The Close AGN Reference Survey (CARS). Long-term spectral variability study of the changing look AGN Mrk 1018

A. L. Coil; A. Markowitz; F. Combes; G. Leung; J. S. Elford; M. Gaspari; M. Krumpe; M. Powell; N. Winkel; R. E. McElroy

REVIEW 1 major objections 1 minor 138 references

The Close AGN Reference Survey (CARS). Long-term spectral variability study of the changing look AGN Mrk 1018

T0 review · 1 major / 1 minor · reviewed 2026-08-07 · deepseek-v4-flash

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Pith's one-line read This paper argues that the changing-look AGN Mrk 1018 flipped its inner accretion flow from a geometrically-thin disk to an ADAF-dominated hot flow, with its warm corona disintegrating and an extended Fe Kα emitter still responding.

desk verdict Careful multi-epoch decomposition supports thin-disk-to-ADAF transition, but the Eddington-ratio axis of the softer-when-fainter claim needs a sensitivity analysis before the BHXRB analogy is quoted. read the letter →

arxiv 2506.03061 v1 pith:DFDGFMGY submitted 2025-06-03 astro-ph.HE

T. Saha , M. Krumpe , A. Markowitz , M. Powell , G. Leung , F. Combes , R. E. McElroy , J. S. Elford

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M. Gaspari N. Winkel A. L. Coil T. Urrutia

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classification astro-ph.HE

keywords changing-lookAGNaccretionstatetransitionADAFsoftX-rayexcessFeKαreverberationMrk1018Eddingtonratiospectralvariability

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

Mrk 1018 is a nearby supermassive black hole that dimmed sharply in the early 2010s, switching from optical type 1 to type 1.9. Drawing on two decades of X-ray, UV, and optical spectra, this paper establishes what changed inside the accretion flow: the UV continuum fell by a factor of roughly 24 while hard X-rays fell by only 8, the soft X-ray excess faded and was gone by 2021, and the hot-corona photon index began to show a 'softer-when-fainter' trend once the Eddington ratio dropped below $\log\lambda_{\rm Edd}\simeq -1.7$. The authors argue this is a genuine accretion-state switch: the inner geometrically-thin, optically-thick disk retreated and was replaced by a hot, radiatively inefficient flow (an ADAF), while the warm corona disintegrated or became energetically negligible. If this reading is right, Mrk 1018 is a direct example of a supermassive black hole undergoing the same kind of state transition seen in stellar-mass black hole X-ray binaries, and its still-fading Fe K$\alpha$ line provides a live probe of how parsec-scale gas responds to the central dimming.

What carries the argument

The load-bearing machinery is the three-component X-ray spectral model $M3 = \mathrm{TBabs}\times(\mathrm{compTT}+\mathrm{zpowerlw}+\mathrm{uxclumpy\_reflect})$, which separates the warm Comptonization soft excess, the hot-corona power law, and distant torus reflection with its Fe K$\alpha$ line. This decomposition lets the authors track each component's flux independently across XMM-Newton, Chandra, Suzaku, and Swift spectra. The broadband picture is carried by the agnsed thermal Comptonization disk model fitted jointly to optical/UV and X-ray data, and by a modified reverse sigmoid function that quantifies the long-term flux drop and its timescale. For the Fe K$\alpha$ response, a reverberation model of a bi-conical reprocessor computes the delayed line response to the ionizing continuum and returns an outer radius $R_{\rm out}\sim10$ pc for the line-emitting region. These components are what convert raw flux changes into the claimed accretion-state switch.

What would settle it

A decisive observation would be a decade-long, high-signal X-ray monitoring campaign on Mrk 1018 that tracks the Fe K$\alpha$ line flux: if the line has not continued declining toward the faint-state equilibrium value (roughly one sixth of its bright-state flux) within that time, the 10 pc extended-emitter interpretation is wrong. A complementary check is a sensitive faint-state radio-to-IR search for ADAF synchrotron emission, whose presence would directly contradict the assumption that the faint-state SED is complete without such components and would shift the Eddington ratios on which the state-switch claim depends.

