{"id":"ce21d9fa-a8c9-44cd-993a-3508dd656046","arxiv_id":"2505.07061","paper_version":1,"verdict":"UNVERDICTED","confidence":"MODERATE","novelty_score":2.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":6,"one_line_summary":"A tidal disruption event's accretion disk is expected to undergo a thermal-viscous instability that makes it collapse from a thick to a thin state, but late-time X-ray observations challenge the timing and universality of that collapse.","lead":"This review paper examines how accretion disks formed in tidal disruption events evolve over years, focusing on a predicted cycle between hot thick and cool thin states. A smart generalist might read it to see how a simple disk theory is being tested against long-term X-ray observations of these cosmic flare events.","discovery_kind":"review","skeptic_critique":{"model":"deepseek-v4-flash","headline":"The instability prediction rests entirely on alpha-viscosity (stress proportional to total pressure); the paper itself concedes beta-viscosity or magnetic support removes it, so the central claim is conditional on an unverified stress law.","rationale":"The reader identified the alpha-viscosity assumption as the weakest link, and my stress-test pass reaches the same conclusion. The review is transparent about this dependence: Section 2 explicitly notes that beta-viscosity removes the instability and that magnetic pressure can stabilize the disk, and the Summary lists the limited scope of the current model. Therefore this is not a hidden internal inconsistency; it is a load-bearing fragility in the underlying model that the review itself highlights. The central claim of the abstract is conditional on a stress law that is plausible but not directly verified for TDE disks, and the cited support from radiation MHD simulations does not fully cover the parameter regime in question. This concern does not change the reader's verdict. The paper is a review rather than a new research claim, so the appropriate verdict remains UNVERDICTED, with the caveat that the scorecard's correctness rating should reflect the model dependence on the viscosity prescription. I also noted a possible numerical tension between Eq. 32 and the combination of Eqs. 24 and 31 for the outer-disk collapse time, but without the model code or Figure 4 data I cannot determine whether disk expansion resolves it; the alpha-dependence concern is the more decisive issue.","tokens_in":17157,"tokens_out":14630,"duration_ms":149021,"concrete_test":"Run a local shearing-box radiation MHD simulation with mid-plane density, temperature, and optical depth typical of a TDE disk near the predicted thick-to-thin transition (e.g., rd ~ 10-100 rg and Mdot ~ 0.1-1 Mdot_Edd), and measure whether the saturated stress is proportional to total pressure or to gas pressure. If the stress tracks gas pressure or is magnetically suppressed, the thermal-viscous instability, and with it the abstract's central claim, disappears; if it tracks total pressure, the concern is resolved. A simpler complementary check is to recompute the one-zone evolution of Section 3 with a beta-viscosity prescription and confirm that no thick-to-thin state transition occurs.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The abstract's central assertion is that the current model predicts a thermal-viscous instability for most TDEs with BH mass below about 10^7 solar masses. In Section 2, immediately after defining the alpha-viscosity prescription (Eq. 5), the paper states that under a beta-viscosity prescription, where stress is only a fraction of gas pressure, the disk does not suffer from the thermal-viscous instability and remains stable throughout its evolution. It also notes that a strong magnetic field can stabilize the disk. Thus the headline prediction is not a robust property of TDE disks; it is a property of the assumed stress law. The paper cites Jiang, Stone, and Davis (2013) radiation MHD simulations as support for alpha-viscosity, but those simulations were shearing-box calculations, not TDE-disk simulations, and the extrapolation to the density, temperature, and optical depth regime of a TDE disk is an assumption. The later comparison with observations (transitions occurring later than predicted, or not at all) does not discriminate between alpha- and beta-viscosity, because the beta-viscosity model would predict no transition in either case. Since the review's most consequential and prominently advertised prediction inherits this least-secure assumption, the central claim is not established unless the total-pressure stress scaling is verified in the TDE regime.