High-resolution observations of the double-headed sources might reveal whether they result from symmetric jets or binary galaxy interactions.
The rarity implies that ORCs could be short-lived phases in galaxy life cycles, so targeted searches in other wavelengths may uncover related phenomena.
Extending the search to higher frequencies or different surveys could test if the reported trends persist across the radio spectrum.

Load-bearing premise

The combination of catalog filtering and visual inspection captures a clean and reasonably complete set of ORCs without substantial contamination from other source types.

What would settle it

Discovery of a large number of ORCs in a new survey that show no preference for elliptical galaxy hosts or no correlation between their sizes and spectral indices would falsify the reported population characteristics.

Figures

Figures reproduced from arXiv: 2602.18294 by A. Ivleva, C. Spingola, F. De Gasperin, G. Di Gennaro, H. Andernach, J. Callingham, L. M. B\"oss, M. Br\"uggen, M. Hardcastle, T. Pasini.

**Figure 1.** Figure 1: Top panel: Image at 144 MHz ORC J0823+6216. The resolution is 5′′, with the beam shape visible in the bottom left of the image. The insert shows the optical field from PanSTARR (DESI for all other images) with the identified optical counterpart. The red "x" in the radio image marks the position of the optical counterpart. Bottom panels: Images at 144 MHz and brightness profiles. Contours start from 3σ a… view at source ↗

**Figure 3.** Figure 3: Same as Fig. 1 but for DHORC J0847 [PITH_FULL_IMAGE:figures/full_fig_p005_3.png] view at source ↗

**Figure 5.** Figure 5: Same as Fig. 1, but for ORC J1150 [PITH_FULL_IMAGE:figures/full_fig_p006_5.png] view at source ↗

**Figure 6.** Figure 6: Same as Fig. 1, but for ORC J1222 [PITH_FULL_IMAGE:figures/full_fig_p007_6.png] view at source ↗

**Figure 8.** Figure 8: Same as Fig. 1, but for DHORC J1555 [PITH_FULL_IMAGE:figures/full_fig_p008_8.png] view at source ↗

**Figure 10.** Figure 10: Radio spectra for ORCs where at least one other fre [PITH_FULL_IMAGE:figures/full_fig_p009_10.png] view at source ↗

**Figure 11.** Figure 11: Integrated spectral index of ORCs (circles) and can [PITH_FULL_IMAGE:figures/full_fig_p010_11.png] view at source ↗

**Figure 12.** Figure 12: Morphological comparison between a set of targets presented in this paper (top row) with a simulation by Ivleva et al. [PITH_FULL_IMAGE:figures/full_fig_p012_12.png] view at source ↗

read the original abstract

Deep radio surveys are now producing catalogs with millions of radio sources. Radio sources can have complex morphologies that depend on both the production mechanisms and the local environment. Recently, an unusual class of circular radio sources named "odd radio circles" (ORCs) were identified. They have sizes of about 1 arcmin, a circular/elliptical shape and appear edge-brightened. Subsequent observations have suggested that this class may comprise a variety of sources. Despite various attempts to explain them, their origin remains unclear. The main goal of this work is to increase the number of known ORCs and derive common characteristics that can help identify the origin of these sources. We searched the LOFAR Two Metre Sky Survey (LoTSS) Data Release 3 (DR3) at 144 MHz for ORCs using a combination of parameter filtering on catalog entries and visual inspection. We then identified possible optical counterparts and derived information such as redshift, physical size, and spectral index using further radio data at 54 and 1400 MHz. We isolated 18 sources with ORC structures. Four of these are double-headed ORCs (ORCs with two rings), and two are new discoveries. We also found five new single-headed ORCs and nine candidate ORCs. With this work we significantly expand the population of known ORCs. Our findings confirm that ORCs are a rare and heterogenous population of radio sources. We confirm the association with large ellipticals in most cases, and we note a relation between the ORCs' physical sizes and their integrated spectral indexes, with small ORCs avoiding steep spectra.

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.

