High-resolution radio observations of TeV candidate sources

E. Kun; J. K\H{o}m\'ives; K. \'E. Gab\'anyi; S. Frey; T. An; Z. Paragi

REVIEW 3 major objections 4 minor 25 references

High-resolution radio observations of TeV candidate sources

T0 review · 3 major / 4 minor · reviewed 2026-08-09 · deepseek-v4-flash

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Pith's one-line read Two optically passive galaxies are found to host faint, compact radio-emitting AGN.

desk verdict New VLBI data suggest faint AGN in two TeV candidates, but the paper's spectral-index reporting contradicts itself and the abstract's 'steep spectra' claim doesn't survive contact with Table 3. read the letter →

arxiv 2502.03097 v1 pith:DSUY36LP submitted 2025-02-05 astro-ph.HE astro-ph.GA

J. Kőmíves , K. É. Gabányi , S. Frey , Z. Paragi , T. An , E. Kun This is my paper · ORCID

classification astro-ph.HEastro-ph.GA

keywords activegalacticnucleiverylongbaselineinterferometryTeVgamma-raycandidatespassiveellipticalgalaxiesbrightnesstemperatureradiospectralindexradio-X-rayluminosityratiocoronalemission

verification ladder T0 review T1 audit T2 compute T3 formal

The pith

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

The reading

Two galaxies whose optical spectra show no sign of nuclear activity, previously classified as passive ellipticals, are found to emit compact, high-brightness-temperature radio cores on milliarcsecond scales, indicating faint active galactic nuclei (AGN) hidden inside them. Using dual-frequency radio observations at 1.7 and 5 GHz, the paper measures brightness temperatures above the $10^5$ K limit for non-AGN galaxies and radio powers above the starburst ceiling, leaving AGN activity as the only viable explanation. The result matters because both objects are on a list of TeV gamma-ray candidates: if they are genuine AGN, the upcoming Cherenkov Telescope Array may detect a population of 'hidden' low-luminosity AGN whose optical spectra give no hint of nuclear activity.

What carries the argument

The argument hinges on the milliarcsecond brightness temperature $T_{\rm b}$ computed from Gaussian model fits to the interferometric visibilities through $T_{\rm b} = 1.22\times10^{12}(1+z) S / (\nu^2 W_1 W_2)$ K, and on comparing that value with the $10^5$ K ceiling for radio emission from non-AGN galaxies set by Condon (1992). The radio powers, computed as $P = 4\pi D_L^2 S (1+z)^{-\alpha-1}$, are compared with the $2\times10^{21}$ W Hz$^{-1}$ limit for starburst-driven emission from Kewley et al. (2000) and Middelberg et al. (2011). The radio–X-ray luminosity ratio is then tested against the coronal relation of Laor & Behar (2008) to identify the likely origin of the emission.

What would settle it

A deep, sub-arcsecond optical or near-infrared image of J1832 that locates the host galaxy's centre would falsify the claim if the VLBI core is offset from that centre, since the radio source could then be an unrelated background object.

Watch

Extended reading notes

Core claim

For J1519 at $z=0.041$ and J1832 at $z=0.046$, the paper finds milliarcsecond-scale radio cores with brightness temperatures of roughly $10^7$–$10^8$ K at 1.7 and 5 GHz, and 1.7 GHz radio powers of about $10^{22}$ W Hz$^{-1}$ — both far above the thresholds expected from stellar or starburst emission. These cores are therefore interpreted as AGN: the optically passive galaxies host faint, compact radio-emitting active nuclei that are outshone by their host galaxies at optical wavelengths. The paper also reports radio-to-X-ray luminosity ratios consistent with the $10^{-5}$ relation for coronal emission, suggesting the radio radiation could be produced in the hot X-ray corona rather than in a relativistic jet.

Load-bearing premise

The conclusion assumes that the compact radio source is physically located at the centre of each optical galaxy; for J1832 this association cannot be verified because the only optical position is imprecise (Gaia astrometric excess noise of 46 mas) and there is no SDSS detection, so the radio emission could in principle belong to an unrelated background object.

