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Changing Look AGN: An X-ray Look

T0 review · 5 major / 5 minor · reviewed 2026-08-12 · deepseek-v4-flash

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Pith's one-line read This paper argues that the changing-look phenomenon, and the difference between Seyfert 1 and Seyfert 2 galaxies, reduces to a single variable: the ratio of X-ray luminosity to Eddington luminosity.

desk verdict A data-rich AGN spectral study whose central single-parameter claim rests on mass estimates that do not survive scrutiny. read the letter →

arxiv 2411.09781 v1 pith:FMKFUBZS submitted 2024-11-14 astro-ph.GA astro-ph.HE

Lev Titarchuk , Elena Seifina , Yegor Mishin This is my paper · ORCID
classification astro-ph.GAastro-ph.HE
keywords changing-lookAGNSeyfert12ComptonizationblackholescalingphotonindexsaturationNGC1566
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

This paper tries to establish that the bewildering variety of active galactic nuclei (AGNs) — Seyfert 1, Seyfert 2, and changing-look objects that flip between them — is not a set of fundamentally different classes but a single continuum of accretion states. The controlling variable, the authors argue, is the ratio of the AGN's X-ray luminosity to its Eddington luminosity, not the inclination of the system or other geometric parameters. The case is built on X-ray spectra of the nearby changing-look galaxy NGC 1566, compared to a type 1 Seyfert (1H 0707–495) and two type 2 Seyferts (NGC 7679 and Mrk 3), all fitted with a Comptonization model. If the claim is right, the long-standing Sy1/Sy2 dichotomy and the puzzling changing-look phenomenon become predictable consequences of where a source sits on one Eddington-scaled luminosity track.

What carries the argument

The central machinery is the BMC (bulk-motion Comptonization) spectral model, a generalized Comptonization model in which the observed X-ray spectrum is a sum of a blackbody-like seed component plus a Comptonized component parameterized by the photon index Γ (= α+1), seed photon temperature kTs, normalization N (proportional to the mass accretion rate), and the Comptonized fraction f. The argument also relies on the index-saturation scaling method: the correlation between Γ and normalization N rises and then saturates at a characteristic level, and scaling the on-set normalization N_t of a target source against reference sources with known masses, distances, and inclinations via s_N = N_r/N_t = (m_r/m_t)($d_t^{2}$/$d_r^{2}$) f_G yields the target black hole mass. Together, the saturated photon index and the derived Lx/LEdd place sources on a classification diagram (Fig. 19) that separates Sy1 and Sy2 regions and shows the changing-look source spanning both.

What would settle it

Observe the next outburst of a changing-look AGN with simultaneous X-ray and optical spectroscopy: if the optical type changes at a different Eddington-scaled luminosity than the ≈3.5×$10^{-4}$ value that separates Sy1 from Sy2 in this paper, the claim that one parameter controls the behavior would be contradicted.

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Extended reading notes

Core claim

Using the bulk-motion Comptonization (BMC) model, the paper shows that the X-ray spectra of NGC 1566 across its changing-look transitions are described by the same Comptonized continuum plus iron-line components used for Sy1 and Sy2 galaxies, with the photon index evolving through low/hard, intermediate, and high/soft states. During outbursts the photon index saturates at a source-specific value (Γ ≈ 2.1 for NGC 1566, ≈ 3 for 1H 0707, ≈ 1.9 for NGC 7679 and Mrk 3), and applying the index-saturation scaling method to these saturation levels yields black hole masses (≈2×$10^{5}$ M_⊙ for NGC 1566, ≈6.8×$10^{7}$ M_⊙ for 1H 0707, ≈8.4×$10^{6}$ M_⊙ for NGC 7679, ≈2.2×$10^{8}$ M_⊙ for Mrk 3). The paper's central discovery is that NGC 1566 behaves like a Sy1 at high Eddington-scaled luminosity and like a Sy2 at low Eddington-scaled luminosity, and that a single variable — Lx/LEdd — can reproduce the range of behavior seen across CL, Sy1, and Sy2 AGNs without invoking inclination differences. The authors conclude that the distinction between these subclasses is therefore blurred, and they note that the X-ray mass of NGC 1566 is one to two orders of magnitude below the optical mass, which they interpret as evidence for a secondary, lower-mass black hole driving the changing-look activity.

