REVIEW 3 major objections 4 minor 61 references
Magnetically arrested discs, not ordinary advection flows, power the jets of low-accretion FR I radio galaxies.
Reviewed by Pith at T0; open to challenge. T0 means a machine referee read the full paper against a public rubric. the ladder, T0–T4 →
T0 review · grok-4.5
2026-07-14 12:20 UTC pith:PRQWRRL7
load-bearing objection Solid sample extension of the MAD-for-FR-I result; the a<0.5 preference and the necessity of MAD both ride on the same free f=1 that the authors already flag as movable. the 3 major comments →
Magnetically Arrested Discs Powering Jets in a Large Sample of Low-Accretion FR I Radio Galaxies
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
In a uniformly selected sample of 289 low-accretion FR I radio galaxies the observed jet powers systematically exceed the maximum Blandford-Znajek output of a standard advection-dominated flow, even for a near-maximally spinning black hole; the magnetically arrested disc solution, in which large-scale poloidal flux saturates near the horizon, fully accounts for the jets and prefers moderate-to-slow spins under the authors' fiducial assumptions.
What carries the argument
The magnetically arrested disc (MAD) field-strength formula that replaces the weaker standard-ADAF magnetic pressure inside the Blandford-Znajek power expression, thereby raising the theoretical jet-power ceiling by the amount needed to match the radio-inferred powers.
Load-bearing premise
The conversion from radio luminosity to jet power is taken at its lowest conventional normalization (f = 1); a larger but still allowed factor would raise every jet power and push the preferred spins higher.
What would settle it
An independent, cavity- or lobe-calorimetry-based jet-power measurement for a statistically useful subset of the same LoTSS FR I sources that returns powers systematically lower than the MAD tracks at a = 0.5 would falsify the claim that magnetically arrested discs are required.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper presents a sample of 289 LoTSS DR1 FR I radio galaxies with SDSS optical data, deriving black-hole masses from host magnitudes and Eddington-scaled accretion rates from nuclear g-band luminosities (median log ṁ ≈ −2.84, vast majority below 0.01). Jet powers are estimated via the Willott et al. (1999) 151 MHz scaling with f = 1. Even for a = 0.95 the standard ADAF Blandford–Znajek ceiling under-predicts the observed Q_jet for ~70 % of sources; the MAD field-strength formula is shown to encompass the data, and under MAD the points prefer a ≲ 0.5. The authors conclude that magnetically arrested discs are common across the FR I luminosity range, extending earlier 3CR results.
Significance. A uniformly selected LoTSS sample an order of magnitude larger than the classic 3CR FR I set is a clear advance. Three independent accretion-rate estimators (g-band core, Hβ, M_BH–σ) all place the bulk of sources in the ADAF regime, and the Q_jet–M_BH comparison is performed with publicly available radio morphologies and SDSS decompositions. If the MAD requirement survives a proper exploration of the free normalization f, the work would establish magnetic-flux saturation as a generic feature of low-accretion FR I engines rather than a peculiarity of the brightest objects.
major comments (3)
- [Section 2, Eq. (2); §4.3] Section 2 and Eq. (2): the entire MAD-required claim and the a < 0.5 preference rest on the single free parameter f = 1 in Q_jet ≃ 3 × 10^38 f^{3/2} L_151^{6/7} W. Raising f to the still-plausible value 10 multiplies every Q_jet by ~30, which (i) shrinks or eliminates the ~70 % ADAF shortfall shown in Fig. 3 and (ii) moves the MAD tracks in Figs. 6–7 into the a ≳ 0.5 regime previously claimed for 3CR sources (explicitly noted by the authors in §4.3). No sample-specific constraint on f is supplied. The abstract and summary statements must be re-phrased to state the range of f over which both conclusions hold, or an independent argument for f ≃ 1 in this LoTSS sample must be provided.
- [Section 3.2, Fig. 3] Section 3.2 and Fig. 3: the ADAF ceiling is drawn only at the sample median log ṁ = −2.84. Because individual sources span more than four decades in ṁ, a fair comparison requires either source-by-source evaluation of L_BZ(ṁ_i, a = 0.95) or a Monte-Carlo realization that folds in the measured ṁ distribution and its uncertainties. The quoted “approximately 70 %” fraction is therefore not yet robust.
- [Section 4.1, Fig. 5] Section 4.1 and Fig. 5: the two spectroscopic estimators systematically return lower ṁ than the photometric values used for the main figures. Lower ṁ raises the required spin under MAD (Eq. 11). The paper should either adopt a joint posterior on ṁ or demonstrate that the a < 0.5 preference survives the lower-ṁ solutions.
minor comments (4)
- [Abstract, §2] Abstract and throughout: several typographical artefacts remain (“iin which”, “sufficiently”, “o the Fundamental”, “1-D profiles are shown for illustration purposes only”). A careful proof-read is needed.
- [Section 3.3, Eq. (6)] Equation (6): the numerical prefactor 1.5 × 10^9 and the adopted values ϵ = 0.01, f_Ω = 0.5 are stated without a short derivation or reference to the precise simulation calibration used; a one-sentence justification would help reproducibility.