Editorial extensions

If this is right

J1519 and J1832 are no longer radio-passive galaxies: their compact, high-brightness-temperature cores mark them as low-luminosity AGN whose optical 'passivity' is an illusion created by host-galaxy light outshining the nucleus.
If these objects are genuine AGN, the TeV candidate list contains a hidden population of weak AGN invisible in optical spectra, and CTA observations will test whether such sources emit at TeV energies.
The radio-to-X-ray ratios matching the $L_R/L_X \approx 10^{-5}$ coronal relation imply the faint radio emission may trace the accretion region rather than a jet, a model that future simultaneous radio and X-ray observations can confirm or rule out.
The significant VLBI-resolved-out flux density (about 11 mJy for J1519) shows that part of the radio emission is extended beyond roughly 750 mas, so the full AGN radio power is higher than the core-alone value and the extended structure remains to be mapped.

Reading between the lines

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

The abstract's claim of 'steep spectra' for both sources appears inconsistent with the paper's own flux densities, which imply $\alpha \approx -0.5$ for J1519 but $\alpha \approx +0.2$ for J1832; a direct multi-frequency spectral-index measurement would settle whether the wording is accurate.
The same brightness-temperature and radio-power diagnostic applied to the other 44 objects in the Balmaverde et al. TeV candidate sample could quickly identify which 'passive' galaxies harbour AGN, creating a far larger and cleaner target list for CTA.
If the coronal origin is confirmed, TeV emission from these sources would be expected to differ in spectral shape and variability from blazar jets, providing a new way to distinguish emission mechanisms in the CTA era.
For J1832, a second-epoch VLBI observation that measures proper motion or parallax of the compact core could kinematically tie the radio source to the host galaxy, circumventing the current astrometric uncertainty.

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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Desk Editor's Note

read the letter

Here's the quick version: the VLBI observations are genuinely new, and the basic AGN identification for J1519 looks fine, but the paper trips over its own spectral-index reporting. The abstract says both targets have steep spectra, Section 4 says they have flat spectra, and the only α values quoted in Section 3 are for the phase-reference calibrators, not the targets. That's a load-bearing inconsistency in the headline claim.

What's new: this is the first mas-scale look at J1519 and J1832, two optically passive TeV candidates from the Te-REX sample. The brightness temperatures (≈10^7 K) and radio powers (≈10^22 W/Hz) are above the thresholds for starburst or passive galaxies, so the faint AGN interpretation is plausible. They also compare with NVSS and VLASS to show how much flux is resolved out, and they're candid about the J1832 positional ambiguity — the Gaia astrometry is poor and there's no SDSS detection, so they can't say whether the radio core is at the galaxy center.

The soft spot is real. From Table 3, the 5/1.7 GHz flux ratios give α(J1519) ≈ −0.5 (steep) and α(J1832) ≈ +0.2 (flat/inverted). The paper never quotes target spectral indices at all; the −0.45 and −0.21 values are for the calibrators. So the abstract's 'steep spectra for both' is not supported by the data, and Section 4's 'flat radio spectra of the targets' is also wrong for at least one of them. This is fixable — a few sentences plus a corrected table, and reconciliation of the abstract — but it needs to happen before this is citable as a result.

The corona interpretation via Laor & Behar is speculative and properly hedged.

Who is this for? People working on TeV candidate identification and VLBI follow-up of faint AGN. It's a modest but legitimate observational contribution, and a serious referee should look at it, mainly to catch exactly this kind of internal inconsistency. I'd accept after a light revision.

Referee Report

3 major / 4 minor

Summary. Kőmíves et al. present dual-frequency (1.7 and 5 GHz) EVN/e-MERLIN observations of two TeV candidate passive elliptical galaxies, J1519 and J1832, selected from the Balmaverde et al. (2020) PEG sample. From Gaussian model fits to the VLBI visibilities they derive component flux densities and sizes, then compute brightness temperatures, radio powers, and spectral indices in order to test whether the compact radio emission is an AGN core. They conclude in the abstract that both optically passive-looking galaxies host faint compact radio-emitting AGN with steep spectra, and in Section 4 they additionally discuss a possible coronal origin based on the radio–X-ray luminosity ratio. The paper is a short proceedings contribution with images, fitted parameters, and standard diagnostic calculations.