Load-bearing premise

The scaling method assumes that the target source and its reference sources saturate at the same photon index and that the normalization at the onset of that saturation can be read reliably from the Γ–N plot; if those saturation levels differ or the onset is misidentified, every derived black hole mass and every Lx/LEdd value in the classification diagram changes.

Editorial extensions

If this is right

  • If the single-parameter picture is correct, Seyfert 1 and Seyfert 2 galaxies are not intrinsically different objects; their classification is determined by where they sit on the Lx/LEdd track, so the same object can appear as Sy1 at high luminosity and Sy2 at low luminosity.
  • The photon-index saturation effect becomes a practical black-hole weighing tool: measuring the saturation level of Γ for an AGN gives its mass by comparison with calibrated reference sources, without needing reverberation mapping.
  • The changing-look phenomenon in NGC 1566 and similar galaxies should be predictable from the observed Eddington-scaled luminosity; an outburst that pushes Lx/LEdd across the critical value ≈3.5×10^-4 should flip the optical classification.
  • The X-ray mass estimate for NGC 1566 predicts the presence of a secondary black hole of ~2×10^5 M_⊙ in its center, a binary interpretation that could be tested by searching for periodic variability.

Reading between the lines

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

  • If the single-parameter claim holds, the scaling method could be applied to other changing-look AGNs to map out the Lx/LEdd threshold between Sy1 and Sy2 behavior, effectively turning the classification diagram into a predictive phase diagram.
  • A testable extension would be to check whether the photon index saturation level for an AGN is independent of the X-ray band used; if saturation levels shift with band, the mass estimates would be biased.
  • The paper's conclusion about inclination has a direct observational consequence: two Seyfert 2 galaxies with identical Lx/LEdd but very different inclinations should show identical Comptonized continuum shapes, which can be measured with current X-ray observatories.
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Editorial analysis

A structured set of objections, weighed in public.

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

Referee Report

5 major / 5 minor

Summary. The paper presents an X-ray spectral study of the changing-look AGN NGC 1566, comparing it with the Seyfert 1 galaxy 1H 0707–495 and the Seyfert 2 galaxies NGC 7679 and Mrk 3 using public Swift, XMM-Newton, NuSTAR, Suzaku, ASCA, BeppoSAX, and RXTE data. All spectra are fitted with a Comptonization (BMC) model plus iron-line components, and the authors identify saturation of the photon index during flares. Scaling this saturation behavior against Galactic and extragalactic reference sources yields black hole masses for the four targets, most notably M_NGC1566 ~ 1.9e5 M_sun, two orders of magnitude below the optical estimate. On this basis the paper concludes that the diversity of CL, Sy1, and Sy2 AGN behavior can be explained by a single parameter, Lx/LEdd, without invoking inclination differences.

Significance. If the mass estimates were reliable, the paper would provide an interesting unification of changing-look and Seyfert subclasses in terms of Eddington-normalized X-ray luminosity, and the analogy with X-ray binary spectral states would be a useful framework. The paper's strengths are its use of public multi-mission data, the systematic application of a uniform Comptonization model, and the explicit tabulation of spectral parameters for many epochs. However, the central claim depends entirely on the scaling-method masses, and those masses rest on assumptions that are not validated and on internal inconsistencies. No independent calibration, machine-checkable derivation, or reproducibility package is provided. The headline conclusion is therefore not supported by the analysis as presented.

