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A New Sample of Closely Separated Dual and Binary AGN Candidates Revealed with the Radio Fundamental Catalog

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Emma Schwartzman, Ryan Pfeifle, Tracy Clarke, Nathan Secrest, Henrique Schmitt, Barry Rothberg

Astrophysics of Galaxies · Submitted 30 Sep 2026 · 106 references

Pith's verdict: Useful archival mining that flags ten promising multi-AGN candidates, but the six dual/binary classifications ride on thin spectral-index evidence, so treat the 6.3 pc record claim as provisional.

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arXiv ID2609.39977 (version 1)
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CitationBarry Rothberg, Emma Schwartzman, Henrique Schmitt, Nathan Secrest, Ryan Pfeifle, Tracy Clarke. "A New Sample of Closely Separated Dual and Binary AGN Candidates Revealed with the Radio Fundamental Catalog." arXiv:2609.39977 (2026). https://pith.science/paper/RA2QTDNZ
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The paper

The paper claims

Archival VLBI imaging resolves ten multi-AGN candidates into parsec-scale pairs, six of which survive as likely dual or binary AGN.

The reading

This paper claims that archival very long baseline interferometry (VLBI) imaging, mined from the Radio Fundamental Catalog, can resolve candidate pairs of active galactic nuclei (AGN) down to parsec separations. Cross-matching the BigMAC literature catalog of multi-AGN candidates against the RFC yields 241 systems with VLBI images, of which ten show two or more compact radio cores. The authors argue that six of these are most consistent with genuinely distinct supermassive black hole pairs, dual or binary AGN, while four are better explained by compact jet structure within a single nucleus. If the interpretation holds, J1305-1033, at a projected separation of about 6.3 parsecs, would be the closest known binary AGN, edging out the current record holder CSO 0402+379 at 7.3 parsecs. The authors also emphasize that all ten sources are International Celestial Reference Frame members whose intrinsic structure could bias astrometry, and recommend removing them from future frame realizations.

What carries the argument

The load-bearing tool is the Radio Fundamental Catalog's archive of VLBA images, which resolve milliarcsecond structure, the only angular scale that can directly separate two AGN cores at parsec separations. For each of the ten systems the authors measure component flux densities with the PyBDSF source finder, then compute brightness temperature, compactness (the peak-to-integrated flux ratio), and radio spectra fit with standard and curved power laws. These diagnostics separate a genuine second AGN core (compact, high brightness temperature, optically thin synchrotron spectrum) from a jet hotspot (extended, compactness above about 2, steeper spectrum). Because components often blur together at the lower S and C bands, the authors recover individual fluxes by assuming component flux ratios of 90/10, 10/90, and 50/50, keeping only fits that produce physically plausible spectral indices; this is where the paper leans on hand-chosen splits that it acknowledges are uncertain.

Extended reading notes

Core claim

The central claim is that a routine archival resource, the four-decade VLBI catalog behind the Radio Fundamental Catalog, can serve as a discovery engine for the closest supermassive black hole pairs. After cross-matching all 5,742 BigMAC confirmed and candidate multi-AGN against the RFC's roughly 22,000 sources, 241 systems have usable images, and ten show multiple compact radio components with no obvious jet morphology. Combining morphology, brightness temperature, compactness ratios, spectral indices, and available multiwavelength data, the authors classify six of these (J0216-0105, J0729-1320, J0745+3142, J1305-1033, J2346+3011, and J2347-1856) as likely dual or binary AGN and four (J0843+4537, J1414+4554, J1451+1343, and J1503+0917) as compact jets. Projected separations range from 6.3 to 204.5 parsecs; J1305-1033, previously known as the periodic-light-curve candidate PG 1302-102, resolves at U band into two components 1.51 milliarcseconds apart, corresponding to roughly 6.34 parsecs at its known redshift of 0.28. A recurring theme is that the parsec-scale radio separations are orders of magnitude smaller than the kiloparsec scales implied by the original selection diagnostics, meaning these systems probe a regime that lower-resolution surveys cannot reach.

Load-bearing premise

The physical separations, and therefore the dual-versus-binary classification, of four systems (J2346+3011, J1451+1343, J1503+0917, and J2347-1856) rest on an assumed redshift of z=0.5 with no spectroscopic confirmation.

