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Multi-epoch monitoring of the faint northern component in 3C 84 during 2016-2021

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Kotaro Niinuma, Motoki Kino, Nozomu Kawakatsu, Hiroshi Nagai, Gabriele Giovannini, Filippo D'Ammando, Monica Orienti, Kazuhiro Hada
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Kiyoaki Wajima, Marcello Giroletti

High Energy Astrophysical Phenomena · Submitted 5 Nov 2026 · 52 references

Pith's verdict: Solid multi-epoch VLBI campaign on 3C 84's N1; main result is absence of systematic motion, which is statistically robust even if the core stability assumption is mildly vulnerable.

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arXiv ID2611.07782 (version 1)
Pith IDARA5OXXA, Pith's permanent record of this paper and its review
CitationFilippo D'Ammando, Gabriele Giovannini, Hiroshi Nagai, Kazuhiro Hada, Kiyoaki Wajima, Kotaro Niinuma, Marcello Giroletti, Monica Orienti, Motoki Kino, Nozomu Kawakatsu. "Multi-epoch monitoring of the faint northern component in 3C 84 during 2016-2021." arXiv:2611.07782 (2026). https://pith.science/paper/ARA5OXXA
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The paper

The paper claims

Multi-epoch VLBI monitoring of the northern jet component N1 in 3C 84 shows it is stationary with jitter and significant flux variability, challenging a simple counter-hotspot interpretation.

The reading

This paper reports 50-epoch 43 GHz VLBI monitoring of the faint northern component N1 in the radio galaxy 3C 84 over 2016–2021. The authors find that N1 exhibits no statistically significant systematic motion; its peak position jitters by approximately 0.2 mas around a mean location 2.5 mas north of the radio core C1. In contrast, its peak flux density varies by roughly a factor of two on timescales of several months, with variability confirmed at >99% confidence. Two classes of explanation are proposed: either N1 is the counter-hotspot of the southern component C3, halted by a jet–cloud collision (Case 1a), or it is part of the northern jet seen through a low-opacity gap in foreground free-free absorption (Cases 2a/2b). The results provide new constraints on the parsec-scale environment of 3C 84 and point to future mm-VLBI observations as a way to distinguish between these scenarios.

What carries the argument

The key machinery is multi-epoch 43 GHz VLBI imaging (KaVA plus archival VLBA data) with uniform angular resolution (0.6 mas beam) and a model-fitting approach that represents the extended N1 component as a single Gaussian to obtain stable centroid positions and peak flux densities. A Monte Carlo error analysis demonstrates that a single-component fit yields positional uncertainties of 0.004–0.023 mas and peak flux systematic uncertainties of ~15%, supporting the reliability of the derived kinematics and light curve. The relative position measurement uses the brightness centroid of C1 as a reference, and the authors cite external evidence that C1 is stable in declination on yearly timescales.

Extended reading notes

Core claim

The central observational discovery is that the global structure of the N1 component in 3C 84 does not show systematic proper motion over five years. A linear fit gives a proper motion of µ_Dec = 0.054 ± 0.019 mas yr⁻¹ (β_app = 0.062 ± 0.021), but this displacement is comparable to the positional scatter (~0.24 mas) and is not statistically significant. Meanwhile, the peak flux density varies by a factor of two on timescales of a few months, with a reduced chi-square of 3.27 and p < 0.01 against constant flux. The authors argue that this combination—stationary mean position plus jitter and variability—cannot be explained by measurement noise or by simple Doppler boosting of a moving hotspot. Instead, the most plausible scenarios involve either a jet–cloud collision with non-uniform free-free absorption (Case 1a) or emission from the northern jet viewed through a gap in foreground free-free absorbing material (Cases 2a/2b).

Load-bearing premise

The analysis assumes that the brightness centroid of the radio core C1 is stable in the north-south direction on yearly timescales, because N1 positions are measured relative to C1; any systematic drift of C1 comparable to the observed N1 scatter (~0.24 mas) would alter the proper motion measurement and could change the conclusion that N1 has no systematic motion.

Editorial extensions

If this is right

  • If N1 is indeed a counter-hotspot halted by a dense cloud (Case 1a), then the cloud must have a number density of ~10⁵–10⁶ cm⁻³ and a size ~5 times larger than the cloud that halted C3, implying a highly inhomogeneous circumnuclear medium.
  • If N1 is a gap-window view of the northern jet (Cases 2a/2b), then the foreground free-free absorber must be patchy on scales of ~0.2–1 mas, and the jet itself must be intrinsically variable on monthly timescales, possibly due to precession or shear-layer instabilities.
  • Future VLBI observations at ≥86 GHz, where free-free absorption is negligible, can directly test both scenarios by revealing whether a moving counter-hotspot exists north of N1 or whether the N1 structure itself shows systematic motion.
  • The absence of systematic motion in N1, together with the asymmetry between L_north and L_south, suggests that simple symmetric ejection models for the 2005 outburst need revision; the northern side may be subject to different environments or geometric effects.
  • The statistically significant flux variability of N1 provides a new diagnostic for the physical conditions of the absorbing and emitting regions, linking radio variability to free-free absorption changes or to intrinsic jet power fluctuations.