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Extended reading notes

Core claim

On the paper's own terms, the central claim is that the bright-to-faint transition of Mrk 1018 was a change in accretion state rather than a change in obscuration or a simple flux dimming. Spectral decomposition into a warm Comptonizing soft excess, a hot-corona power law, and torus reflection shows that each component dropped by a different factor—$R_{\rm UV}\sim24$ for the UV band, $R_{\rm HX}\sim8$ for 2–10 keV hard X-rays, $R_{\rm SX}\sim12$ for the soft excess, and $R_{\rm FeK\alpha}\sim3.4$ for the iron line—so the broadband spectrum hardened. In the faint state, with $\log\lambda_{\rm Edd}<-1.7$, the photon index $\Gamma$ anticorrelates with Eddington ratio ($\Gamma=(-0.45\pm0.09)\log\lambda_{\rm Edd}+(0.79\pm0.16)$), a 'softer-when-fainter' behavior like that in black hole X-ray binaries and low-luminosity AGN. The paper interprets this as the inner flow switching from a geometrically-thin, optically-thick disk to an ADAF-dominated hot flow: the warm corona disintegrates or becomes energetically negligible by 2021, while the hot X-ray-emitting flow becomes energetically dominant. The narrow Fe K$\alpha$ line has dropped by only half the factor of its ionizing continuum, which the authors attribute to an emitting region extending to roughly 10 pc that has not yet had time to respond.

Load-bearing premise

The interpretation rests on converting measured luminosities into Eddington ratios with one X-ray bolometric correction applied in both states and on modeling the faint state without the synchrotron, cyclotron, and bremsstrahlung emission an ADAF is expected to produce; if those unmodeled components contribute substantially to the X-ray flux, or if the bolometric correction differs in the faint state, the derived $\lambda_{\rm Edd}$ values, the $-1.7$ threshold, and the claimed 'softer-when-fainter' trend would shift.

Editorial extensions

If this is right

Mrk 1018 becomes an observational precedent: a supermassive black hole whose long-term spectral transition is read as the same thin-disk-to-ADAF state switch invoked for stellar-mass black hole X-ray binaries.
The disappearance of the soft X-ray excess by 2021 connects warm-corona disintegration to low Eddington ratio, supporting models in which the radiatively efficient inner flow and its warm corona vanish below $\lambda_{\rm Edd}\simeq0.02$.
The Fe K$\alpha$ line should keep fading for roughly another decade, approaching the faint-state equilibrium set by the ionizing continuum drop.
If the source re-brightens, the UV band should increase more strongly than the X-ray band, because the hot flow's increased covering fraction or additional ADAF emission would partially compensate the X-rays.
The measured transition timescale ($t_{\rm sc}\sim1$ year) and the 30-year bright phase match the thermal and viscous timescales of a geometrically thick disk, supporting an intrinsic accretion-disk instability as the trigger.

Reading between the lines

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

If the faint-state ADAF contributes significant synchrotron, cyclotron, or bremsstrahlung emission, the quoted Eddington ratios and the $\log\lambda_{\rm Edd}<-1.7$ threshold would shift; the paper itself flags this as a caveat.
The same component decomposition could be applied to other recurrent changing-look AGN to test whether the warm-corona/ADAF switch is a universal sequence rather than a peculiarity of Mrk 1018.
A testable extension is to monitor Mrk 1018's Fe K$\alpha$ flux over the next decade: if it does not decline toward the predicted faint-state value, the assumed 10 pc extent or the single-scattering reverberation model would need revision.

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Editorial analysis

A structured set of objections, weighed in public.

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

Desk Editor's Note

read the letter

Plain take: this is the most careful multi-epoch spectral decomposition of Mrk 1018 to date, and the component-resolved behavior—soft excess vanishing by 2021, R_UV ≈ 3 R_HX, Fe Kα lag—makes a genuinely new contribution. The qualitative accretion-state switch story survives scrutiny; the quantitative Eddington-ratio axis does not, and the abstract slightly oversells it.