Editorial extensions

If this is right

  • Confirmation of even one of the six candidate systems would double the known binary AGN population, which currently consists of a single confirmed source, CSO 0402+379.
  • J1305-1033, if confirmed, would become the smallest known binary AGN at about 6.3 parsecs, just beating the current record.
  • The RFC is established as an archival resource capable of parsec-scale multi-AGN searches without requiring new observations for every candidate.
  • All ten sources carry intrinsic radio structure that can shift apparent VLBI centroids, so the authors recommend excluding them from future realizations of the International Celestial Reference Frame.
  • Because measured radio separations are often factors of several to orders of magnitude smaller than the scales that motivated original selection, VLBI follow-up is shown to reveal much more compact SMBH pairs than lower-resolution diagnostics suggest.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • The fact that ten of 241 imaged systems (about four percent) show multiple compact cores hints that a full systematic analysis of the remaining 231 BigMAC-RFC sources could yield a substantially larger census of parsec-scale pairs.
  • The dual-versus-jet distinction rests partly on two-point spectral fits with zero degrees of freedom and hand-chosen flux-ratio splits, so the six 'likely' classifications are better read as well-motivated candidates for dedicated high-frequency VLBI follow-up than as confirmed binaries.
  • If these parsec-separation systems are real binaries, they sit precisely in the regime where the 'final parsec problem' predicts stalled inspiral, giving empirical anchor points for models of SMBH merger timescales.
  • The astrometric warning generalizes beyond these ten sources: any ICRF quasar showing milliarcsecond structure is a candidate for centroid jitter, which strengthens the case for structural monitoring programs feeding future frame realizations.
Original abstract

The Big Multi-AGN Catalog (BigMAC) provides the first literature-complete compilation of confirmed and candidate dual, binary, and recoiling active galactic nuclei (AGN). The BigMAC provides a basis for the first population-wide radio study of multiple AGN (multi-AGN). The Radio Fundamental Catalog (RFC) is composed of four decades of Very Long Baseline Interferometry (VLBI) observations capable of resolving parsec-scale structure. All BigMAC candidate and confirmed multi-AGN were cross-matched to the RFC, highlighting a total of 241 BigMAC sources with available RFC imaging. We present the first detailed analysis of the ten systems exhibiting multiple compact components, most likely to be radio multi-AGN. Using multi-frequency radio and supporting multiwavelength information, we assess whether each system is more consistent with dual/binary AGN or intrinsic jet structure. The dual/binary nature of six systems is further supported by our radio analysis, while the four remaining systems are more plausibly explained by compact jet activity. Several candidate multi-AGN systems exhibit projected separations of only a few to tens of parsecs. In many cases, the parsec-scale radio separations are orders of magnitude smaller than the separations inferred from the observations that originally motivated their selection, emphasizing the importance of VLBI follow-up. Because all ten sources are members of the International Celestial Reference Frame (ICRF) and display significant intrinsic radio structure, they also represent potential sources of astrometric error. This work establishes the RFC as an archival resource for characterizing the closest SMBH pairs while improving future ICRF realizations.

The compiled record

The paper's own sentences, arranged, kept beside the review and separate from it. Each label is drawn from a closed vocabulary; the number beside it is the probability the classifier gave that label. Numbers in the tables are the authors', checked by code. No sentence in this record was written by a model.

Compiled by classifier-1.13.0 via TypeSafe API, question bank abstract-p1-20260923, 2026-10-01: 1 request, 8,280 input tokens.

What the abstract itself states, as probabilities: claims first 0.93 · states limitation 0.65 · human participants 0.01 · controlled comparison 0.09 · randomized assignment 0.01 · states quantitative result 0.98 · compares with prior results 0.61 · mentions code or data availability 0.04.

The abstract, sentence by sentence

arXiv listing abstract v1. Results and the main claim are underlined; .

The Big Multi-AGN Catalog (BigMAC) provides the first literature-complete compilation of confirmed and candidate dual, binary, and recoiling active galactic nuclei (AGN). The BigMAC provides a basis for the first population-wide radio study of multiple AGN (multi-AGN). The Radio Fundamental Catalog (RFC) is composed of four decades of Very Long Baseline Interferometry (VLBI) observations capable of resolving parsec-scale structure. All BigMAC candidate and confirmed multi-AGN were cross-matched to the RFC, highlighting a total of 241 BigMAC sources with available RFC imaging. We present the first detailed analysis of the ten systems exhibiting multiple compact components, most likely to be radio multi-AGN. Using multi-frequency radio and supporting multiwavelength information, we assess whether each system is more consistent with dual/binary AGN or intrinsic jet structure. The dual/binary nature of six systems is further supported by our radio analysis, while the four remaining systems are more plausibly explained by compact jet activity. Several candidate multi-AGN systems exhibit projected separations of only a few to tens of parsecs. In many cases, the parsec-scale radio separations are orders of magnitude smaller than the separations inferred from the observations that originally motivated their selection, emphasizing the importance of VLBI follow-up. Because all ten sources are members of the International Celestial Reference Frame (ICRF) and display significant intrinsic radio structure, they also represent potential sources of astrometric error. This work establishes the RFC as an archival resource for characterizing the closest SMBH pairs while improving future ICRF realizations.