Significance. If the result holds, it provides direct evidence that at least some optically passive TeV-candidate galaxies harbour compact, high-brightness-temperature radio cores, supporting the idea that low-luminosity AGN are present in this population and can contribute to the TeV source counts. The paper's strengths are its direct mas-scale VLBI imaging of two faint targets, the explicit computation of T_b and P from measured model parameters using standard formulas, and the comparison with well-established physical thresholds (Condon 1992; Kewley et al. 2000). These quantitative diagnostics are reproducible from Table 3. However, the central claim as stated in the abstract is compromised by an internal inconsistency in the reported spectral indices and by the positional ambiguity for J1832 that the authors themselves acknowledge.

major comments (3)

[Abstract and Section 3] The abstract's headline assertion that both targets have 'steep spectra' is not supported by the reported data. In Section 3, the only spectral indices quoted, alpha = -0.45 +/- 0.13 for J1516+1932 and alpha = -0.21 +/- 0.07 for J1816+5307, are explicitly values for the two phase-reference calibrators listed in Table 2, not for the targets J1519 and J1832. No target spectral indices are given or derived in the text. Recomputing from the target flux densities in Table 3 gives alpha(J1519) = ln(2.5/4.2)/ln(5/1.7) ~ -0.48, which is steep, but alpha(J1832) = ln(9.0/7.2)/ln(5/1.7) ~ +0.21, which is flat or slightly inverted. Section 4 then states that 'flat radio spectra of the targets' strongly indicate an AGN origin, directly contradicting the abstract. The authors should quote the target spectral indices explicitly, correct the abstract/Discussion mismatch, and adjust the conclusions to what the data actually show for each source.
[Section 4 (J1832)] The paper itself states that 'due to the lack of precise optical positions, we cannot ascertain whether the radio emission in J1832 indeed originates at the centre of its host galaxy.' The Gaia DR3 astrometric excess noise of 46 mas with significance 116 and the absence of an SDSS detection mean that the compact radio source could in principle be unrelated to the optical galaxy. Because the conclusion that J1832 hosts the AGN is one of the two central claims in the abstract, this limitation is load-bearing. The discussion and abstract should be explicitly conditional for J1832, or the claim should be restricted to J1519 if the association cannot be secured.
[Section 3 (radio power)] The text states that the 1.7 GHz radio powers are 'an order of magnitude higher' than the starburst-related limiting value of ~2 x 10^21 W Hz^-1. From Table 3, P(J1519) = 1.3 x 10^22 W Hz^-1, which is only a factor of ~6.5 above that limit, while P(J1832) = 2.9 x 10^22 W Hz^-1 is a factor of ~15. The 'order of magnitude' phrasing is inaccurate for J1519 and should be revised to a quantitative statement.

minor comments (4)

[Section 3] The sentence 'These values (-0.45 +/- 0.13 for J1516+1932 and -0.21 +/- 0.07 for J1816+5307) are indicating flat radio spectra' should be reworded to make unambiguous that these indices belong to the phase-reference calibrators, not to the targets; the current wording is easy to misread as reporting target properties.
[Section 3, Eq. (2)] The radio-power formula uses alpha as the spectral index, but no target alpha is provided in the paper. The authors should either state the assumed alpha value used in the calculation or compute alpha directly from the Table 3 flux densities.
[Figure 1 caption] The caption contains a duplicated word ('at at') and the paper header contains the typo 'sourc es'; these should be corrected in the final version.
[Section 4] The comparison with the Laor & Behar (2008) radio–X-ray luminosity relation would be easier to evaluate if the adopted ROSAT X-ray flux densities and their uncertainties were listed; currently only a qualitative statement that the ratio 'aligns with the 10^-5 threshold' is given.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the radio-derived conclusions are computed from measured quantities using standard formulae and tested against external benchmarks.