major comments (5)
  1. [Section 3.5, Eqs. (3)-(9)] The entire mass-determination chain rests on N_t values that are chosen 'at the beginning of the Γ-saturation part' by inspection, with no objective algorithm and no systematic uncertainty. Equation (6) propagates only the mr and dr errors, ignoring the uncertainty in N_t and in fG, even though Eq. (3) shows that mt is inversely proportional to N_t. A 10-30% error in N_t would change all four masses and consequently every Lx/LEdd value in Fig. 19. The quoted 20-27% errors in Tables 5 and 6 therefore do not represent the actual accuracy of the method.
  2. [Section 3.5.4 and Table 5] For 1H 0707-495 the reference sources ESO 243-49, M101 ULX-1, OJ 287, and SDSS J0752 have masses taken from the authors' own scaling-method papers (Titarchuk & Seifina 2016a,b; Titarchuk et al. 2023). Using these as references to derive the target mass does not provide an independent test of the scaling method, and it makes the 1H 0707 mass estimate circular in the sense that the method is calibrated on its own outputs. The large dispersion between references (±45% per reference in Table 5) reinforces the concern that the references do not share a common, independently established mass scale.
  3. [Section 3.5.1 and Table 6] The inclination used for NGC 1566 is internally inconsistent: Section 3.5.1 states it = 60°, while Table 6 adopts it = 37.5° in the final estimate. Since fG = cos ir / cos it enters Eq. (3), this difference shifts mt by roughly 60%, with no justification or sensitivity test provided. A similar issue affects the other target sources, for which trial inclinations are not given in the text of Sections 3.5.2 and 3.5.3.
  4. [Sections 4 and 5, NGC 1566 mass] The X-ray scaling mass for NGC 1566 (1.9e5 M_sun) is two orders of magnitude below the optical mass quoted in Table 1 (0.8-1.3e7 M_sun). Rather than treating this as a failure of the scaling assumptions, the paper invokes a secondary, lower-mass BH in a binary system. That entity is not required by any direct observation presented in the paper and is used to reconcile the mass estimate rather than to test it. Because the Lx/LEdd classification in Fig. 19 and the single-parameter conclusion in Section 5 are built on these masses, the central claim is not established.
  5. [Section 5 and Fig. 19] The conclusion that CL, Sy1, and Sy2 AGN diversity can be explained by a single variable 'without any need for additional differences in Sy AGN parameters, such as its inclination' is not supported by the analysis. The scaling formula itself includes inclination through fG, and no test is shown that inclination does not affect the placement of sources in the Γ versus Lx/LEdd diagram. The paper demonstrates only that a particular model with a particular set of derived masses can be organized this way, not that inclination is physically irrelevant.
minor comments (5)
  1. [Section 3.5.4] The text uses z0707 = 0.004 in Eq. (12), while Table 1 lists z = 0.041 for 1H 0707-495; this inconsistency changes the distance and therefore the mass estimate and must be corrected.
  2. [Section 3.5.1] The text refers to 'NGC 1655' when the target is NGC 1566.
  3. [Fig. 12 caption] The caption contains 'Suzanne' (likely Suzaku) and a duplicated 'IS (ID=707002010, IS (ID=00014923002' phrase; both should be fixed.
  4. [Abstract and Section 3.5.1] The abstract quotes M1566 ~ 2e5 M_sun while Section 3.5.1 gives 1.9e5 M_sun; the rounding should be consistent.
  5. [Tables 5 and 6] The tables repeat the same target mass for every reference source, which can mislead a reader into thinking each reference independently determines mt; the text should state explicitly that these are identical results of one scaling calculation with different references, and the reported uncertainty is only the dispersion of the mean.

Circularity Check

2 steps flagged · score 6.0 of 10

The central Lx/LEdd classification is built on scaling-method BH masses whose reference masses are themselves outputs of the same authors' prior scaling papers, making the 'single parameter' conclusion partly manufactured by the method.

  1. self citation load bearing [Section 3.5.4, Table 5 (BH mass scaling for 1H 0707–495)]
    "We found that SDSS J0752, OJ 287, M101 ULX–1 and ESO 243 HLX–1 can be used as the reference sources because these sources met all aforementioned requirements to estimate a BH mass of the target source 1H 0707. Table 5 footnotes: (1) Titarchuk and Seifina (2016b); (2) Titarchuk and Seifina (2016a); and (3) Titarchuk et al. (2023)."

    The reference masses mr inserted into Eq. (4) (mt = fG mr/sN dt^2/dr^2) come from the same authors' own scaling-method papers, not from independent dynamical or reverberation calibrators. The target mass M_0707 ~ 6.8e7 Msun is therefore a ratio of scaling-method outputs, with the method's saturation assumption built in at both ends. This mass is then used to compute Lx/LEdd for Fig. 19, so the paper's claim that Lx/LEdd alone separates Sy1, Sy2, and CL-AGN is not an independent test but inherits the self-referential calibration.