What's new and good: The X-ray decomposition with warm Comptonization soft excess + hot powerlaw + uxclumpy reflection is well executed, with Bayesian evidence and proper handling of pileup (readout streak for C1, annular extraction for Swift). The authors go beyond Lyu et al. (2021) by resolving the soft excess flux evolution and showing its parameters (kT, tau) stay roughly constant while its luminosity collapses, then disappears. The UV vs hard X-ray drop factor mismatch (24 vs 8) is measured carefully, including host-galaxy subtraction and a non-parametric correction for variable Swift standard stars. The Fe Kα reverberation argument is the most interesting piece: R_Fe ~3.4 vs R_ion ~6.8, with a simple bi-cone reprocessor yielding R_out ~10 pc and a concrete prediction that the line will continue to fade over ~9 years. Testable, falsifiable, honest.

Soft spots: The 'softer-when-fainter' Γ–λ_Edd relation and the log λ_Edd < −1.7 threshold rest on λ_Edd values computed with Duras et al. (2020) bolometric corrections, calibrated for radiatively efficient AGN. The authors themselves flag in §3.5 that the faint state may be ADAF-dominated with synchrotron/cyclotron/bremsstrahlung not in the model. The stress-test note is on point: Table 3 and Table 4 λ_Edd values differ by 0.2–0.35 dex, and the offset is not constant, so the slope/intercept of Eq. 2 and the −1.7 boundary could shift if agnsed-based values were used. The Chandra points, which drive much of the faint-state correlation, have no simultaneous UV and rely entirely on the X-ray-based Duras correction. This doesn't kill the qualitative picture—the drop factors, soft excess disappearance, and Fe Kα lag are independent of the bolometric correction—but it does mean the specific BHXRB/LLAGN analogy with a fitted slope is on shaky quantitative ground. The 'half the factor' Fe Kα claim is also 1–2σ, so the abstract is optimistic. The 9-year prediction inherits the simplified reprocessing geometry, though caveats are explicit.

Bottom line: worth a serious referee. I'd accept it with a request for a sensitivity analysis: re-derive Eq. 2 using the Table 4 (agnsed) λ_Edd values and show whether the trend and threshold survive. The paper is a strong archive study and a benchmark for Mrk 1018 regardless of how that exercise turns out.

Referee Report

1 major / 1 minor

Summary. The paper presents a long-term (2005–2021) multi-wavelength spectral variability study of the changing-look AGN Mrk 1018. Archival X-ray spectra from XMM-Newton, Chandra, Suzaku, and Swift are fitted with a physically motivated three-component model (warm Comptonization soft excess, hot-corona power law, and torus reflection), and broadband optical-to-X-ray SEDs are fitted with the agnsed model. The authors report that during the bright-to-faint transition the UV flux dropped by a factor of roughly 24, the 2–10 keV X-ray flux by roughly 8, and the soft excess disappeared by 2021. They identify a 'softer-when-fainter' trend in the hot-corona photon index versus Eddington ratio below log λ_Edd ≈ −1.7 and interpret the transition as a change from a geometrically thin disk plus warm corona to an ADAF-dominated hot flow. The Fe Kα line is found to have dropped by less than its driving continuum, which they model as delayed response from a parsec-scale reprocessor with an outer radius of about 10 pc, and they predict the line will take roughly another 9 years to reach equilibrium.

Significance. If the main conclusions hold, Mrk 1018 would be one of the best-documented cases of an accretion-state switch in a changing-look AGN, with measured component drop factors, a vanished soft excess, and a concrete, falsifiable prediction for future Fe Kα monitoring. The paper's strengths include the systematic use of Bayesian model comparison (BXA) across all high signal-to-noise spectra, explicit reporting of 90% posterior intervals, treatment of pileup in the bright Chandra observation via the readout streak, and several honest caveats about model limitations, such as the absence of ADAF-specific radiative processes in the SED model. The analysis is empirical rather than circular: the spectral decomposition and the variability ratios are fits to independent data, and the team's prior Mrk 1018 papers are used only to supply outburst history and host-galaxy constraints. The main risk is that the quantitative anchor of the BHXRB/LLAGN analogy—the Eddington-ratio scale in Fig. 7—depends on a bolometric correction that is calibrated for radiatively efficient AGN but is applied to a state that the paper itself argues may be ADAF-dominated.