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"This manuscript is a review of the long-term evolution of accretion disks in tidal disruption events (TDEs). It presents a local thermal-equilibrium analysis of an alpha-viscosity disk annulus, derives the thermal stability condition, and builds a one-zone model for the global evolution of disk mass and radius with ongoing fallback. The central claims are that (i) under total-pressure alpha-viscosity the disk undergoes a thermal-viscous instability for most TDEs with black-hole mass below about 10^7 solar masses, causing thick-to-thin state transitions and order-of-magnitude accretion-rate drops, and (ii) the current one-zone model is highly incomplete when compared with late-time X-ray observations, which show transitions later than predicted or none at all. The paper also discusses possible resolutions, including shock heating, direct fallback onto the inner disk, and magnetic pressure support.","tokens_in":17383,"tokens_out":12352,"duration_ms":118132,"significance":"If the central prediction is correct, alpha-viscosity TDE disks would provide a new setting for thermal-viscous limit cycles and explain sharp X-ray drops in several TDEs. The review has clear strengths: the local stability derivation in Section 2 is standard and internally consistent; the paper explicitly lists its own caveats (unknown shock-heating efficiency fsh, absence of a global disk model, uncertain wind cooling); and the comparison with AT2018fyk, AT2021ehb and jetted TDEs is concrete and falsifiable in principle. However, the headline instability is conditional on the total-pressure alpha-viscosity prescription, and the paper itself notes that beta-viscosity or magnetic pressure support removes the instability. Since the cited numerical support comes from shearing-box simulations rather than TDE-specific disk simulations, the central claim is not established as a robust property of TDE disks. The review is nevertheless a useful and honest synthesis; with explicit reframing of the conditional claims it can be made defensible.","major_comments":[{"comment":"The headline prediction that \"the current model predicts a thermal-viscous instability for most TDEs with BH mass ≲ 10^7 M⊙\" is conditional on the total-pressure α-viscosity prescription in Eq. (5). The paper itself states in Section 2 that under a β-viscosity prescription, or with a strong magnetic field, the disk remains stable, and the cited radiation-MHD support (Jiang, Stone & Davis 2013) comes from shearing-box simulations at parameters different from TDE disks. The Section 3.2 comparison with observed transitions (or their absence) does not break this degeneracy: a stably evolving β-viscosity disk would produce no transition, so both outcomes are consistent with either stress law. I ask that the central claims be systematically qualified as conditional on α-viscosity, and that the review include an explicit statement of what observations would discriminate between α- and β-type stress prescriptions.","section":"Abstract; Section 2 (Eq. 5)"},{"comment":"The timing comparison between model and observations is built on two quantities with different epistemic status. Equation (32) has its normalization \"numerically calibrated based on Fig. 4\", so it is a fit to the one-zone model rather than an independent analytic prediction, while Eq. (33) assumes a flat angular-momentum distribution dMfb/dℓ ∝ ℓ^0, which is introduced as speculation. Consequently, the claimed mismatch between the predicted ≈80 d collapse and the observed ≈1 yr transitions should be presented not as a robust prediction of the model but as a property of the particular one-zone implementation with fsh = 0 and a δ-function circularization radius. Please mark these dependencies explicitly, and indicate how a global 1D calculation would test whether the timing discrepancy is real.","section":"Section 3.2, Eqs. (32)–(33)"}],"minor_comments":[{"comment":"The caption says \"unstable (∂ g/∂ T < 0) ones in red\"; since Eq. (19) defines thermal stability as ∂ g/∂ T < 0, the unstable condition should be ∂ g/∂ T > 0.","section":"Figure 1 caption"},{"comment":"The typeset expression appears to be missing a division symbol; it should read Ω ≃ ΩK/(1+θ²) for a sub-Keplerian disk, and the subsequent use in Eq. (26) is consistent with that reading.","section":"Eq. (4)"},{"comment":"Several references (e.g., [38], [40], [98]) are cited in arXiv e-print form; please update to the published versions where available.","section":"References"},{"comment":"The sentence \"These are in disagreement with the predictions from the one-zone model\" would benefit from stating explicitly that the disagreement is evaluated for fsh = 0 and for the adopted α values; otherwise it reads as a stronger statement than the model permits.","section":"Section 3.2, item (3)"}],"recommendation":"major_revision","confidential_remarks":"The manuscript is a single-author invited review with a reasonable citation balance. The main risk is not circularity but overstatement of a stress-law-dependent prediction; the author already acknowledges most caveats, so the revision should largely be a matter of reframing the abstract and the model-observation comparison rather than adding new calculations."