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arxiv: 1906.08342 · v1 · pith:H3D3M5U7new · submitted 2019-06-19 · 🌌 astro-ph.HE

Gamma-ray emission in radio galaxies under the VLBI scope -- I. Parsec-scale kinematics and high-energy properties of γ-ray detected TANAMI radio galaxies

R. Angioni , E. Ros , M. Kadler , R. Ojha , C. M\"uller , P.G. Edwards , P.R. Burd , B. Carpenter
show 17 more authors
M.S. Dutka S. Gulyaev H. Hase S. Horiuchi F. Krau{\ss} J.E.J. Lovell T. Natusch C. Phillips C. Pl\"otz J.F.H. Quick F. R\"osch R. Schulz J. Stevens A.K. Tzioumis S. Weston J. Wilms J.A. Zensus
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Pith reviewed 2026-05-25 19:50 UTC · model grok-4.3

classification 🌌 astro-ph.HE
keywords radio galaxiesgamma-ray emissionVLBIparsec-scale jetsDoppler boostingFermi-LATTANAMIjet kinematics
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The pith

Gamma-ray luminosity in radio galaxies shows no dependence on Doppler boosting indicators from parsec-scale jets

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

The paper examines multi-epoch 8.4 GHz VLBI images of five Fermi-LAT detected radio galaxies from the TANAMI sample to derive jet kinematics and intrinsic parameters. It combines these with Fermi-LAT gamma-ray data and published MOJAVE results for a total of fifteen sources. A correlation appears between VLBI core flux density and gamma-ray flux, yet gamma-ray luminosity shows no dependence on core brightness temperature or core dominance. These two quantities are standard proxies for Doppler boosting, so their lack of correlation implies that gamma-ray emission is not driven by orientation-dependent effects. A sympathetic reader would care because the result separates the high-energy behavior of radio galaxies from the beaming-dominated picture that applies to blazars.

Core claim

The authors find that the VLBI core flux density correlates with the gamma-ray flux. In contrast, the gamma-ray luminosity does not depend on core brightness temperature or core dominance. They therefore conclude that gamma-ray emission in radio galaxies is not driven by orientation-dependent effects.

What carries the argument

Multi-epoch VLBI imaging at 8.4 GHz to measure core flux density, brightness temperature, and core dominance, tested for correlation against Fermi-LAT gamma-ray flux and luminosity

If this is right

  • The first gamma-ray detection of Pictor A may coincide with a new VLBI component passing through the radio core.
  • PKS 0521-36 exhibits subluminal parsec-scale motions together with gamma-ray variability on timescales as short as six hours.
  • PKS 0625-35 shows confirmed superluminal motion reaching beta_app approximately 3.
  • A firm lower limit can be placed on the age of the compact symmetric object PKS 1718-649.
  • The core-flux to gamma-ray-flux correlation holds across the combined TANAMI and MOJAVE sample.

Where Pith is reading between the lines

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

  • Gamma-ray production regions in radio galaxies may lie outside the zone where the radio core's Doppler factor dominates.
  • Intrinsic jet power rather than viewing angle could set the gamma-ray output in misaligned sources.
  • Longer-term monitoring of component ejections versus gamma-ray flares could distinguish radio-galaxy behavior from blazar patterns.
  • Repeating the test at higher radio frequencies closer to the black hole would directly check the proxy assumption.

Load-bearing premise

Core brightness temperature and core dominance measured at 8.4 GHz on parsec scales serve as reliable proxies for the Doppler factor that would affect gamma-ray emission produced closer to the black hole.

What would settle it

A statistically significant positive correlation between gamma-ray luminosity and either core brightness temperature or core dominance in a larger sample of radio galaxies would falsify the central claim.