major comments (1)

[§4.5] The timescale argument that identifies the viscous timescale with t_bright uses α = 0.01 and H/R = 0.001–0.2 as illustrative parameters and reaches a range of 10–30 years versus a measured t_bright of roughly 30 years. This is reasonable as a plausibility argument, but it is presented with enough certainty in Section 5 that a reader could mistake an order-of-magnitude estimate for a constraint on the disk structure. I suggest adding one sentence clarifying that the quoted values are illustrative and that no unique disk model is being claimed.

minor comments (1)

[§4.1] There is a duplicated article in 'Meanwhile, the the hot corona photon index Γ'; the second 'the' should be deleted.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the spectral decomposition, flux-drop factors, and Fe Kα response are empirical fits independent of the concluded ADAF transition.

full rationale

The central results are measured quantities from independent spectral decompositions: Γ is the power-law photon index from M3 fits; λEdd in Table 3 is computed from the observed 2–10 keV flux using the external Duras et al. (2020) bolometric correction; the drop factors and Fe Kα fluxes come from direct model fitting to the data. The 'softer-when-fainter' correlation (Eq. 2) is a fit to these measured values, not a quantity defined to equal the inputs. The ADAF/thin-disk interpretation is an external-model comparison (Esin et al. 1997; Noda & Done 2018) applied to the measured spectral changes. The paper explicitly cautions in §3.5 that agnsed is approximate in the faint ADAF-like state and may miss synchrotron/cyclotron/bremsstrahlung; this is a stated modeling caveat that could affect λEdd, but it is not an instance of the conclusion being built into the input. The Fe Kα reverberation analysis fits Rout to the observed line light curve and predicts a ~9 yr settling time from the fitted response; the 'not fully responded' statement follows directly from the measured RFe≈3.4 vs Rion≈6.8 mismatch. The internal λEdd differences between Tables 3 and 4 are model/systematic uncertainties in the Eddington-ratio scale, not a circular construction. Self-citations to Krumpe et al. (2017), Brogan et al. (2023), and McElroy et al. (2016) provide observational support (outburst light curves, standard-star calibration, black-hole mass) that is external to this paper's fitted values and does not assume the ADAF conclusion. No load-bearing step reduces by construction to its own inputs.

Assumptions & free parameters 6 free parameters · 7 assumptions · 0 invented entities

The central claims rest on standard AGN spectral models, a phenomenological variability function, an adopted black hole mass, and a bolometric correction. No entities beyond standard AGN components (warm corona, hot corona, ADAF, torus) are introduced. The main fitted quantities feeding the claims are the MRS drop factors, the Γ-λEdd regression coefficients, and the Fe Kα reprocessor radius.

free parameters (6)

MRS X-ray drop factor R_HX = 7.9 ± 0.5
Fitted to the 2-10 keV light curve with a modified reverse sigmoid; directly feeds the claimed hard X-ray drop factor of ~8 (Sect. 3.6).
MRS UV drop factor R_UVM2 = 28 ± 2
Fitted to the UVM2 light curve; supports the claimed ~24x UV drop in Table 5.
Γ-λEdd slope = -0.45 ± 0.09
Linear regression of photon index on log λEdd in the faint state, Eq. 2; underpins the 'softer-when-fainter' claim.
Γ-λEdd intercept = 0.79 ± 0.16
Intercept of Eq. 2; quoted with the slope to define the trend.
Fe Kα reprocessor outer radius Rout = 10 +17/-6 pc
Fitted to the Fe Kα light curve with a single-scattering reverberation model; drives the ~9 year response-time prediction.
XMM5 soft-excess frozen parameters (kBTe, τ) = 0.17 keV, 14 (frozen to XMM4 values)
Chosen to compute the 2021 soft-excess upper limit; affects the 'soft excess not detected by 2021' claim.