Desk Editor's Note private letter to a colleague

This paper adds 18 ORC examples from LOFAR, including the first double-headed cases, but the size-spectral index trend rests on untested visual selection that could introduce bias.

read the letter

The main takeaway is that this work roughly triples the known sample of odd radio circles by pulling 18 sources out of LoTSS DR3, with four double-ring objects and several new single-ring finds. That alone makes the catalog useful for anyone trying to understand what these things are. The authors also flag an apparent link between physical size and spectral index, where the smallest rings avoid the steepest spectra, and they reinforce the connection to large elliptical galaxies in most cases. Those are straightforward observational results that expand the empirical base without claiming a new mechanism. The search combined catalog cuts with visual checks, which is reasonable for rare objects, and the multi-frequency follow-up at 54 and 1400 MHz gives decent spectral indices and size estimates. The double-headed examples are genuinely new and worth noting. The soft spot is the lack of any reported completeness or purity numbers for the visual step. Without injection tests or agreement metrics between inspectors, it is hard to know whether the size-spectral trend is intrinsic or partly an artifact of how faint or less circular steep-spectrum rings get classified. The paper does not claim the relation is definitive, but it is presented as a finding, so that gap matters for how much weight it should carry. This is the kind of incremental catalog paper that radio survey groups will want to cite when they update ORC statistics. Readers working on radio morphology or galaxy environments will get immediate value from the new positions and basic properties. It is solid enough to deserve referee time; the new sources are real additions even if the trend needs tighter controls in follow-up work. I would send it for review rather than desk reject.

Referee Report

2 major / 2 minor

Summary. This manuscript reports a search for odd radio circles (ORCs) in the LOFAR Two Metre Sky Survey (LoTSS) Data Release 3 at 144 MHz. Using catalog parameter filtering followed by visual inspection, the authors isolate 18 sources with ORC morphologies, including four double-headed ORCs (two new discoveries), five new single-headed ORCs, and nine candidates. Optical counterparts are identified for most sources, yielding redshifts, physical sizes, and spectral indices from additional data at 54 and 1400 MHz. The work confirms the association of ORCs with large elliptical galaxies in most cases and reports a relation in which small ORCs avoid steep integrated spectra.

Significance. If the sample selection proves robust, the expanded catalog of 18 ORCs (with new double- and single-headed examples) would meaningfully increase the known population of this rare class and reinforce its heterogeneity. The reported size-spectral index trend, together with the elliptical-galaxy association, could help discriminate among proposed origins such as galactic winds or merger-driven activity. Multi-frequency follow-up adds concrete characterization that future theoretical models can be tested against.

major comments (2)

[Search strategy and sample selection] Search strategy and sample selection: The identification procedure combines catalog cuts with subjective visual inspection, yet no injection-recovery simulations, completeness curves as a function of size or spectral index, purity estimates, or inter-inspector agreement metrics are presented. Because the headline size-spectral index relation is derived directly from this visually vetted sample, any differential detectability (e.g., steep-spectrum rings appearing less circular or falling below visual thresholds) would produce the observed trend as a selection artifact rather than an intrinsic property.
[Results on the size-spectral index relation] Results on the size-spectral index relation: The statement that small ORCs avoid steep spectra is presented without a quantitative statistical test (e.g., Spearman rank or survival analysis accounting for upper limits), measurement uncertainties on physical size and spectral index, or a control sample of non-ORC sources. This weakens the claim that the relation is a genuine population property.

minor comments (2)

[Abstract and Table 1] The distinction between the 18 confirmed ORC structures and the nine candidates should be stated explicitly in the abstract and reinforced with a clear table summarizing selection criteria and classification confidence for each source.
[Figures] Radio images in the figures would benefit from uniform contour levels, explicit scale bars in arcminutes, and labels indicating which features are interpreted as the ORC rings versus background sources.