Figures

Figures reproduced from arXiv: 1906.08342 by A.K. Tzioumis, B. Carpenter, C. M\"uller, C. Phillips, C. Pl\"otz, E. Ros, F. Krau{\ss}, F. R\"osch, H. Hase, J.A. Zensus, J.E.J. Lovell, J.F.H. Quick, J. Stevens, J. Wilms, M. Kadler, M.S. Dutka, P.G. Edwards, P.R. Burd, R. Angioni, R. Ojha, R. Schulz, S. Gulyaev, S. Horiuchi, S. Weston, T. Natusch.

Figure 1
Figure 1. Figure 1: First-epoch 8.4 GHz contour maps of γ-ray detected TANAMI radio galaxies. The blue ellipse indicates the convolving beam, while the black bar shows the linear scale. Top left to bottom right: Pictor A, PKS 0521−36, PKS 0625−35, Centaurus B, PKS 1718−649 (the B1950 name is indicated in the top right corner of each image). The full set of multi-epoch images and the associated map parameter tables are present… view at source ↗
Figure 2
Figure 2. Figure 2: Spectral index map of Pictor A (0518−458) between 8.4 GHz and 22.3 GHz for epoch 2008 Nov 27. Black contours are from the 8.4 GHz image. The convolving beam is represented in grey in the lower￾left corner. gle, speed, and Doppler factor using the Atacama Large Mil￾limeter Array (ALMA). Their results suggest a jet viewing an￾gle in the range 16◦ ≤ θ ≤ 38◦ , from the jet-to-counterjet ratio. Their detection … view at source ↗
Figure 5
Figure 5. Figure 5: Spectral index map of PKS 0625−35 (0625−354) between 8.4 GHz and 22.3 GHz for epoch 2008 Nov 27. The black contours are from the 8.4 GHz image. The convolving beam is represented in grey in the lower-left corner. 1343−601 (Centaurus B) This classic FR I radio galaxy was added to the TANAMI sample after being detected by Fermi￾LAT in the second source catalog (Nolan et al. 2012). TANAMI imaging reveals a sm… view at source ↗
Figure 4
Figure 4. Figure 4: Spectral index map of PKS 0521−36 (0521−365) between 8.4 GHz and 22.3 GHz for epoch 2008 Mar 28. Black contours are from the 8.4 GHz image. The convolving beam is represented in grey in the lower-left corner. 0625−354 This is an FR I radio galaxy, but shows an opti￾cal spectrum similar to a BL Lac object. Its γ-ray properties (see Section 4.3) also suggest a moderately aligned jet, similar (but less extrem… view at source ↗
Figure 6
Figure 6. Figure 6: Spectral index map of PKS 1718−649 between 8.4 GHz and 22.3 GHz for epoch 2008 Feb 07. The black contours are from the 8.4 GHz image. The convolving beam is represented in grey in the lower-left corner. 4.2. Radio kinematic analysis results The main results of the kinematic analysis for Pictor A, PKS 0521−36, PKS 0625−35 and PKS 1718−649, following the procedure described in Section 3.1, are summarized by … view at source ↗
Figure 7
Figure 7. Figure 7: Jet kinematics of our radio galaxies: core distance of jet features as a function of time. The solid lines represent a least squares fit to their positions (the slope is the apparent speed). Top left to bottom right: Pictor A, PKS 0521−36, PKS 0625−35, PKS 1718−649. The B1950 name is given above each image. Article number, page 10 of 41 [PITH_FULL_IMAGE:figures/full_fig_p010_7.png] view at source ↗
Figure 8
Figure 8. Figure 8: Jet kinematics of PKS 0521−36 (0521−365): core distance of jet features as a function of time, including the previous VLBI dataset of Tingay & Edwards (2002). The solid lines represent a least squares fit to their positions (the slope is the apparent speed). Compare with top-right panel of [PITH_FULL_IMAGE:figures/full_fig_p011_8.png] view at source ↗
Figure 9
Figure 9. Figure 9: Multi-epoch images of Centaurus B (1343−601). The colored crossed circles represent the circular Gaussian components that have been fitted to the clean maps. Article number, page 11 of 41 [PITH_FULL_IMAGE:figures/full_fig_p011_9.png] view at source ↗
Figure 10
Figure 10. Figure 10: Parameter space of intrinsic jet speed β and viewing angle θ allowed by our observations. The blue shaded area is the one allowed by the measurement of R (θ as function of β given R, i.e., θR), while the red shaded area is the one allowed by the observed βapp (θ as function of β given βapp, i.e., θβapp ). For each source we provide a minimum, maximum and (except for PKS 0521−36) a central estimate of R an… view at source ↗
Figure 11
Figure 11. Figure 11: Light curve of Pictor A (0518−458) between 0.1-300 GeV over 103 months of Fermi-LAT data. Blue points are detections, red arrows are upper limits. Top to bottom: 1-month, 3-months and 6-months binning, respectively. 2009 2010 2011 2012 2013 2014 2015 2016 2017 Time [Decimal year] 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Flux (weekly) [Photons cm − 2 s − 1 ] ×10−6 0521−365 0 1 2 3 4 Flux (monthly) [Photons cm − 2 s − 1… view at source ↗
