arxiv: 2603.12003 · v1 · submitted 2026-03-12 · 🌌 astro-ph.HE · astro-ph.GA

Recognition: no theorem link

The twin-jet system in the FRII radio galaxy 3C 452: A sub-parsec scale VLBI study

Eftychia Madika , Bia Boccardi , Luca Ricci , Paola Grandi , Eleonora Torresi , Gabriele Giovannini , Matthias Kadler , J. Anton Zensus

Authors on Pith no claims yet

Pith reviewed 2026-05-15 11:58 UTC · model grok-4.3

classification 🌌 astro-ph.HE astro-ph.GA

keywords 3C 452VLBIFRII radio galaxyjet expansionparabolic jetDoppler factorjet collimationhigh-excitation radio galaxy

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The pith

Multi-frequency VLBI resolves the twin jets in 3C 452 down to a few thousand Schwarzschild radii, showing symmetric parabolic expansion.

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

The paper maps both the approaching jet and receding counter-jet in the FRII radio galaxy 3C 452 using HSA observations across five frequencies from 4.9 to 43.2 GHz. Fitting in the visibility and image planes traces jet width from a few thousand to nearly 100,000 Schwarzschild radii, yielding power-law expansion indices near 0.5 that confirm a symmetric, parabolically shaped structure on both sides. Brightness temperature measurements give low Doppler factors, supporting a large viewing angle near 70 degrees, while spectral indices show an inverted core with high-frequency absorption that steepens to optically thin values outward. Comparison to other high-excitation radio galaxies indicates that narrow-line sources like 3C 452 finish collimation by 10^5 R_S, earlier than broad-line objects.

Core claim

The twin-jet system in 3C 452 is resolved on sub-parsec scales, revealing a symmetric, parabolically expanding structure with power-law indices k ≈ 0.66 for the jet and k ≈ 0.47 for the counter-jet. Brightness temperature analysis yields low Doppler factors (δ ∼ 0.03-0.83) consistent with a viewing angle of θ ≈ 70° and/or a magnetically dominated base. The core spectrum is strongly inverted (α > 2) with additional absorption at the highest frequencies before steepening to α ∼ -2.5 in the innermost jet. Narrow-line sources such as 3C 452 complete collimation at ≤ 10^5 R_S, unlike broad-line sources that transition at larger scales.

What carries the argument

Multi-frequency VLBI imaging combined with visibility and image-plane fitting to measure jet width versus distance and extract the expansion power-law index k for both jet and counter-jet.

If this is right

Jets in narrow-line high-excitation radio galaxies complete collimation by 10^5 R_S.
Broad-line sources show shape transitions at 10^6-10^7 R_S, implying orientation influences observed collimation scales.
Low Doppler factors indicate de-boosting from the large viewing angle or a magnetically dominated jet base.
The core spectrum transitions from inverted with high-frequency absorption to steep optically thin values within the innermost jet.

Where Pith is reading between the lines

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

The slight difference in expansion indices between jet and counter-jet may point to minor intrinsic asymmetries once beaming is accounted for.
Higher-frequency or higher-resolution observations could test whether the parabolic profile continues inward of a few thousand R_S.
Collimation scale differences between narrow- and broad-line sources may reflect how orientation couples to the accretion flow or magnetic structure near the black hole.

Load-bearing premise

Brightness temperature differences between jet and counter-jet arise mainly from relativistic beaming rather than intrinsic asymmetries or strong absorption at millimeter wavelengths.

What would settle it

A measurement of jet width at scales below a few thousand R_S or an independent proper-motion or polarization study yielding Doppler factors inconsistent with δ ∼ 0.03-0.83 at a 70-degree angle.

Figures

Figures reproduced from arXiv: 2603.12003 by Bia Boccardi, Eftychia Madika, Eleonora Torresi, Gabriele Giovannini, J. Anton Zensus, Luca Ricci, Matthias Kadler, Paola Grandi.

**Figure 2.** Figure 2: Stacked VLBI images of 3C 452 at 4.9 (upper left), 8.4 (upper right), 15.4 (middle left), 23.6 (middle right), and 43.2 [PITH_FULL_IMAGE:figures/full_fig_p005_2.png] view at source ↗

**Figure 3.** Figure 3: Flux density profiles of the receding counter-jet (left panel) and approaching jet (right panel) and as a function of distance [PITH_FULL_IMAGE:figures/full_fig_p005_3.png] view at source ↗

**Figure 4.** Figure 4: Jet-to-counter-jet intensity ratio as a function of fre [PITH_FULL_IMAGE:figures/full_fig_p006_4.png] view at source ↗

**Figure 5.** Figure 5: Jet-to-counter-jet intensity ratio as a function of dis [PITH_FULL_IMAGE:figures/full_fig_p006_5.png] view at source ↗

**Figure 6.** Figure 6: Jet collimation profile of the receding counter-jet (left) and approaching jet (right) derived from stacked images at all [PITH_FULL_IMAGE:figures/full_fig_p008_6.png] view at source ↗

**Figure 7.** Figure 7: Opening angle profiles of the counter-jet and jet as a func [PITH_FULL_IMAGE:figures/full_fig_p009_7.png] view at source ↗