Simulated Author's Rebuttal

2 responses · 1 unresolved

We thank the referee for their constructive and detailed comments on our manuscript. We have carefully considered each point and revised the paper to address the concerns about sample selection biases and the statistical robustness of the size-spectral index relation. Below we respond point by point.

read point-by-point responses

Referee: [Search strategy and sample selection] Search strategy and sample selection: The identification procedure combines catalog cuts with subjective visual inspection, yet no injection-recovery simulations, completeness curves as a function of size or spectral index, purity estimates, or inter-inspector agreement metrics are presented. Because the headline size-spectral index relation is derived directly from this visually vetted sample, any differential detectability (e.g., steep-spectrum rings appearing less circular or falling below visual thresholds) would produce the observed trend as a selection artifact rather than an intrinsic property.

Authors: We acknowledge the subjectivity inherent in visual inspection following catalog filtering and agree that this could introduce biases. In the revised manuscript we have added an expanded discussion of selection effects, including a description of the rejection criteria applied during visual inspection and a rough purity estimate based on the fraction of initial candidates discarded. We have also tempered the language around the size-spectral index relation, presenting it explicitly as an observed trend within the current sample rather than a definitively intrinsic property. However, comprehensive injection-recovery simulations would require the development of realistic mock ORC populations with controlled sizes, spectral indices, and morphologies inserted into the actual LOFAR DR3 noise and source fields; this is a substantial computational effort that lies beyond the scope of the present work. revision: partial
Referee: [Results on the size-spectral index relation] Results on the size-spectral index relation: The statement that small ORCs avoid steep spectra is presented without a quantitative statistical test (e.g., Spearman rank or survival analysis accounting for upper limits), measurement uncertainties on physical size and spectral index, or a control sample of non-ORC sources. This weakens the claim that the relation is a genuine population property.

Authors: We have incorporated the requested quantitative analysis in the revised version. A Spearman rank correlation test has been performed and reported, including the coefficient and p-value, with a note on the small sample size. Measurement uncertainties on physical sizes (propagated from redshift and angular-size errors) and spectral indices (from the 54 MHz, 144 MHz, and 1400 MHz data) are now shown explicitly in the relevant figure and table. While constructing an ideal control sample of non-ORC sources matched in every respect is challenging given the distinctive morphology, we have added a comparison to a flux- and size-selected sample of extended radio galaxies from the same LoTSS DR3 catalog and discussed the differences. These additions strengthen the presentation of the observed trend. revision: yes

standing simulated objections not resolved

Full injection-recovery simulations, completeness curves as a function of size and spectral index, and inter-inspector agreement metrics for the visual search strategy

Circularity Check

0 steps flagged

No circularity in observational sample expansion and correlation reporting

full rationale

The paper reports an observational search in the LoTSS DR3 catalog at 144 MHz, applying parameter filtering followed by visual inspection to isolate 18 ORC sources (including new single- and double-headed cases), cross-matching with optical data for redshifts and sizes, and noting an empirical trend between physical size and spectral index. No equations, model derivations, fitted parameters, or predictions appear; the central claims are direct counts and a data-derived correlation with no reduction to inputs by construction. Self-citations to prior ORC work are present but serve only as background and do not load-bear any uniqueness theorem or ansatz. The identification method is acknowledged as partly subjective, yet this does not trigger any of the enumerated circularity patterns.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The work is purely observational and relies on standard radio-astronomy catalog processing and visual morphology classification; no free parameters, ad-hoc axioms, or new entities are introduced beyond the existing definition of ORCs.

axioms (1)

domain assumption Standard assumptions of radio source detection and morphological classification in interferometric surveys apply without significant bias
Invoked implicitly in the catalog filtering and visual inspection steps described in the abstract

pith-pipeline@v0.9.0 · 5644 in / 1395 out tokens · 32368 ms · 2026-05-15T20:40:06.607468+00:00 · methodology

discussion (0)

Forward citations

Cited by 1 Pith paper

Reviewed papers in the Pith corpus that reference this work. Sorted by Pith novelty score.

A Compact Radio Ring with a Diffuse Envelope in LOFAR: Odd Radio Circle or Distinct Phenomenon?
astro-ph.GA 2026-05 unverdicted novelty 6.0

J1248+4826 is a newly identified compact ORC candidate with a ~30 kpc ring radius and ~200 kpc total extent at assumed z=0.2, consistent with the ORC population but with the host galaxy on the ring edge and no ongoing...

Reference graph

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