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Quick take: this is a well-executed invited review, not a research paper. It restates the standard alpha-viscosity thermal-viscous instability model for TDE disks and then does something useful: it confronts the model with recent X-ray data (AT2018fyk, AT2021ehb, ASASSN-14li, jetted TDEs) and is unusually honest about where the model fails. The reader's UNVERDICTED verdict is right; there is no new observational or numerical result to accept or reject.\n\nWhat it does well: Section 2's local stability derivation is standard but clean, and the dimensionless S-curves in Figs. 1-3 are a good teaching tool. The paper explicitly flags its own soft spots — f_sh is unknown, the one-zone model is crude, beta-viscosity or a strong magnetic field would suppress the instability entirely, and the late-time discrepancies are listed in the summary plus/minus format. That is the right way to write a review. The only genuinely new element is the outside-in collapse timing argument (eq. 33): if a fraction of fallback gas lands near the ISCO, the inner disk collapses around t ~ 1 yr, which better matches the observed X-ray drops. It is presented as a speculation, which is appropriate.\n\nSoft spots, in proportion: the headline claim — most TDEs with M ≲ 1e7 Msun should undergo a thermal-viscous instability — depends entirely on α-viscosity with stress proportional to total pressure. The paper concedes that under beta-viscosity or with magnetic support the instability goes away. The Jiang+13 shearing-box simulations support alpha-viscosity, but they are not TDE disk simulations, so the extrapolation is an assumption. The stress-test note is fair: the observational comparisons cannot discriminate alpha from beta, because beta predicts no transition in either case. Also, eqs. (32) and (33) are fits to the author's own model curves, not derived predictions; they need global disk simulations before being used as diagnostics. The citation pattern is fine: prior work is properly credited, and self-citations correspond to real contributions.\n\nWho it is for: graduate students or observers wanting a compact map of the current model and its observational tensions. It deserves a serious referee and to be published as a review. I would bring it to reading group and would cite it as the current review on late-time TDE disk evolution.","headline":"A solid, honest invited review — no new result, but the observation comparison and the outside-in collapse timing idea are worth engaging; the instability claim is conditional on an unverified stress law.","tokens_in":18010,"tokens_out":2854,"would_cite":true,"duration_ms":29862,"reading_group":"yes","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":[],"model":"deepseek-v4-flash","headline":"A tidal disruption event's disk should undergo a thermal-viscous collapse from a thick hot state to a thin cool one, yet many observed X-ray transitions come later or not at all.","keywords":["tidal disruption events","accretion disk evolution","thermal-viscous instability","alpha viscosity","fallback","X-ray lightcurves","state transitions","super-Eddington accretion"],"falsifier":"Measure a statistical sample of TDEs with long-term X-ray coverage and estimated black hole masses below $10^7\\,M_\\odot$: if most do not show an abrupt order-of-magnitude X-ray decline within about two years, or if the decline epochs do not correlate with the predicted fallback-rate threshold, the thermal-viscous collapse is not operating. A direct test of the mechanism would be to determine in radiation-dominated accretion flows whether the viscous stress is proportional to total pressure; a $\\beta$-viscosity scaling would remove the instability.","tokens_in":16855,"feed_emoji":"📉","tokens_out":8072,"duration_ms":80286,"temperature":0.7,"pith_summary":"This review argues that the long-term evolution of a tidal disruption event (TDE) disk is set by viscous angular-momentum transport plus ongoing mass fallback, and that under the standard $\\alpha$-viscosity law the disk is thermally unstable for most events with black hole mass below about $10^7\\,M_\\odot$. As fallback declines, the disk should collapse from a geometrically thick, radiation-pressure-dominated state to a thin, gas-pressure-dominated state, producing sudden order-of-magnitude drops in the accretion rate and X-ray luminosity. The paper reads the steep X-ray declines seen in jetted TDEs and in AT2018fyk and AT2021ehb as evidence for such state transitions, but notes the model predicts an outer-disk collapse at roughly 80 days whereas observed transitions cluster near one year, and some X-ray bright TDEs show no transition up to 500 days. It concludes that