assumptions (7)

domain assumption Black hole mass MBH = 10^7.9 M_sun (McElroy et al. 2016) is adopted throughout.
Used for Eddington ratios, radii, and variability timescales; if wrong by a factor of a few, the quantitative interpretation shifts.
domain assumption The three-component X-ray model M3 (compTT + zpowerlw + uxclumpy) correctly isolates soft excess, hot corona, and torus reflection.
The entire component-resolved variability analysis depends on this decomposition.
ad hoc to paper The modified reverse sigmoid (MRS) functional form describes the long-term flux drop.
A phenomenological choice; sparse 2014-2016 sampling means t0 and tsc are uncertain.
domain assumption The Duras et al. (2020) bolometric correction applies in both bright and faint states.
Faint state may be ADAF-dominated with unmodeled radiative contributions; the authors note this caveat in Sect. 3.5.
ad hoc to paper The Fe Kα reverberation geometry (biconical reprocessor, Rin=0.01 pc, θo=45°, single scattering) is adequate.
Rout=10 pc and the ~9 year lag are derived from this geometry; the authors list caveats (a)-(d) in Sect. 4.6.2.
domain assumption agnsed with a=0, kBTe=200 keV, hx=10 Rg describes the broadband SED.
Used to derive λEdd and component luminosities; faint state is an approximate application.
ad hoc to paper Timescale estimates use α=0.01 and H/R between 0.001 and 0.2 as illustrative disk parameters.
Sect. 4.5 uses these to match tsc and tbright; they are not measured.

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Cite this review

Pith. "Pith review of The Close AGN Reference Survey (CARS). Long-term spectral variability study of the changing look AGN Mrk 1018." pith.science (2026). https://pith.science/paper/DFDGFMGY

@misc{pith2026250603061,
  author       = {Pith},
  title        = {Pith review of: The Close AGN Reference Survey (CARS). Long-term spectral variability study of the changing look AGN Mrk 1018},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/DFDGFMGY}},
  note         = {Machine review of arXiv:2506.03061}
}

read the original abstract

Changing-look AGNs (CLAGN) are accreting supermassive black hole systems that undergo variations in optical spectral type, driven by major changes in accretion rate. Mrk 1018 has undergone two transitions, a brightening event in the 1980s and a transition back to a faint state over the course of 2-3 years in the early 2010s. We characterize the evolving physical properties of the source's inner accretion flow, particularly during the bright-to-faint transition, as well as the morphological properties of its parsec-scale circumnuclear gas. We model archival X-ray spectra from XMM-Newton, Chandra, Suzaku, and Swift, using physically-motivated models to characterize X-ray spectral variations and track Fe Kalpha line flux. We also quantify Mrk 1018's long-term multi-wavelength spectral variability from optical/UV to the X-rays. Over the duration of the bright-to-faint transition, the UV and hard X-ray flux fell by differing factors, roughly 24 and 8, respectively. The soft X-ray excess faded, and was not detected by 2021. In the faint state, when the Eddington ratio drops to log Lbol/LEdd < -1.7, the hot X-ray corona photon index shows a 'softer-when-fainter' trend, similar to that seen in some black hole X-ray binaries and samples of low-luminosity AGNs. Finally, the Fe Kalpha line flux has dropped by only half the factor of the drop in the X-ray continuum. The transition from the bright state to the faint state is consistent with the inner accretion flow transitioning from a geometrically-thin disk to an ADAF-dominated state, with the warm corona disintegrating or becoming energetically negligible, while the X-ray-emitting hot flow becoming energetically dominant. Meanwhile, narrow Fe Kalpha emission has not yet fully responded to the drop in its driving continuum, likely because its emitter extends up to roughly 10 pc.

Figures

Figures reproduced from arXiv: 2506.03061 by the authors.

**Figure 1.** XMM-Newton EPIC-pn pattern-0 data−model residuals from the X-ray spectral fitting using the models described in Sect. 3.1. Left: bright state observation XMM2 (2008); right: faint state observation XMM4 (2019). Significant improvement in the residuals can be seen in both of the multi-component models M2 and M3 compared to the single power-law model, M1. 0.5 1.0 2.0 5.0 10.0 Energy [keV] 10 6 10 5 10 4 10 3 10 2 10 1… view at source ↗