Reading between the lines

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

  • The paper implicitly assumes that the radio core C1 provides a stable positional reference. If future observations reveal that C1 itself drifts north–south by more than ~0.1 mas on yearly timescales (e.g., due to core-shift or structural changes), the conclusion that N1 has no systematic motion could be overturned.
  • The favored scenarios (Case 1a and Cases 2a/2b) are not mutually exclusive: N1 could be a stationary feature in the counter-jet that is also viewed through a gap window, with its variability arising from both cloud non-uniformity and intrinsic jet changes. Disentangling these requires simultaneous multi-frequency observations.
  • If N1 is a cloud-stalled counter-hotspot, the lack of a systematic flux decline (unlike C3) implies that the free-free absorption from the cloud itself varies, rather than the cloud merely extinguishing the hotspot. This points to a clumpy ionized medium with column densities fluctuating by ~factors of 2 on monthly timescales.
Original abstract

We present the results of multi-epoch very long baseline interferometry (VLBI) observations of the sub-parsec-scale northern jet component of 3C 84 (known as the N1 component) at 7 mm from 2016 to 2021. The peak position of the N1 component exhibits positional variations of approximately 0.2 mas at a distance of about 2.5 mas from the central radio core (C1) throughout the observation period. Although a linear fit yields a small proper motion of $\mu_{\rm{Dec}}=0.054\pm0.019~\rm{mas}~\rm{yr}^{-1}$, corresponding to an apparent velocity of $\beta_{\rm{app}}=0.062\pm0.021$, this displacement is comparable to the observed positional scatter, and the hypothesis that N1 exhibits systematic motion is not statistically supported. In contrast, the peak flux density varies by approximately a factor of two on timescales of several months. We propose two possible explanations for the observed behavior of N1. One is that N1 is the counterpart of the southern hotspot C3, but its motion has been halted by a collision with a compact and dense cloud along its path; in this case, the flux variability may be caused by changes in the free-free absorption opacity of the non-uniform cloud. The other possibility is that N1 is part of the northern jet that is heavily obscured by foreground free-free absorption and is observed through a gap in the absorbing medium; in this case, the variability may reflect intrinsic changes in the jet and its interaction with a non-uniform ambient medium. These results provide new constraints on the physical origin of N1 and on the circumnuclear environment of 3C 84.

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, 2026-11-06: 1 request, 6,676 input tokens.

What the abstract itself states, as probabilities: claims first 0.03 · states limitation 0.79 · human participants 0.01 · controlled comparison 0.04 · randomized assignment 0.01 · states quantitative result 0.99 · compares with prior results 0.12 · mentions code or data availability 0.02.

The abstract, sentence by sentence

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

We present the results of multi-epoch very long baseline interferometry (VLBI) observations of the sub-parsec-scale northern jet component of 3C 84 (known as the N1 component) at 7 mm from 2016 to 2021. The peak position of the N1 component exhibits positional variations of approximately 0.2 mas at a distance of about 2.5 mas from the central radio core (C1) throughout the observation period. Although a linear fit yields a small proper motion of $\mu_{\rm{Dec}}=0.054\pm0.019~\rm{mas}~\rm{yr}^{-1}$, corresponding to an apparent velocity of $\beta_{\rm{app}}=0.062\pm0.021$, this displacement is comparable to the observed positional scatter, and the hypothesis that N1 exhibits systematic motion is not statistically supported. In contrast, the peak flux density varies by approximately a factor of two on timescales of several months. We propose two possible explanations for the observed behavior of N1. One is that N1 is the counterpart of the southern hotspot C3, but its motion has been halted by a collision with a compact and dense cloud along its path; in this case, the flux variability may be caused by changes in the free-free absorption opacity of the non-uniform cloud. The other possibility is that N1 is part of the northern jet that is heavily obscured by foreground free-free absorption and is observed through a gap in the absorbing medium; in this case, the variability may reflect intrinsic changes in the jet and its interaction with a non-uniform ambient medium. These results provide new constraints on the physical origin of N1 and on the circumnuclear environment of 3C 84.

