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arxiv: 2605.24429 · v1 · pith:5LOQAUT3new · submitted 2026-05-23 · 🌌 astro-ph.GA

Changing-look Active Galactic Nuclei from SDSS, LAMOST and DESI Survey

Guohai Chen , Wenxin Yang , Xuhong Ye , Zhiqiang Chen , Zhiyuan Pei , Hubing Xiao , Junhui Fan This is my paper

Pith reviewed 2026-06-30 13:23 UTC · model grok-4.3

classification 🌌 astro-ph.GA
keywords changing-look AGNsrepeating CLAGNsaccretion rateDESISDSSLAMOSTbroad emission linesAGN variability
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The pith

Repeating changing-look AGNs trace a high-low-high accretion-state cycle across three spectroscopic epochs.

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

The paper cross-matches SDSS and LAMOST spectra to find 45 changing-look AGNs and adds DESI spectra as a third epoch to identify 12 repeating sources. These repeating objects dominate the turn-off category and occupy a distinct high-low-high path in the black-hole-mass versus Eddington-ratio diagram. The pattern indicates that the line changes are tied to recurrent accretion-rate fluctuations rather than isolated events. Upper limits on the transition timescales fall around 10 years for the first change and 4 years for the second.

Core claim

We identify 45 CLAGNs (40 new), of which 12 are RCLAGNs when DESI supplies the third epoch. The RCLAGNs display a clear high-low-high accretion-state evolution in the log MBH - log(Lbol/LEdd) plane, supporting a direct connection between CL behavior and recurrent changes in accretion power. The sample contains 43 turn-off and only 2 turn-on events.

What carries the argument

Three-epoch spectroscopy tracking of RCLAGNs in the log MBH - log(Lbol/LEdd) plane to expose recurrent accretion-state changes.

If this is right

  • CL transitions arise from recurrent physical processes such as accretion-rate fluctuations or disk instabilities.
  • RCLAGNs exhibit a clear high-low-high accretion-state evolution.
  • The high detection rate of repeated CL behavior indicates that the underlying mechanism operates recurrently.
  • Rest-frame upper limits place the first transition at roughly 10 yr and the second at roughly 4 yr.

Where Pith is reading between the lines

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

  • Future multi-epoch surveys could use the same three-epoch method on known CLAGNs to measure the fraction that repeat and thereby constrain the typical recurrence timescale.
  • The strong preference for turn-off over turn-on events may reflect either intrinsic differences in how accretion changes affect Type 1 versus Type 2 objects or simply the relative ease of detecting the loss of broad lines.
  • If accretion fluctuations drive the repeats, the same sources should show correlated X-ray or UV variability on similar timescales.

Load-bearing premise

That the appearance or disappearance of broad emission lines signals genuine changes in accretion rate or broad-line-region visibility rather than variable dust obscuration.

What would settle it

A sample of RCLAGNs that fail to show the high-low-high accretion evolution or direct evidence that line changes in these objects are produced by changing line-of-sight dust.

Figures

Figures reproduced from arXiv: 2605.24429 by Guohai Chen, Hubing Xiao, Junhui Fan, Wenxin Yang, Xuhong Ye, Zhiqiang Chen, Zhiyuan Pei.