background contribution method result interpretation main claim

What the classifier reports for each sentence. The number is the probability it gave the label.
#sentencerolethe authors' own workstates the main conclusionstrength
s0 The Big Multi-AGN Catalog (BigMAC) provides the first literature-complete compilation of confirmed and candidate dual, binary, and recoiling active galactic nuclei (AGN). background 0.91 0.15 0.05 asserted 1.00
s1 The BigMAC provides a basis for the first population-wide radio study of multiple AGN (multi-AGN). background 0.54 0.24 0.05 asserted 1.00
s2 The Radio Fundamental Catalog (RFC) is composed of four decades of Very Long Baseline Interferometry (VLBI) observations capable of resolving parsec-scale structure. background 0.55 0.09 0.02 asserted 1.00
s3 All BigMAC candidate and confirmed multi-AGN were cross-matched to the RFC, highlighting a total of 241 BigMAC sources with available RFC imaging. method 0.76 0.84 0.05 asserted 1.00
s4 We present the first detailed analysis of the ten systems exhibiting multiple compact components, most likely to be radio multi-AGN. contribution 1.00 0.91 0.10 asserted 0.57
s5 Using multi-frequency radio and supporting multiwavelength information, we assess whether each system is more consistent with dual/binary AGN or intrinsic jet structure. method 0.96 0.94 0.07 asserted 0.60
s6 The dual/binary nature of six systems is further supported by our radio analysis, while the four remaining systems are more plausibly explained by compact jet activity. result 0.88 0.95 0.46 asserted 0.63
s7 Several candidate multi-AGN systems exhibit projected separations of only a few to tens of parsecs. result 1.00 0.77 0.09 asserted 0.98
s8 In many cases, the parsec-scale radio separations are orders of magnitude smaller than the separations inferred from the observations that originally motivated their selection, emphasizing the importance of VLBI follow-up. interpretation 0.54 0.77 0.13 asserted 0.94
s9 Because all ten sources are members of the International Celestial Reference Frame (ICRF) and display significant intrinsic radio structure, they also represent potential sources of astrometric error. interpretation 0.79 0.53 0.05 hedged possibility 0.82
s10 This work establishes the RFC as an archival resource for characterizing the closest SMBH pairs while improving future ICRF realizations. interpretation 0.56 0.75 0.61 asserted 1.00

What kind of paper the classifier reports this is: empirical study 0.95 · evidence from analysis of existing data 0.99.

What was examined, and what was not

sources read
  • arXiv listing abstract v1
not examined
  • full text
  • figures and tables
  • references and citing sentences
  • relations to other papers
levels run
  • P abstract
not run
  • S sections
  • T body sentences
  • C citations
  • R cross-paper relations

Every quoted sentence on this record is the authors'. Every label is from Pith's closed vocabulary, with the probability shown beside it. Every number in the measurements table is the authors', checked by code. Record as JSON