Figure 12
Figure 12. Figure 12: Light curve of PKS 0521−36 (0521−365) between 0.1-300 GeV over 103 months of Fermi-LAT data, with weekly (blue points) and monthly (red points) binning. Upper limits are indicated by arrows of the respective colors. The left y-axis reports the weekly flux values, the right one reports the monthly flux values. a doubling of the VLBI core flux density during the first γ-ray flaring periods of 2010-2011 (see… view at source ↗
Figure 13
Figure 13. Figure 13: Light curve of PKS 0521−36 (0521−365) between 0.1-300 GeV over 103 months of Fermi-LAT data, with 6-hours (blue points) and daily (red points) binning. Upper limits are indicated by arrows of the respective colors. The left y-axis reports the 6-hours flux values, the right one reports the daily flux values. 2009 2010 2011 2012 2013 2014 2015 2016 2017 Time [Decimal year] 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 Fl… view at source ↗
Figure 14
Figure 14. Figure 14: Light curve of PKS 0625−35 (0625−354) between 0.1-300 GeV over 103 months of Fermi-LAT data, with monthly binning. Blue points are detections, red arrows are upper limits. 0625−354 The estimated range of viewing angles (θ < 38◦ ) from our VLBI data does not allow us to settle the uncertain classification of PKS 0625−35, as it is consistent with jet orien￾tations typical of radio galaxies (see e.g., Kim et… view at source ↗
Figure 15
Figure 15. Figure 15: Light curve of Centaurus B between 0.1-300 GeV over 103 months of Fermi-LAT data, with monthly binning. Blue points are detections, red arrows are upper limits. 261.0 ◦ 262.0 ◦ 263.0 ◦ RA (J2000) −65.4 ◦ −65.2 ◦ −65.0 ◦ −64.8 ◦ −64.6 ◦ −64.4 ◦ −64.2 ◦ −64.0 ◦ Dec (J2000) 3FGL J1728.0-6446 PKS 1718-649 PS J1724.2-6459 PKS 1718−649 0 5 10 15 20 25 30 35 40 TS [PITH_FULL_IMAGE:figures/full_fig_p017_15.png] view at source ↗
Figure 16
Figure 16. Figure 16: Fermi-LAT map of excess TS in the inner region of the ROI centered on PKS 1718−649, after removing the unidentified catalog source 3FGL J1728.0−6446. The ellipses represent the 68% and 95% confidence positional errors on the catalog source, while the black circles represent the same errors for the new source PS J1724.2−6459. The map size is 1.6 ◦ × 1.6 ◦ . Each pixel corresponds to 0.1◦ . 5.2. Sample prop… view at source ↗
Figure 17
Figure 17. Figure 17: Maximum measured apparent speed βapp as a function of me￾dian total VLBI luminosity for our TANAMI γ-ray radio galaxies (blue circles) and MOJAVE γ-ray radio galaxies (red squares). The down￾ward arrows indicate upper limits on the apparent speed, for sources with speed consistent with zero within the uncertainties. 10−1 100 < SVLBI core > [Jy] 10−8 10−7 Fγ [photons cm − 2 s −1 ] 0518-458 0521-365 0625-35… view at source ↗
Figure 18
Figure 18. Figure 18: Fermi-LAT flux above 100 MeV as a function of median ra￾dio VLBI core flux density for our TANAMI γ-ray radio galaxies (blue circles) and MOJAVE γ-ray radio galaxies (red squares). extended at > 5σ confidence level. The contribution of the ex￾tended lobes to the high-energy flux is dominant, while the radio core contribution is no more than 14%. Interestingly, Fornax A has not been detected with VLBI obse… view at source ↗
Figure 19
Figure 19. Figure 19: Fermi-LAT flux above 100 MeV as a function of median ra￾dio VLBI jet flux density for our TANAMI γ-ray radio galaxies (blue circles) and MOJAVE γ-ray radio galaxies (red squares). energy, and therefore depends not only on flux and luminosity distance, but also on photon index. In [PITH_FULL_IMAGE:figures/full_fig_p019_19.png] view at source ↗
Figure 21
Figure 21. Figure 21: Median radio VLBI core brightness temperature as a function of Fermi-LAT luminosity above 100 MeV for our TANAMI γ-ray radio galaxies (blue circles) and MOJAVE γ-ray radio galaxies (red squares). A correction factor has been applied to the MOJAVE data to account for the difference in frequency, but this might still imply a bias. that it is not driven by Doppler boosting. In the first systematic study of t… view at source ↗
Figure 22
Figure 22. Figure 22: Median radio VLBI core dominance as a function of Fermi￾LAT luminosity above 100 MeV for our TANAMI γ-ray radio galaxies (blue circles) and MOJAVE γ-ray radio galaxies (red squares). radio galaxies, allowing us to robustly study variations in the pc-scale jet structure for the first time. We complemented the VLBI analysis with Fermi-LAT γ-ray light curves, and investi￾gated whether there are physical conn… view at source ↗
read the original abstract