  2. fitted input called prediction [Section 3.5.1, Eqs. (3)-(4), Figure 14]
    "A value of Nt = 1.04×10^-4, Nr in units of L39/d10^2 is determined in the beginning of the Γ-saturation part (see Fig. 6, ST07, ST09, Titarchuk et al. (2014); Titarchuk and Seifina (2016a,b, 2009)). ... mt = fG mr/sN dt^2/dr^2."

    The target normalization Nt is read by inspection from the Γ versus N plot at the assumed onset of saturation, and Eq. (4) converts that fitted Nt into the BH mass M_1566 ~ 1.9e5 Msun. The quoted 20% uncertainty is only the dispersion among reference sources, not the error on Nt or on the same-saturation assumption. This fitted mass is then used to place NGC 1566 on the Lx/LEdd diagram, so the central 'single variable parameter' conclusion depends directly on an unvalidated, eyeballed input rather than on an independent prediction.

full rationale

The paper's headline conclusion is that CL, Sy1, and Sy2 diversity follows from a single parameter, Lx/LEdd, with no need for inclination differences. That conclusion is read off Fig. 19, whose x-axis is computed from the scaling-method BH masses in Tables 5 and 6. The scaling method's reference masses for 1H 0707 are taken from the authors' own prior scaling papers (Titarchuk & Seifina 2016a,b; Titarchuk et al. 2023), so the 1H 0707 mass is calibrated on the same technique rather than on independent measurements. For NGC 1566, the mass is a direct transformation of an Nt value selected by eye at the 'beginning of the Γ-saturation part,' and the resulting M ~ 1.9e5 Msun conflicts with the optical mass by two orders of magnitude, prompting an unobserved binary companion interpretation rather than a re-examination of the scaling assumptions. The inclination enters the scaling factor fG = cos ir/cos it, and the paper is internally inconsistent about the target inclination (60° in Section 3.5.1 versus 37.5° in Table 6), so the claim that inclination is unnecessary is not demonstrated. These are genuine load-bearing circularities: the classification inherits the method's fitted inputs and self-citations. However, the spectral fits, the measured photon indices, and the comparison with independent literature masses give the paper some empirical content beyond pure circularity, so a score of 6 rather than 8 or 10 is appropriate.

Assumptions & free parameters 3 free parameters · 4 assumptions · 2 invented entities

The central claims rest on a chain of fitted or hand-chosen parameters (Nt, inclinations, fixed NH), on the uncalibrated validity of the BMC scaling relation, and on the assumption that reference masses from the authors' own prior applications of the same method are reliable. The binary-black-hole explanation for NGC 1566 is an invented entity without independent evidence.

free parameters (3)
  • Target BMC normalization at saturation onset Nt = 1.04e-4 (NGC1566), 5e-4 (NGC7679), 0.1 (Mrk3), 3.5e-3 (1H0707)
    Chosen by eye at the beginning of the gamma-saturation branch in the gamma-N plots (Sect. 3.5, Figs. 14, 16); enters Eq. (3) linearly and sets the mass scale.
  • Target inclination it = 60 deg (NGC1566), 55 deg (1H0707), 30 deg (NGC7679), 50 deg (Mrk3)
    Trial inclinations used to compute fG = cos ir / cos it in Eq. (3), which scales the mass estimate.
  • Fixed absorbing column NH = 2.5e20 (NGC1566), 3e21 (1H0707), 4e21 (NGC7679)
    Fixed values in the tbabs*(BMC+Gaussian/Laor) fits (Table 4 footnote); the choice affects the best-fit photon index and normalization that feed the gamma-N scaling.
assumptions (4)
  • domain assumption The BMC Comptonization model provides a physically correct description of the X-ray spectra of these AGNs across all states.
    Invoked in Sect. 3.3.1; the model is the authors' own generalized Comptonization model and is assumed to yield true spectral indices and normalizations.
  • domain assumption The scaling relation sN = Nr/Nt = (mr/mt)(dt^2/dr^2) fG (Eq. 3) correctly converts normalization ratios into black hole masses.
    Taken from Shaposhnikov and Titarchuk 2009 and earlier author papers; no independent calibration is provided in this paper.
  • ad hoc to paper The target and reference sources saturate at the same photon index gamma_sat, and the chosen Nt marks the same physical point on each track.
    Asserted in Sect. 3.5 and implemented in Figs. 14 and 16; the Nt values are selected by inspection, not by a formal criterion.
  • ad hoc to paper For 1H 0707, the masses and distances of the reference sources (ESO 243, M101 ULX-1, OJ 287, SDSS J0752) are reliable, including masses taken from the authors' own scaling-method papers.
    Table 5; three of four reference values come from Titarchuk and Seifina 2016a,b and Titarchuk et al. 2023, so the target mass is not independent of the method being used.
invented entities (2)
  • Secondary smaller black hole in NGC 1566
    purpose: Explains why the X-ray scaling mass (1.9e5 solar masses) is two orders below the optical mass (about 1e7 solar masses)
    Proposed in Sect. 4 as a 'duality' of the SMBH; no periodic signal, timing signature, or direct imaging evidence is provided.
  • Additional soft X-ray source in 1H 0707
    purpose: Explains why the scaling mass of 1H 0707 is higher than reverberation estimates
    Mentioned in Sect. 4 as possibly associated with star formation; no independent detection is presented.