Figure 1
Figure 1. Figure 1: Left: Comparison between the mean log10(λFλ) values derived from the gri-based and [O III]-based flux recalibration methods for LAMOST spectra. The blue points show individual sources with uncertainties, while the black contours represent the kernel density estimation (KDE). The dashed line indicates the one-to-one relation. Right: An example spectrum of J103228.85+350207.0. The gray line shows the origina… view at source ↗
Figure 2
Figure 2. Figure 2: Spectral fitting results for J135406.42+232549.4 from two different epochs. Top: SDSS spectrum in the high state, showing prominent broad Hβ and Hα emission lines. Bottom: LAMOST spectrum in the low state, where the broad Hβ component has disappeared and the broad Hα emission is significantly weakened. The observed spectra are plotted in black. The continuum model is shown in orange, the broad and narrow e… view at source ↗
Figure 3
Figure 3. Figure 3: Multiwavelength variability and spectral comparisons of two CLAGNs. (a) shows the CLAGN J135406.42+232549.4, with SDSS and LAMOST spectroscopic observations, while (b) shows the RCLAGN J162151.68+472756.1, for which SDSS, LAMOST, and DESI spectra are available. In each panel, the top subplot presents mid-infrared light curves from ALLWISE (W1 red; W2 dark red) and NEOWISE (W1 blue; W2 dark blue). Faint poi… view at source ↗
Figure 4
Figure 4. Figure 4: Rest-frame distributions of the upper-limit transition timescales for CLAGNs and RCLAGNs. The blue histogram represents the first spectral-state transition for the 45 CLAGNs, calculated as |MJDLAMOST − MJDSDSS|/[365.25(1 + z)]. The orange histogram shows the second transition for the 12 RCLAGNs, calculated as |MJDDESI − MJDLAMOST|/[365.25(1 + z)]. transition are clustered around ∼10 yr, while those of the … view at source ↗
Figure 5
Figure 5. Figure 5: Left: Comparison of black hole masses derived from Hα and Hβ BELs for CLAGNs and RCLAGNs. Blue circles represent single-transition CLAGNs, while red squares and red triangles denote the first and second high states of repeating CLAGNs, respectively. The black dashed line indicates the one-to-one relation (y = x), and the black solid line shows the best linear fit (slope = 1.03, R 2 = 0.84). Right: Distribu… view at source ↗
read the original abstract

Although more than 1000 optical changing-look active galactic nuclei (CLAGNs) have been reported to date, their physical origin remains unclear, and repeating CLAGNs (RCLAGNs) are still rare. Expanding the CLAGN sample, especially RCLAGNs, is therefore important for constraining the underlying mechanism. We systematically search for CLAGNs by cross-matching spectroscopic observations from the Sloan Digital Sky Survey (SDSS) and the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST), and further use spectra from the Dark Energy Spectroscopic Instrument (DESI) to investigate repeating CL behavior. We identify 45 CLAGNs, including 40 newly reported sources. The sample is dominated by turn-off events, with 43 turn-off and 2 turn-on sources, possibly because Type 2 AGNs either lack a detectable broad-line region or have their broad emission lines obscured by circumnuclear dust. Using DESI as a third spectroscopic epoch, we identify 12 RCLAGNs. This high detection rate of repeated CL behavior suggests that CL transitions may arise from recurrent physical processes, such as accretion-rate fluctuations or disk instabilities. In the log MBH - log(Lbol/LEdd) plane, RCLAGNs further show a clear high-low-high accretion-state evolution, supporting a close link between CL behavior and recurrent changes in accretion power. Finally, the rest-frame upper limits on the transition timescales are about 10 yr for the first transition and about 4 yr for the second, reflecting different survey time baselines rather than intrinsic differences in physical transition timescales.

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.

Referee Report

1 major / 2 minor

Summary. The manuscript reports a systematic cross-match of SDSS, LAMOST, and DESI spectroscopic observations to search for changing-look AGNs (CLAGNs). It identifies 45 CLAGNs (40 new), dominated by turn-off events (43 turn-off, 2 turn-on), and uses DESI as a third epoch to find 12 repeating CLAGNs (RCLAGNs). The authors interpret the repeat rate as evidence that CL transitions arise from recurrent processes such as accretion-rate fluctuations or disk instabilities, report a high-low-high evolution for RCLAGNs in the log M_BH–log(L_bol/L_Edd) plane, and give rest-frame upper limits on transition timescales of ~10 yr (first) and ~4 yr (second).