In the framework of the TANAMI multi-wavelength and VLBI monitoring, we study the evolution of the parsec-scale radio emission in radio galaxies in the southern hemisphere and their relationship to the $\gamma$-ray properties. In this first paper, we focus on Fermi-LAT-detected sources. We perform a kinematic analysis for five $\gamma$-ray detected radio galaxies using multi-epoch 8.4 GHz VLBI images, deriving limits on intrinsic jet parameters. We analyzed Fermi-LAT data in order to study possible connections between the $\gamma$-ray properties and the pc-scale jets of Fermi-LAT-detected radio galaxies, both in terms of variability and average properties. We discuss the individual source results and draw preliminary conclusions on sample properties including published VLBI results from the MOJAVE survey, with a total of fifteen sources. We find that the first $\gamma$-ray detection of Pictor A might be associated with the passage of a new VLBI component through the radio core. For the peculiar AGN PKS 0521-36, we detect subluminal parsec-scale jet motions, and we confirm the presence of fast $\gamma$-ray variability in the source down to timescales of 6 hours. We robustly confirm the presence of significant superluminal motion, up to $\beta_{app}\sim$3, in the jet of the TeV radio galaxy PKS 0625-35. Finally, we place a lower limit on the age of the Compact Symmetric Object (CSO) PKS 1718-649. We draw some preliminary conclusions on the relationship between pc-scale jets and $\gamma$-ray emission in radio galaxies. We find that the VLBI core flux density correlates with the $\gamma$-ray flux, as seen in blazars. On the other hand, the $\gamma$-ray luminosity does not show any dependence on the core brightness temperature and core dominance, two indicators of Doppler boosting, suggesting that $\gamma$-ray emission in radio galaxies is not driven by orientation-dependent effects.