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Cite this review

Pith. "Pith review of Changing Look AGN: An X-ray Look." pith.science (2026). https://pith.science/paper/FMKFUBZS

@misc{pith2026241109781,
  author       = {Pith},
  title        = {Pith review of: Changing Look AGN: An X-ray Look},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/FMKFUBZS}},
  note         = {Machine review of arXiv:2411.09781}
}
read the original abstract

To date, a number of changing-look (CL) active galactic nuclei (AGNs) are known. We studied, in detail what happens to the X-ray spectrum during the CL events using the example of the nearby CL Seyfert NGC1566, which was observed by Swift, NuSTAR, XMM-Newton, and Suzaku. We applied the Comptonization model to describe an evolution of NGC~1566 X-ray spectra during outbursts and compared these results with a typical behavior for other AGNs to identify some differences and common properties that can ultimately help us to better understand the physics of the CL phenomenon. We found that changes in the X-ray properties of NGC1566 are characterized by a different combination of Sy1 (using 1H0707-495 as a representative) and Sy2 properties (using NGC7679 and Mrk3 as their representatives). At high X-ray luminosities NGC1566 exhibits the behavior typical for Sy1, and at low luminosities we see a transition of NGC1566 from the Sy1 behavior to the Sy2 pattern. We revealed the saturation of the spectral indices, \=a for these four AGNs during outbursts (\=a_1566~1.1, \=a _0707~2, \=a _7679~0.9 and \=a_mrk3~0.9) and determined the masses of the black holes (BHs) in the centers of these AGNs namely, M_0707~6.8x10^7 M_sol, M_7679~8.4x10^6 M_sol, M_mrk3~2.2x10^8 M_sol and M_1566~2x10^5 M_sol, applying the scaling method. Our spectral analysis shows that the changing-look of NGC1566 from Sy1.2 to Sy1.9 in 2019 was accompanied by the transition of NGC1566 to an accretion regime which is typical for the intermediate and highly soft spectral states of other BHs. We also find that when going from Sy2 to Sy1, the spectrum of NGC1566 shows an increase in the soft excess accompanied by a decrease in the Comptonized fraction (0.1<f<0.5), which is consistent with the typical behavior of BH sources during X-ray outburst decay.

Figures

Figures reproduced from arXiv: 2411.09781 by the authors.