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 We present the results of multi-epoch very long baseline interferometry (VLBI) observations of the sub-parsec-scale northern jet component of 3C 84 (known as the N1 component) at 7 mm from 2016 to 2021. contribution 0.96 0.97 0.03 asserted 0.99
s1 The peak position of the N1 component exhibits positional variations of approximately 0.2 mas at a distance of about 2.5 mas from the central radio core (C1) throughout the observation period. result 1.00 0.92 0.08 asserted 1.00
s2 Although a linear fit yields a small proper motion of $\mu_{\rm{Dec}}=0.054\pm0.019~\rm{mas}~\rm{yr}^{-1}$, corresponding to an apparent velocity of $\beta_{\rm{app}}=0.062\pm0.021$, this displacement is comparable to the observed positional scatter, and the hypothesis that N1 exhibits systematic motion is not statistically supported. result 0.72 0.95 0.33 asserted 0.98
s3 In contrast, the peak flux density varies by approximately a factor of two on timescales of several months. result 1.00 0.94 0.06 asserted 1.00
s4 We propose two possible explanations for the observed behavior of N1. interpretation 0.98 0.64 0.04 asserted 0.45
s5 One is that N1 is the counterpart of the southern hotspot C3, but its motion has been halted by a collision with a compact and dense cloud along its path; in this case, the flux variability may be caused by changes in the free-free absorption opacity of the non-uniform cloud. interpretation 1.00 0.20 0.07 hedged possibility 0.91
s6 The other possibility is that N1 is part of the northern jet that is heavily obscured by foreground free-free absorption and is observed through a gap in the absorbing medium; in this case, the variability may reflect intrinsic changes in the jet and its interaction with a non-uniform ambient medium. interpretation 1.00 0.25 0.07 hedged possibility 0.96
s7 These results provide new constraints on the physical origin of N1 and on the circumnuclear environment of 3C 84. interpretation 0.77 0.71 0.38 asserted 0.97

What kind of paper the classifier reports this is: empirical study 1.00 · evidence from observation or experiment 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. Examples of uv–coverage for observations of 3C 84. Left: KaVA. Center: VLBA, restricted to spatial frequencies up to 350 Mλ. Right: full VLBA uv–coverage. The VLBA data are archival data from the blazar monitoring program led by the Boston University group. 2.4 Modelfit procedure The N1 component exhibits an extended structure elongated in the east–west direction, in contrast to the compact C1 and C3 com￾1 https://www.bu.edu/blazars/VLBA_GLAST/0316.html
Figure 2. KaVA and VLBA images of the sub-parsec-scale structure of 3C 84 from 2016 August 16 to 2021 May 28. All images were restored with a circular Gaussian beam of 0.6 mas, indicated by the yellow filled circle in the bottom corner of the panel for 2021 May 28. This represents the typical angular resolution achieved by KaVA observations at 43 GHz and corresponds to a linear scale of 0.21 pc. The intensity range is shown by the color scale in the bottom-right corner with contours at levels of 30 mJy beam−1× (1, 4, 16, 64, 256). The horizontal yellow solid, white dashed, and white solid…
Figure 3. The (left) panel shows the location of the brightest peak of N1 at each epoch overlaid on the VLBA image obtained on 2021 May 28. The contours are shown at levels of 30 mJy beam−1× 1 and 2. The green circle and the blue square indicate the first epoch (2016 September 12) and the last epoch (2021 May 28), respectively. The (right) panel shows the pc-scale structure of the jet base in 3C 84 observed on 2021 May 28, and is the same as that shown in Figure2. The contours start at levels of 30 mJy beam−1× (1, 4, 16, 64, 256). 3.2 The peak intensity of N1
Figure 4. Time variation of the peak flux density of N1, derived from a single-component fit. 4 Discussion Our results show that N1 exhibits no systematic motion and in￾stead displays only jittery behaviour. Here, we discuss possible explanations for its nature (e.g. a counter-hotspot of C3, a par￾tially obscured jet, etc.). For this purpose, we define the south length (Lsouth) as the radial distance between C1 and C3, and the north length (Lnorth) as that between C1 and N1. When the N1 component was first identified, it was suggested that N1 is the counter hotspot of C3, since Lsouth and…
Figure 5. A sketch of jet–cloud interactions in 3C 84 corresponding to Cases 1a: self-FFA from the invisible cloud, 1b: fast motion of FFA-foreground, and 1c: FFA-foreground and large B-field. In these cases, an invisible cloud halts the propagation of the northern hotspot. The observed variability may be attributed to the nonuniform density of the cloud (see
Figure 6. An example of simulated visibilities (panel A) and the corresponding reconstructed image with a model fit using a single elliptical Gaussian component (panel B). The simulated visibilities were generated using the uv-coverage and visibility noise level of the KaVA observation performed on 2016 October 11. Alt text: Schematic illustrations of five possible scenarios for the environment surrounding the N1 component, as discussed in this paper
Figure 7. Representative results of the Monte Carlo simulations using the uv-coverage and visibility noise level of the KaVA observation conducted on 2016 October 11. The intrinsic model consists of two circular Gaussian components used to generate the simulated visibilities. The simulations were performed with 1000 trials for two model-fitting schemes: (1) a modelfit using single elliptical Gaussian component (upper panels) and (2) a modelfit using two circular Gaussian components (lower panels). Each panel shows, from left to right, the distributions of the recovered total flux density,…

Figures reproduced from arXiv:2611.07782 by the authors.

References

Discussion

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