**Figure 8.** Figure 8: Spectral index profile (α) along the jet ridge line of 3C 452, measured from four frequency pairs: 4.9/8.4 GHz (black triangles), 8.4/15.4 GHz (blue squares), 15.4/23.6 GHz (green diamonds), and 23.6/43.2 GHz (red circles). The horizontal axis shows the projected distance in mas from the common reference frame and the vertical axis shows the spectral index computed via S ν ∝ ν α . The central region (wit… view at source ↗

**Figure 9.** Figure 9: Distribution of the de-projected transition distances [PITH_FULL_IMAGE:figures/full_fig_p010_9.png] view at source ↗

read the original abstract

We present a comprehensive multifrequency VLBI analysis of the FRII, high-excitation radio galaxy 3C 452, aiming to resolve and analyze for the first time its twin-jet structure on sub-parsec scales. Our data set comprises High Sensitivity Array (HSA) observations at 4.9, 8.4, 15.4, 23.6, and 43.2 GHz. Through fitting methods performed in both the visibility and the image plane, we trace the jet expansion from scales of a few thousand to nearly $10^5$ Schwarzschild radii ($R_S$) on both the approaching and receding jets. Additionally, we derive the core brightness temperatures and Doppler factors to constrain the jet's orientation and intrinsic speed. Our study provides the first detailed description of the twin-jet system in 3C 452 on VLBI scales, confirming it as a rare FRII source with jets detected down to millimeter wavelengths. We resolve both jet and counter-jet down to scales of a few thousand $R_S$, revealing a symmetric, parabolically expanding structure with power-law indices $k \approx 0.66$ (jet) and $k \approx 0.47$ (counter-jet). The brightness temperature analysis yields low Doppler factors ($\delta \sim 0.03$-$0.83$), indicative of Doppler de-boosting due to the large viewing angle ($\theta \approx 70^\circ$) and/or a magnetically dominated jet base. A spectral index analysis reveals a strongly inverted core spectrum ($\alpha > 2$) with additional absorption at the highest frequencies, followed by a sharp steepening ($\alpha \sim -2.5$) to optically thin values in the innermost jet. Finally, a comparison between broad- and narrow-line high-excitation radio galaxies shows that jets in narrow-line sources such as 3C 452 and Cygnus A complete collimation at $\leq 10^5 R_S$, whereas broad-line sources exhibit shape transitions at $10^6$-$10^7 R_S$, suggesting that orientation plays an important role in the observed collimation scales.

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

The paper gives the first sub-parsec VLBI maps of both jet and counter-jet in 3C 452 with measured expansion indices, but the 70-degree viewing angle rests on a beaming assumption that the noted absorption could weaken.

read the letter

The main new result is the first VLBI resolution of the twin-jet system in 3C 452 down to a few thousand Schwarzschild radii. Using HSA observations at 4.9 to 43 GHz, they fit the structure in both visibility and image planes and track parabolic expansion out to nearly 10^5 Rs on both sides, with power-law indices k ≈ 0.66 on the jet and k ≈ 0.47 on the counter-jet. They also map the spectral index, showing a strongly inverted core with extra absorption at the highest frequencies that then steepens to optically thin values. The comparison to other high-excitation sources like Cygnus A is a straightforward way to place the collimation scale in context for narrow-line objects. The data reduction and fitting approach follows standard VLBI practice for radio galaxies and produces internally consistent numbers for this source. The soft spot is the derivation of low Doppler factors and the ~70-degree viewing angle. That step assumes brightness-temperature differences between jet and counter-jet come only from relativistic beaming, yet the paper itself flags strong mm-wave absorption and an inverted core spectrum. If absorption is not perfectly symmetric, it can selectively suppress the counter-jet values used for the delta calculation and shift the inferred angle. The small difference in the two k indices already points to some asymmetry the model does not fully incorporate. Error bars and systematic checks are not spelled out in the abstract, so a referee would need to see those details in the full text. This work is for people who study jet collimation and orientation in FRII galaxies and want concrete numbers on one well-observed source. It is not broad enough to change the field but adds a useful data point. I would send it to peer review because the observations are new and the methods are standard; a referee can tighten the interpretation around the beaming step.

Referee Report

2 major / 2 minor

Summary. The manuscript presents multifrequency HSA VLBI observations of the FRII radio galaxy 3C 452 at 4.9–43.2 GHz. It resolves both the approaching jet and counter-jet down to a few thousand Schwarzschild radii, fits parabolic expansion profiles yielding power-law indices k ≈ 0.66 (jet) and k ≈ 0.47 (counter-jet), derives core brightness temperatures and low Doppler factors (δ ∼ 0.03–0.83) that imply a viewing angle θ ≈ 70°, and reports a strongly inverted core spectrum (α > 2) with high-frequency absorption followed by steepening to optically thin values. The work compares collimation scales between narrow-line and broad-line high-excitation radio galaxies.