Figure 1
Figure 1. — Evolution of NGC 1566 during 2007–2023 observations with Swift/ [PITH_FULL_IMAGE:figures/full_fig_p043_1.png] view at source ↗
Figure 2
Figure 2. — X-ray image of NGC 1566 (in center), accumulated by [PITH_FULL_IMAGE:figures/full_fig_p044_2.png] view at source ↗
Figure 3
Figure 3. — X-ray image of 1H 0707–495 (2SXPS J070841.4-493306 – according to the [PITH_FULL_IMAGE:figures/full_fig_p045_3.png] view at source ↗
Figures from the paper (18 more)
Figure 4
Figure 4. Figure 4: — X-ray image of Mrk 3 (in center), accumulated by [PITH_FULL_IMAGE:figures/full_fig_p046_4.png]
Figure 5
Figure 5. Figure 5: — X-ray image of NGC 7679 (LSXPS J232846.7 [PITH_FULL_IMAGE:figures/full_fig_p047_5.png]
Figure 6
Figure 6. Figure 6: — From top to bottom: evolution of the Swift/ [PITH_FULL_IMAGE:figures/full_fig_p048_6.png]
Figure 7
Figure 7. Figure 7: — Evolution of the Swift/XRT count rate during 1996 – 2010 observations of 1H 0707–495 [PITH_FULL_IMAGE:figures/full_fig_p049_7.png]
Figure 8
Figure 8. Figure 8: — A suggested geometry for NGC 1566, 1H 0707, Mrk 3 and NGC 7679 sources. Disk [PITH_FULL_IMAGE:figures/full_fig_p050_8.png]
Figure 9
Figure 9. Figure 9: — From top to bottom: evolution of the Swift/ [PITH_FULL_IMAGE:figures/full_fig_p051_9.png]
Figure 10
Figure 10. Figure 10: — The distribution of NGC 7679 observations by [PITH_FULL_IMAGE:figures/full_fig_p052_10.png]
Figure 11
Figure 11. Figure 11: — From top to bottom: evolution of the RXTE/PCA count rate, fluxes in 3–10 keV (blue points) and 10–20 keV (crimson points) bands, Comptonized fraction f , and BMC normalization during 1997–1997 flare events of Mrk 3. In the last bottom panel, we present an evolution …
Figure 12
Figure 12. Figure 12: — Four representative spectra of NGC 1566 from [PITH_FULL_IMAGE:figures/full_fig_p054_12.png]
Figure 13
Figure 13. Figure 13: — Representative E ∗ F(E) spectral diagrams that are related to different spectral states for 1H 0707–495 (top left) using Suzaku observation 00091623 (black, LHS) combined with RXTE/HEXTE observation 20309010100 (black, LHS), ASCA observations 73043000 (brigh blue, I…
Figure 14
Figure 14. Figure 14: — Scaling of photon index Γ for NGC 1566 (with brown line for target source) and NGC 7679 (with violet line for target source) with NGC 4051, GX 339–4, GRO J1655–40, Cyg X– 1 and 4U 1543–47 as reference sources) [PITH_FULL_IMAGE:figures/full_fig_p056_14.png]
Figure 15
Figure 15. Figure 15: — The best-fit spectrum of 1H 0707–495 in [PITH_FULL_IMAGE:figures/full_fig_p057_15.png]
Figure 16
Figure 16. Figure 16: — Scaling of photon index Γ for 1H 0707–495 (with black line for target source) and SDSS J0752, OJ 287 and M101 ULX–1 (as reference sources) [PITH_FULL_IMAGE:figures/full_fig_p057_16.png]
Figure 17
Figure 17. Figure 17: — Three representative spectra of NGC 7679 with the best-fit modeling for the LHS [PITH_FULL_IMAGE:figures/full_fig_p058_17.png]
Figure 18
Figure 18. Figure 18: — Four representative spectra of Mrk 3 in units of [PITH_FULL_IMAGE:figures/full_fig_p058_18.png]
Figure 19
Figure 19. Figure 19: — Photon index, Γ plotted versus Lx/LEdd for Sy 1 (red box), Sy 2 (green box) and CL￾AGN (violet box). The grey dotted arrow indicates the critical value of Lx/LEdd, separating Sy1 and Sy2. It is evident that the CL-AGN box covers both the Sy1 and Sy2 AGN regions [PI…
Figure 20
Figure 20. Figure 20: — Photon index, Γ plotted versus a BH mass for Sy 1 (blue squares) and Sy 2 (red stars) AGNs [PITH_FULL_IMAGE:figures/full_fig_p059_20.png]
Figure 21
Figure 21. Figure 21: — Photon index, Γ plotted versus BMC normalization (which is proportional to M˙ ) for 1H 0707 (top left panel), NGC 1566 (top right panel), Mrk 3 (bottom left panel) and NGC 7679 (bottom right panel) [PITH_FULL_IMAGE:figures/full_fig_p060_21.png]

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