**Figure 2.** Unfolded best-fitting models to the XMM-Newton EPIC-pn pattern 0 and Suzaku XIS-0 data. The spectral model is the best-fitting, physically motivated model M3 (Sect. 3.1). Solid curves: model evaluated at best fit value; dotted curves: model evaluated at the upper limit; blue markers: unfolded data; red curve: soft X-ray excess modeled by compTT; magenta curve: hot-corona powerlaw; green curve: uxclumpy torus reflect… view at source ↗

**Figure 3.** Overview of UV and X-ray spectral components. (a) de-reddened and host-galaxy subtracted UVM2 flux from combined XMM-Newton OM and Swift UVOT data. (b) integrated total hard X-ray flux. (c) photon index (Γ) of the hot-corona power law (zpowerlw in M3; Sect. 3.1). (d) integrated flux of the soft excess flux component (0.3–2.0 keV) (e) integrated flux of the reflection component (2.0–10.0 keV) (f) integrated Fe Kα emi… view at source ↗

Figures from the paper (8 more)

**Figure 4.** Figure 4: agnsed broadband SED fits. (a) The absorbed total broadband model. The circular markers indicate XMM-Newton EPIC-pn+OM datasets for the XMM3, and XMM5 observations and XMM-Newton EPIC-pn + OM + Swift-UVOT datasets for XMM2. The solid lines of corresponding color repres…

**Figure 5.** Figure 5: (a) UVM2 light-curve data and best-fit MRS function normalized by the faint state flux (b) corresponding UVM2 residual (c) 2-10 keV X-ray light-curve data and best-fit normalized by the faint state flux(d) corresponding X-ray residual. The circular, diamond, triangular…

**Figure 6.** Figure 6: Energy resolved bright and faint state flux ratio. The solid black line is the ratio of the average X-ray spectra (R(E)) in the bright and the faint phase and the dashed lines demarcate the upper and lower error. The plot is based on the M3 model compTT + zpowerlw + ux…

**Figure 7.** Figure 7: Evolution of photon index (Γ) of the intrinsic hot corona power law with respect to Eddington ratio (λedd = Lbol/Ledd). The different colors represent different missions: green–Chandra, red–Chandra observation from 2010 evaluated from readout streak, blue–XMM-Newton, …

**Figure 8.** Figure 8: Illustration of one possible geometrical change in AGN accretion structures due to the major CLAGN transition occurring after 2013. Length scales as illustrated here are approximate only. (a) bright state configuration: warm corona present in the inner accretion struct…

**Figure 9.** Figure 9: Response of an extended parsec scale structure with inner radius of Rin = 0.01 pc line response and outer radius Rout = 10 pc. The dashed lines correspond to the upper and lower errors of Rout. Inset: reducedχ 2 from the Fe Kα light-curve fit best estimate of external…

**Figure 11.** Figure 11: Time evolution of Fe Kα equivalent width, which exhibits a mild increase with time. Black circular markers: XMM-Newton; blue diamond marker: Suzaku; unfilled green square marker: Chandra from 2016 (C2); green square markers: all other Chandra spectra. 4.6.3. Equivalen…

**Figure 10.** Figure 10: Spectral fits to the Fe Kα line from C2 (2016), XMM3 (2018), XMM4 (2019), and XMM5 (2021) (datasets are grouped for clarity). The dashed lines indicate the profile corresponding to the upper and lower errorbars of σFeKα. The spectra here are normalized using the unde…

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Pith tools

Reviewed August 7, 2026 · model on record in the stance chip above.

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, " * write output.state after.block = add.period write newline

ENTRY address archiveprefix author booktitle chapter edition editor howpublished institution eprint journal key month note number organization pages publisher school series title type volume year label extra.label sort.label short.list INTEGERS output.state before.all mid.sentence after.sentence after.block FUNCTION init.state.consts #0 'before.all := #1 ...

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write newline

" write newline "" before.all 'output.state := FUNCTION n.dashify 't := "" t empty not t #1 #1 substring "-" = t #1 #2 substring "--" = not "--" * t #2 global.max substring 't := t #1 #1 substring "-" = "-" * t #2 global.max substring 't := while if t #1 #1 substring * t #2 global.max substring 't := if while FUNCTION word.in bbl.in " " * FUNCTION format....

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