the current one-zone model is highly incomplete and identifies shock heating from fallback, a broad angular momentum distribution of the fallback gas, and a possible minimum disk thickness as the missing physics.","feed_headline":"TDE disks should crash into a thin state; X-ray drops arrive late","feed_subtitle":"Alpha-viscosity predicts a thermal-viscous collapse within ~80 days; observed TDEs transition near one year, if at all.","key_machinery":"The central object is the S-curve of local thermal equilibrium solutions $g(T;\\Sigma,r_d)=0$, built from the $\\alpha$-viscosity prescription $\\nu_{\\rm vis}=\\alpha H^2\\Omega_K$, with viscous and shock heating balanced by radiative, advective, and wind cooling. The unstable middle branch of the S-curve is what makes the disk collapse from a thick radiation-pressure state to a thin gas-pressure state, and the critical accretion rates at the two S-curve boundaries set the collapse and revival thresholds. The overall evolution model is a one-zone tracking of disk mass and radius under mass and angular momentum conservation, supplemented by a spherization radius and a Bernoulli-limited wind to estimate the accretion rate reaching the black hole.","core_discovery":"On the paper's own terms, the central claim is that a radiation-pressure-dominated TDE disk built by fallback and governed by local alpha viscosity is subject to a thermal-viscous instability: for fixed surface density and radius the thermal equilibrium curve $g(T)=0$ is S-shaped, with an unstable branch separating a thick, hot stable solution from a thin, cool one. As the fallback rate declines over months to years, the disk moves onto the unstable branch and collapses vertically, dropping the accretion rate by a few orders of magnitude; after the collapse, mass accumulation can push it back onto the thick branch, producing repeated accretion bursts in a limit cycle that eventually stops when fallback is too weak. The paper uses a piecewise steady-state one-zone model with this instability to compute the outer-disk radius and mass, then compares the predicted accretion and shock luminosities with X-ray and UV/optical lightcurves. The model reproduces qualitative features such as a super-Eddington early phase, sharp X-ray drops, and possible limit-cycle behavior, but fails on timing: the outer disk should collapse near 80 days, while observed steep X-ray declines occur near one year, and some systems show no decline. The proposed resolution is that a large spread in the angular momentum of fallback gas delivers a fraction directly to the inner disk, whose collapse time of about one year matches the observations.","pith_inferences":["A population-level test follows from the paper's inner-disk collapse scenario: if the one-year collapse time is real, the distribution of X-ray-drop epochs across a sample should shift systematically with black hole mass and stellar mass; a null correlation would point to a different trigger, such as an external obscuration event.","The model's tension with no-transition sources like ASASSN-14li could be resolved if the true circularization shock efficiency is high enough to stabilize the disk, suggesting a testable connection between observed optical/UV reprocessing luminosity and the disk's stability.","The limit-cycle prediction implies that very late-time (5-10 year) UV plateaus could be a signature of a thin disk that retains nearly all the fallback mass rather than a separate mechanism; multi-band late-time monitoring could distinguish that from a magnetically arrested state.","The paper leaves the angular-momentum distribution of fallback gas as an open input; if it is broad as the self-crossing shock picture suggests, the same model should predict a smooth radial mass-infall profile, and the ratio of early to late X-ray timing would indirectly measure that distribution."],"forward_implications":["If the alpha-viscosity picture is right, most TDEs around black holes below about $10^7\\,M_\\odot$ should show abrupt, order-of-magnitude X-ray drops as the disk collapses, so long-term X-ray monitoring of optically and X-ray selected samples directly tests the instability.","The observed steep declines in Swift J1644+57, Swift 2058+05, AT2018fyk, and AT2021ehb are explained as state transitions; the two rapid drops in AT2021ehb tentatively support the predicted limit cycle.","If the inner-disk collapse picture holds, the timing of the steep X-ray drop encodes the fallback rate and thereby the disrupted star's mass and the black hole mass, since the inner-disk collapse time scales as $M^{-1/5}M_*^{3/5}$.","The model predicts rapid accretion flares after the first collapse; their absence in the current sample means either the disk is kept thick by fallback interactions, magnetic pressure, or Lense-Thirring misalignment, or the limit cycle is yet to be seen.","Under beta