Significance. If the spectral changes are shown to be intrinsic, the addition of 12 RCLAGNs and the three-epoch analysis would meaningfully expand the known repeating sample and provide observational support for recurrent accretion-driven models of CL behavior. The multi-survey cross-match and explicit reporting of concrete counts (45 total, 12 repeats) are strengths that facilitate future statistical studies of CL duty cycles.

major comments (1)
  1. [Abstract and RCLAGN results/discussion] Abstract and discussion of RCLAGNs: The central inference that the 12 RCLAGNs indicate recurrent physical processes (accretion-rate fluctuations or disk instabilities) and exhibit a 'clear high-low-high accretion-state evolution' in the log M_BH–log(L_bol/L_Edd) plane is load-bearing for the conclusions, yet the manuscript explicitly lists circumnuclear dust obscuration as a possible alternative explanation for the strong dominance of turn-off events (43 vs. 2). No quantitative multi-wavelength tests (e.g., mid-IR color variability, X-ray hardness ratios, or extinction diagnostics) are presented to exclude line-of-sight effects specifically for the repeating subset, leaving the intrinsic-accretion interpretation conditional.
minor comments (2)
  1. [Abstract] The abstract states a 'high detection rate' of repeats but does not quote the total number of three-epoch sources monitored or the selection function; adding this context would allow readers to evaluate the rate quantitatively.
  2. [Sample construction / methods] The manuscript should clarify the exact line-flux or equivalent-width thresholds and redshift cuts used to classify turn-off vs. turn-on events, as these directly affect the reported 43:2 ratio.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for the constructive feedback and for recognizing the value of the multi-survey cross-match and the identification of 12 RCLAGNs. We address the single major comment below.

read point-by-point responses

  1. Referee: [Abstract and RCLAGN results/discussion] Abstract and discussion of RCLAGNs: The central inference that the 12 RCLAGNs indicate recurrent physical processes (accretion-rate fluctuations or disk instabilities) and exhibit a 'clear high-low-high accretion-state evolution' in the log M_BH–log(L_bol/L_Edd) plane is load-bearing for the conclusions, yet the manuscript explicitly lists circumnuclear dust obscuration as a possible alternative explanation for the strong dominance of turn-off events (43 vs. 2). No quantitative multi-wavelength tests (e.g., mid-IR color variability, X-ray hardness ratios, or extinction diagnostics) are presented to exclude line-of-sight effects specifically for the repeating subset, leaving the intrinsic-accretion interpretation conditional.

    Authors: We acknowledge that the manuscript notes circumnuclear dust obscuration as one possible contributor to the overall dominance of turn-off events. However, the 12 RCLAGNs were selected precisely because they exhibit repeated spectral changes across three independent epochs, and these sources trace a coherent high-low-high trajectory in the log M_BH–log(L_bol/L_Edd) plane. Such a specific, recurrent evolutionary path is more naturally explained by changes in accretion rate than by variable line-of-sight obscuration, which would require finely tuned and repeatable dust geometry on the observed timescales. We therefore maintain that the repeating behavior and the accretion-state evolution provide observational support for recurrent accretion-driven processes. At the same time, we agree that the optical-only data set does not include the quantitative multi-wavelength diagnostics needed to fully exclude obscuration for the RCLAGN subset. We will revise the discussion (and, if space permits, the abstract) to state explicitly that the intrinsic-accretion interpretation is favored by the observed patterns but remains conditional pending future multi-wavelength follow-up. revision: partial

Circularity Check

0 steps flagged

No circularity: purely observational sample construction from survey cross-matches

full rationale

The paper identifies CLAGNs and RCLAGNs solely by cross-matching spectra from SDSS, LAMOST, and DESI, classifying turn-off/turn-on events and plotting sources in the log MBH-log(Lbol/LEdd) plane using standard derived quantities. No equations, fitted parameters, or self-citations reduce the reported counts (45 CLAGNs, 12 RCLAGNs), detection rate, or high-low-high evolution to quantities defined or fitted inside the paper itself. The derivation chain consists of direct observational selection and empirical plotting with no self-definitional, fitted-input, or self-citation-load-bearing steps.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The work rests on standard domain assumptions about what broad-line visibility means for accretion state; no free parameters or new entities are introduced in the abstract.

axioms (1)
  • domain assumption Spectral classification of AGN types based on presence or absence of broad emission lines accurately reflects changes in accretion rate or broad-line-region visibility.
    Invoked when counting turn-off versus turn-on events and when interpreting the high-low-high accretion evolution.

pith-pipeline@v0.9.1-grok · 5846 in / 1304 out tokens · 44236 ms · 2026-06-30T13:23:06.258731+00:00 · methodology

discussion (0)

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