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 paper reports multi-epoch 8.4 GHz VLBI kinematic analysis for five Fermi-LAT detected TANAMI radio galaxies, combined with Fermi-LAT light-curve and spectral analysis, and extends the sample to 15 sources by including published MOJAVE results. It finds a correlation between VLBI core flux density and gamma-ray flux but no correlation between gamma-ray luminosity and either core brightness temperature or core dominance (taken as Doppler-boosting proxies), leading to the conclusion that gamma-ray emission in radio galaxies is not driven by orientation-dependent effects.

Significance. If the central interpretation holds, the result would indicate that gamma-ray production in misaligned radio galaxies differs from the Doppler-boosted blazar paradigm, with implications for jet emission models and the location of the high-energy zone. The work benefits from direct VLBI component tracking, Fermi variability analysis down to 6-hour timescales in one source, and the combined 15-source sample that allows preliminary statistical statements.

major comments (1)
  1. [Abstract and concluding discussion] Abstract and final paragraph: the claim that the lack of correlation between gamma-ray luminosity and the two radio indicators implies gamma-ray emission is not orientation-driven rests on the untested assumption that 8.4 GHz parsec-scale core brightness temperature and core dominance faithfully trace the Doppler factor applicable to gamma-ray production (modeled as occurring on sub-parsec scales). No multi-frequency VLBI, jet-acceleration modeling, or comparison to sub-pc indicators is presented to support commensurability of the proxies across scales.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for the constructive review. We address the single major comment below, agreeing where the manuscript requires qualification and indicating the planned revisions.

read point-by-point responses

  1. Referee: [Abstract and concluding discussion] Abstract and final paragraph: the claim that the lack of correlation between gamma-ray luminosity and the two radio indicators implies gamma-ray emission is not orientation-driven rests on the untested assumption that 8.4 GHz parsec-scale core brightness temperature and core dominance faithfully trace the Doppler factor applicable to gamma-ray production (modeled as occurring on sub-parsec scales). No multi-frequency VLBI, jet-acceleration modeling, or comparison to sub-pc indicators is presented to support commensurability of the proxies across scales.

    Authors: We agree that the interpretation in the abstract and final paragraph assumes the 8.4 GHz parsec-scale core brightness temperature and core dominance serve as faithful proxies for the Doppler factor at the gamma-ray emission site. The manuscript contains no multi-frequency VLBI data, jet-acceleration modeling, or sub-parsec comparisons to test this assumption directly. In the revised version we will qualify the relevant statements to present the lack of correlation as suggestive rather than conclusive, explicitly note the scale mismatch as a limitation, and add a short paragraph in the discussion section outlining the need for future multi-scale observations to strengthen the proxy validation. revision: yes

Circularity Check

0 steps flagged

No circularity: empirical correlations derived directly from VLBI and Fermi measurements

full rationale

The paper's central results consist of direct observational correlations computed from independently measured quantities: VLBI core flux densities, brightness temperatures, and core dominance parameters extracted from 8.4 GHz multi-epoch images, together with Fermi-LAT gamma-ray fluxes and luminosities. No equations, fitted parameters, or self-citations reduce the reported absence of dependence on Doppler indicators to the inputs by construction. Kinematic limits are obtained via standard proper-motion analysis of component positions across epochs; the sample is augmented with published MOJAVE results treated as external data. The interpretation that gamma-ray emission is not orientation-driven follows from the observed lack of correlation and does not loop back to any definitional or fitted step within the paper.

Axiom & Free-Parameter Ledger

0 free parameters · 2 axioms · 0 invented entities

The paper rests on standard VLBI assumptions (stationary core, no significant acceleration between epochs, cosmological parameters for distance conversion) and the domain assumption that 8.4 GHz core properties trace the same jet region responsible for gamma rays. No free parameters are fitted to produce the headline correlation; no new entities are postulated.

axioms (2)
  • standard math Standard flat Lambda-CDM cosmology for converting angular to linear scales and apparent to intrinsic speeds
    Invoked implicitly when reporting beta_app values and source ages
  • domain assumption Core brightness temperature and core dominance at 8.4 GHz are monotonic proxies for Doppler factor
    Used to interpret the lack of correlation with gamma-ray luminosity as evidence against orientation effects

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