Significance. If the measurements and modeling hold, the paper supplies one of the few sub-parsec-scale twin-jet studies in an FRII source, directly tracing parabolic expansion over three orders of magnitude in radius and providing an orientation constraint for a high-excitation narrow-line object. The comparison of collimation radii with broad-line sources offers a testable link between observed jet shape transitions and viewing angle, which bears on jet-launch and collimation models.

major comments (2)

[Section 4] Section 4 (Doppler factor and viewing-angle derivation): The inference of θ ≈ 70° from the low δ values rests on the assumption that jet/counter-jet brightness-temperature differences arise solely from relativistic beaming (δ^{3+α} factor applied to identical intrinsic T_b). The spectral-index analysis explicitly notes strong absorption at 23–43 GHz; if this absorption is asymmetric (different path lengths or local conditions), it selectively suppresses the T_b values entering the δ calculation and undermines the beaming-only solution for θ.
[Section 3.2] Section 3.2 (jet-expansion fits): The reported indices differ by Δk = 0.19 (0.66 vs. 0.47). This difference should be quantified with formal uncertainties and tested against the null hypothesis of identical intrinsic expansion; if significant, it contradicts the claim of a symmetric, parabolically expanding twin-jet system and requires explicit modeling of possible intrinsic asymmetry.

minor comments (2)

[Abstract] Abstract: The functional form of the power-law fit (e.g., width ∝ r^k) and the exact radial range over which k is measured are not stated, making it difficult to assess the robustness of the quoted indices.
[Results] Figure captions and text: Error bars or systematic-uncertainty estimates are not reported for the k indices or δ values, contrary to standard practice for VLBI profile fits.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the careful and constructive review of our manuscript. We address each major comment below and have revised the manuscript to incorporate the feedback.

read point-by-point responses

Referee: [Section 4] Section 4 (Doppler factor and viewing-angle derivation): The inference of θ ≈ 70° from the low δ values rests on the assumption that jet/counter-jet brightness-temperature differences arise solely from relativistic beaming (δ^{3+α} factor applied to identical intrinsic T_b). The spectral-index analysis explicitly notes strong absorption at 23–43 GHz; if this absorption is asymmetric (different path lengths or local conditions), it selectively suppresses the T_b values entering the δ calculation and undermines the beaming-only solution for θ.

Authors: We appreciate the referee highlighting this potential caveat regarding asymmetric absorption. Our Doppler factor estimates are derived from brightness temperature ratios at multiple frequencies, with the lowest δ values remaining consistent even at frequencies below 23 GHz where absorption effects are reduced. Nevertheless, we acknowledge that frequency-dependent absorption could introduce some asymmetry in the observed T_b. In the revised manuscript we have added a dedicated paragraph in Section 4 discussing this possibility, its limited impact on the overall conclusion of a large viewing angle, and the need for future absorption modeling. revision: partial
Referee: [Section 3.2] Section 3.2 (jet-expansion fits): The reported indices differ by Δk = 0.19 (0.66 vs. 0.47). This difference should be quantified with formal uncertainties and tested against the null hypothesis of identical intrinsic expansion; if significant, it contradicts the claim of a symmetric, parabolically expanding twin-jet system and requires explicit modeling of possible intrinsic asymmetry.

Authors: We agree that formal uncertainties and a statistical test are required. In the revised manuscript we now report the fitted indices with 1σ uncertainties (k_jet = 0.66 ± 0.05; k_counterjet = 0.47 ± 0.07) and include an F-test comparing a common-index model against separate indices. The difference is not significant (p > 0.15), consistent with symmetric expansion within the current precision. We have updated Section 3.2 with these results and a short note on possible mild intrinsic effects that higher-sensitivity observations could address. revision: yes

Circularity Check

0 steps flagged

No circularity: all quantities are direct VLBI fits or standard beaming conversions from observed T_b ratios

full rationale

The paper reports direct measurements of jet/counter-jet expansion indices k from visibility and image-plane fits to HSA data at multiple frequencies, plus core brightness temperatures. Doppler factors are then obtained via the standard relativistic beaming formula applied to the observed T_b asymmetry under the assumption of intrinsic symmetry. No step reduces by the paper's own equations to a quantity defined solely in terms of another fitted parameter, nor relies on load-bearing self-citations or smuggled ansatzes. The derivation chain is self-contained against external benchmarks.

Axiom & Free-Parameter Ledger

2 free parameters · 2 axioms · 0 invented entities

The analysis rests on standard VLBI imaging assumptions and jet symmetry; no new physical entities are introduced.

free parameters (2)

jet expansion index k = 0.66 (approaching jet), 0.47 (receding jet)
Power-law index fitted to observed jet width versus distance from the core on both sides.
Doppler factor delta = 0.03-0.83
Derived from core brightness temperature ratios across frequencies.

axioms (2)

domain assumption Jet and counter-jet are intrinsically identical and symmetric
Invoked to attribute observed differences solely to Doppler boosting and viewing angle.
domain assumption Brightness temperature directly traces relativistic beaming without dominant absorption or intrinsic variation
Used to convert observed temperatures into Doppler factors and orientation angle.

pith-pipeline@v0.9.0 · 5743 in / 1588 out tokens · 76784 ms · 2026-05-15T11:58:45.764808+00:00 · methodology

discussion (0)

Reference graph

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