viscosity or a magnetically stabilized stress law the instability disappears and the disk evolves on decade-long timescales, which would conflict with the observed rapid X-ray evolution; radiation MHD simulations of shearing flows support the alpha-like total-pressure stress, so the instability is expected to operate."],"supporting_citations":[{"why":"Supplies the alpha-viscosity prescription $\\nu_{\\rm vis}=\\alpha H^2\\Omega_K$ on which the thermal-viscous instability analysis rests.","marker":"[86]"},{"why":"Introduces the piecewise steady-state one-zone model of a viscously spreading TDE disk with the outer-disk collapse and S-curve equilibrium solutions.","marker":"[88]"},{"why":"Provides the early long-term disk evolution model with viscous spreading and no ongoing fallback, which the review extends.","marker":"[17]"},{"why":"Supplies the numerical stellar fallback rate used in the evolution calculations and the late-time $t^{-5/3}$ scaling.","marker":"[53]"},{"why":"Radiation MHD shear-flow simulations supporting the claim that viscous stress tracks total pressure, so the radiation-pressure disk is thermally unstable.","marker":"[42]"},{"why":"Provides the X-ray and optical lightcurves of AT2018fyk showing steep flux drops near one year that the paper compares with predicted state transitions.","marker":"[97]"},{"why":"Provides the AT2021ehb observations with two rapid X-ray drops separated by about 50 days, used as tentative support for the limit-cycle prediction.","marker":"[98]"},{"why":"Demonstrates that the self-crossing shock broadens the angular momentum distribution of fallback gas, the basis for the inner-disk direct fallback scenario.","marker":"[59]"}],"fun_headline_variants":["TDE disk collapse runs late: predicted 80 days, seen near one year","Thermal-viscous instability can't explain TDE X-ray timing","TDE accretion disk instability: collapse delayed by angular momentum spread","Why TDE disks ignore the 80-day collapse deadline","X-ray drops in TDEs arrive late: disk instability timing mismatch"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The instability prediction rests on the assumption that the viscous stress is proportional to the total (gas plus radiation) pressure with a constant alpha; if the stress instead scales with gas pressure alone, or magnetic pressure stabilizes the disk, the predicted collapse does not occur.","fun_headline_variants_meta":{"raw":{"variants":["TDE disk collapse runs late: predicted 80 days, seen near one year","Thermal-viscous instability can't explain TDE X-ray timing","TDE accretion disk instability: collapse delayed by angular momentum spread","Why TDE disks ignore the 80-day collapse deadline","X-ray drops in TDEs arrive late: disk instability timing mismatch"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000755,"raw_usage":{"total_tokens":3334,"prompt_tokens":897,"completion_tokens":2437,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":513,"completion_tokens_details":{"reasoning_tokens":2344}},"tokens_in":513,"tokens_out":2437,"duration_ms":17603,"temperature":1.0,"reasoning_tokens":2344,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-15T22:26:42.650236+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Measure a statistical sample of TDEs with long-term X-ray coverage and estimated black hole masses below $10^7\\,M_\\odot$: if most do not show an abrupt order-of-magnitude X-ray decline within about two years, or if the decline epochs do not correlate with the predicted fallback-rate threshold, the thermal-viscous collapse is not operating. A direct test of the mechanism would be to determine in radiation-dominated accretion flows whether the viscous stress is proportional to total pressure; a $\\beta$-viscosity scaling would remove the instability.","supporting_citations":[{"cited_title":"D., 2014, ApJ, 784, 87","cited_arxiv_id":null,"evidence_quote":"Introduces the piecewise steady-state one-zone model of a viscously spreading TDE disk with the outer-disk collapse and S-curve equilibrium solutions."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Supplies the numerical stellar fallback rate used in the evolution calculations and the late-time $t^{-5/3}$ scaling."},{"cited_title":"M., Davis S","cited_arxiv_id":null,"evidence_quote":"Radiation MHD shear-flow simulations supporting the claim that viscous stress tracks total pressure, so the radiation-pressure disk is thermally unstable."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Provides the X-ray and optical lightcurves of AT2018fyk showing steep flux drops near one year that the paper compares with predicted state transitions."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Demonstrates that the self-crossing shock broadens the angular momentum distribution of fallback gas, the basis for the inner-disk direct fallback scenario."}],"review_version":1}