- [Figure 1] Figure 1 caption: the one-dimensional profiles are described twice; the second description can be deleted for clarity.
- [Table 1] Table 1: only 20 of 289 sources are listed; either expand the electronic table or state that the full catalogue will be released.
Circularity Check
No derivation reduces to its inputs by construction; only minor non-load-bearing self-citation to the authors' prior 3CR MAD analysis.
specific steps
-
self citation load bearing
[Abstract; §1 (Introduction); §4.3; §5 (Summary point 4)]
"This large, uniformly selected LoTSS sample extends the MAD requirement previously established for the bright 3CR FR I population, indicating that magnetically arrested discs are common in FR I radio galaxies across a wide range of luminosities. ... He et al. (2024) applied this MAD framework to a sample of 17 FR I radio galaxies from the 3CR catalog and demonstrated that it successfully accounts for their unexpectedly powerful jets."
The claim that MADs are a 'common feature' of the FR I population rests in part on the prior He et al. (2024) result (same first/second authors). That prior result is not re-derived or independently verified here; it is simply cited. The circularity is mild because the present sample's own ADAF-deficit and MAD-coverage calculations stand alone and do not logically require the 3CR paper.
full rationale
The paper's central chain is: (i) independent LoTSS+SDSS measurements of L_151, host magnitude and nuclear g-band luminosity yield Q_jet (Willott et al. 1999 with fixed f=1), M_BH (McLure & Dunlop 2002) and ṁ (McLure & Dunlop 2004); (ii) these are compared to the external Blandford–Znajek formula evaluated first with the standard ADAF B-field scaling and then with the MAD B-field scaling of Narayan et al. (2003). The MAD tracks cover the data while the ADAF tracks do not; spin is then read off by eye against the same external tracks. None of these steps is tautological: Q_jet, ṁ and M_BH are measured quantities, the MAD/ADAF formulae are taken from the literature, and no free parameter is fitted to the present sample and then re-used as a prediction. The sole self-citation (He et al. 2024, overlapping authors) is used only for historical comparison and to claim that the new LoTSS result “extends” the earlier 3CR finding; it is not invoked as a uniqueness theorem or as the sole justification that MAD is required for the present sample. The acknowledged sensitivity to the free factor f is a systematic uncertainty, not circularity. Hence the score is 2 (one minor self-citation that is not load-bearing).
Axiom & Free-Parameter Ledger
free parameters (4)
- f (Willott jet-power normalization) =
1 (fiducial; range 1–20 discussed)
- ϵ (radial-to-Keplerian velocity ratio in MAD) =
0.01
- f_Ω (angular-velocity ratio Ω/Ω_K) =
0.5
- a = 0.95 (upper-limit spin for ADAF ceiling) =
0.95
axioms (5)
- domain assumption Willott et al. (1999) scaling Q_jet ≃ 3 × 10^38 f^{3/2} L_151^{6/7} W converts 151 MHz luminosity into time-averaged jet power.
- domain assumption McLure & Dunlop (2002) host-galaxy magnitude–BH mass relation yields log(M_BH/M_⊙) from absolute R-band magnitude.
- domain assumption MAD magnetic-field strength scales as B_MAD ∼ 1.5 × 10^9 (1−f_Ω)^{1/2} ϵ^{-1/2} m_BH^{-1/2} ṁ^{1/2} R^{-5/4} G (Narayan et al. 2003).
- domain assumption Optical g-band nuclear luminosity can be converted to bolometric luminosity via the McLure & Dunlop (2004) quasar relation L_bol = 10 L_B.
- domain assumption Sources with ṁ < 0.01 are in the ADAF regime and the standard ADAF magnetic-pressure scaling applies.
read the original abstract
We study a sample of 289 Fanaroff-Riley type I (FR I) radio galaxies selected from the LOFAR Two-Metre Sky Survey (LoTSS) DR1, identified by their edge-darkened radio morphologies. Using Sloan Digital Sky Survey (SDSS) DR17 optical photometry and spectroscopy, we derive Eddington-scaled accretion rates spanning -6.84 < log $\dot{m}$ < -0.87 (median $\approx$ -2.84). The vast majority of sources lie below $\dot{m}$ = 0.01, indicating that their central engines are well described by advection-dominated accretion flows (ADAFs). However, even for a rapidly spinning black hole with a = 0.95, the maximum jet power predicted by the Blandford-Znajek mechanism in the standard ADAF regime is lower than the observed jet power (estimated from 151 MHz radio luminosity) for approximately 70% of the sample. We demonstrate that the magnetically arrested disc (MAD) scenario, in which large-scale poloidal magnetic flux accumulates near the event horizon, can fully account for the powerful jets observed in these low-accretion systems. Within the MAD framework, the data are consistent with slow-spinning black holes with $a < 0.5$. This large, uniformly selected LoTSS sample extends the MAD requirement previously established for the bright 3CR FR I population, indicating that magnetically arrested discs are common in FR I radio galaxies across a wide range of luminosities.
Figures
Reference graph
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discussion (0)
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