Figures
Figure 1. Separation (in kiloparsecs) versus redshift plot for BigMAC-RFC candidate multi-AGN, plotted against the Rank 0.5 and 1 BigMAC confirmed and candidate dual and binary AGN. BigMAC rank 0.5 candidates are marked with grey circles. BigMAC rank 1 dual AGN candidates are marked with red diamonds. BigMAC rank 1 binary AGN candidates are marked with blue stars. BigMAC-RFC targets with known redshifts are marked in green, while those with assumed redshifts of 0.5 are marked with right-facing triangles denoting their possibly higher redshifts. attempts produced spectral indices that were…
Figure 2. VLBA 2.27 (Left) and 8.65 (Right) GHz images of J0216-0105. Scalebars are located in the lower left hand corners, beams are shown in the lower right hand corners. Colorbars in Jys are displayed to the right of each image. Contours are drawn in light grey. Positive contours are solid at levels: 3σ, 15σ, 27σ, 39σ, and 51σ. Negative contours are dashed at levels: -9σ, -6σ, and -3σ. The blue and green crosses note the positions of the two components at the highest available frequency (X- or U-bands). Axes show angular offsets in right ascension and declination relative to the reference…
Figure 3. VLBA 2.25 (Left) and 8.65 (Right) GHz images of J0729-1320. Scalebars are located in the lower left hand corners, beams are shown in the lower right hand corners. Colorbars in Jys are displayed to the right of each image. Contours are drawn in light grey. Positive contours are solid at levels: 3σ, 15σ, 27σ, 39σ, and 51σ. Negative contours are dashed at levels: -9σ, -6σ, and -3σ. The blue and green crosses note the positions of the two components at the highest available frequency (X- or U-bands). Axes show angular offsets in right ascension and declination relative to the reference…
Figure 4. VLBA 2.28 (Top Left), 4.84 (Top Right), and 8.65 (Bottom) GHz images of J0745+3142. For plot description, see
Figure 5. VLBA 2.25 (Top Left), 4.84 (Top Right), and 8.65 (Bottom) GHz images of J0843+4537. For plot description, see
Figure 6. VLBA 2.28 (Top Left), 4.35 (Top Right), 7.61 (Bottom Left), and 15.36 (Bottom Right) GHz images of J1305-1033. For plot description, see
Figure 7. VLBA 2.28 (Top Left), 4.84 (Top Right), and 8.65 (Bottom) GHz images of J1414+4554. For plot description, see
Figure 8. VLBA 2.25 (Top Left), 4.34 (Top Right), and 8.65 (Bottom) GHz images of J1451+1343. For plot description, see
Figure 9. VLBA 2.25 (Left) and 8.65 (Right) GHz images of J1503+0917. For plot description, see
Figure 10. VLBA 2.28 (Top Left), 4.34 (Top Right), and 8.65 (Bottom) GHz images of J2346+3011. For plot description, see
Figure 11. VLBA 2.25 (Left) and 8.65 (Right) GHz images of J2347-1856. For plot description, see
Figure 12. Radio spectra for J0216-0105 showing the multi-frequency VLBA data. Due to the limited S-band resolution, three configurations have been shown. In all configurations, the X-band flux densities remain constant for both components. At S-band, they have been varied: 10% South/90% North (Upper Left), 90% South/10% North (Upper Right), and 50% South/50% North. Components are differentiated by shape (squares, triangles, and circles), while colors are attributable to observational frequencies: S-band in purple, C-band in green, X-band in brown, and U-band in grey. Error bars have been…
Figure 13. Radio spectra for J0729-1320 showing the multi-frequency VLBA data. Due to the limited S-band resolution, three configurations have been shown. In all configurations, the X-band flux densities remain constant for both components. At S-band, they have been varied: 10% East/90% West (Upper Left), 90% East/10% West (Upper Right), and 50% East/50% West. For plot description, see
Figure 14. Radio spectra for J0745+3142 showing the multi-frequency VLBA data. Due to the limited S and C-band resolution, two configurations have been shown. In all configurations, the X-band flux densities remain constant for both components. At S-band, they have been varied: 50% North/50% South (Left) and 90% North/10% South (Right). At C-band, they have been varied: 50% North/50% South (Left) and 90% North/10% South (Right). For plot description, see
Figure 15. Radio spectra for J0843+4537 showing the multi-frequency VLBA data. Due to the limited S-band resolution, two configurations have been shown. In all configurations, the C and X-band flux densities remain constant for both components. At S-band, they have been varied: 10% South/90% North (Left) and 50% South/50% North (Right). For plot description, see
Figure 16. Radio spectra for J1305-1033 showing the multi-frequency VLBA data. Due to the limited S, C, and X-band resolution, two configurations have been shown. In both configurations, the U-band flux densities remain constant for both components. At S-band, they have been varied: 50% North/50% South (Left) and 50% North/50% South (Right) . At C-band, they have been varied: 10% North/90% South (Left) and 50% North/50% South (Right). At X-band, they have been varied: 10% North/90% South (Left) and 50% North/50% South (Right). For plot description, see
Figure 17. Radio spectra for J1414+4554 showing the multi-frequency VLBA data. For plot description, see
Figure 18. Radio spectra for J1451+1343 showing the multi-frequency VLBA data. For plot description, see
Figure 19. Radio spectra for J1503+0917 showing the multi-frequency VLBA data. For plot description, see
Figure 20. Radio spectra for J2346+3011 showing the multi-frequency VLBA data. For plot description, see
Figure 21. Radio spectra for J2347-1856 showing the multi-frequency VLBA data. For plot description, see
Figure 22. Optical tricolor ugz images drawn from the DESI LS Legacy Viewer for the full sample. The tricolor ugz image for J0729-1320 uses the PanSTARRS observations, as it is outside the DESI LS sky coverage. The scale bar in the bottom right corner indicates 5”. Notice in several cases that the sources appear elongated rather than pointlike, which may potentially point to underlying complex host morphologies and/or a multiplicity of sources, neither of which are resolvable in these images. 8. APPENDIX C: OPTICAL IMAGING

Figures reproduced from arXiv:2609.39977 by the authors.

References

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