full rationale

The derivation of the central claim proceeds from measured mas-scale VLBI flux densities and component sizes (Table 3) via standard relations: spectral index from S proportional to nu^alpha, brightness temperature from Eq. (1), radio power from Eq. (2), and radio-to-X-ray ratio from ROSAT catalog data and the 5 GHz flux density. These are not fitted parameters renamed as predictions; they are direct calculations from the reported measurements, and the conclusions are compared with external thresholds: Tb about 1e5 K from Condon 1992, P about 2e21 W/Hz from Kewley et al. 2000, and LR/LX about 1e-5 from Laor and Behar 2008. The observational data are new, and no quantity needed for the conclusion is imported from the conclusion itself. The self-citations (Gabanyi et al. 2019 and Mosoni et al. 2006 for the coherence-loss correction; Kun et al. 2014 and Schinzel 2011 for error formulae) are calibration and systematic-uncertainty inputs, not the load-bearing premise, and the qualitative conclusion is robust to the associated uncertainties. Separately, the manuscript appears internally inconsistent about target spectral indices: the alpha values quoted in Section 3 are for the phase-reference calibrators, and Section 4 refers to flat radio spectra of the targets while the abstract says steep spectra. That is a correctness or reporting issue, not circular derivation. No circular step can be exhibited from the paper's equations or citations.

Assumptions & free parameters 1 free parameters · 3 assumptions · 0 invented entities

The paper introduces no new particles, forces, or physical mechanisms. It relies on standard cosmological parameters, standard radio-AGN empirical thresholds, and an external coronal relation. The only hand-chosen numerical factor is the 20% coherence-loss correction.

free parameters (1)

coherence_loss_correction = 20% recovered flux density
Applied uniformly to all measured flux densities in Section 3 to account for missing phase correction. It directly scales brightness temperature and radio power and therefore influences whether the sources pass the AGN threshold.

assumptions (3)

domain assumption Flat Lambda-CDM cosmology with H0 = 70 km/s/Mpc, Omega_m = 0.27, Omega_vac = 0.73.
Used in Section 1 to compute luminosity distances and angular scale conversions for both sources.
domain assumption Brightness temperatures above roughly 10^5 K indicate non-thermal AGN activity in galaxies.
Invoked in Section 3 with the threshold from Condon (1992) to classify the compact cores as AGN rather than star formation.
domain assumption The empirical relation LR/LX approximately 10^-5 for coronally active stars also applies to radio-quiet AGN coronae.
Used in Section 4 to suggest the radio emission might come from an X-ray corona, following Laor & Behar (2008).

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Cite this review

Pith. "Pith review of High-resolution radio observations of TeV candidate sources." pith.science (2026). https://pith.science/paper/DSUY36LP

@misc{pith2026250203097,
  author       = {Pith},
  title        = {Pith review of: High-resolution radio observations of TeV candidate sources},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/DSUY36LP}},
  note         = {Machine review of arXiv:2502.03097}
}

abstract

Radio-loud active galactic nuclei (AGN) with their jets pointed close to our line of sight constitute the majority of extragalactic $\gamma$-ray sources and significantly contribute to the radiation observed in the even higher energy regime. The upcoming Cherenkov Telescope Array (CTA) is expected to detect fainter TeV objects, leading to an anticipated increase in the proportion of non-blazar extragalactic high-energy sources. Here we present the results of our dual-frequency (1.7 and 5~GHz) European VLBI Network (EVN) and enhanced Multi Element Remotely Linked Interferometer Network (e-MERLIN) observations of two faint radio sources from the list of TeV candidate sources. They do not show signs of nuclear activity in their optical spectra, but they were hypothesized to contain faint AGN that is outshone by the host galaxy. We used the mas-scale resolution radio data to try to pinpoint the location of the compact radio emitting feature, determine its spectral index, radio power, brightness temperature and radio-X-ray luminosity ratio and thus identify the origin of the radio emission. Our results suggest that both optically passive-looking galaxies host faint compact radio-emitting AGN with steep spectra.

Figures

Figures reproduced from arXiv: 2502.03097 by the authors.

**Figure 1.** VLBI images of sources J1832 (top) and J1519 (bottom) at at 1.7 GHz (left) and 5 GHz (right). Panel (a): The peak intensity is 4.41 mJy beam−1 . The lowest contours are at ±0.31 mJy beam−1 . The restoring beam size is 16.9 mas × 9.0 mas. The position angle of the major axis is PA = 11.1 ◦ . Panel (b): The peak intensity is 0.48 mJy beam−1 . The lowest contours are at ±0.48 mJy beam−1 . The restoring beam size is 5… view at source ↗

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Pith tools

Reviewed August 9, 2026 · model on record in the stance chip above.

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