{"id":"4ec38d17-0e69-419a-bfed-49667a495cd5","arxiv_id":"2607.18919","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":6.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":5,"one_line_summary":"Echoes from an accreting exotic compact object are progressively compressed and suppressed, then vanish when the event horizon overtakes the reflecting surface, after which the signal becomes ordinary black-hole ringdown.","lead":"This paper models a star-like exotic compact object that is swallowing matter and slowly turning into a black hole, and simulates how its gravitational-wave echoes change during the transition. It finds that the echoes get squashed and dimmer and then stop once the growing event horizon overtakes the object's surface.","discovery_kind":"new_application","skeptic_critique":{"model":"deepseek-v4-flash","headline":"Echo disappearance is largely encoded in the hand-imposed surface trajectory (Eq. 15) and boundary switch, not derived from accretion physics; the claimed signature is not yet robust.","rationale":"The paper's central claim is that accretion-driven horizon formation suppresses echoes and produces a smooth transition to BH ringdown. The logic is internally consistent: if the surface is eventually engulfed by the event horizon, no further reflections can escape (assuming standard GR causality). However, the model does not derive the surface response from any accretion physics; Eq. (15) is a kinematic choice that ensures the surface is timelike and eventually crosses the horizon. The causal bound from Ref. [13] is only a necessary condition for avoiding horizon formation; it does not determine the actual trajectory. Thus the qualitative result—echo suppression and termination—is contingent on an unconstrained parameter (epsilon) and the initial surface radius. In addition, the numerical implementation replaces the reflecting boundary with an absorbing horizon at the crossing time, which by construction ends echoes; the 'smooth' character of the transition is not verified against a gradual absorption model or an interior evolution. While the paper is a reasonable first proof-of-concept, the headline result is not yet a robust quantitative prediction. The authors themselves call for more realistic matter distributions (Sec. VI). These issues justify retaining the reader's conditional verdict: the framework is promising, but the central signature must be tested against a range of surface responses before being presented as characteristic of accreting ECOs.","tokens_in":10913,"tokens_out":6468,"duration_ms":65481,"concrete_test":"Vary epsilon in Eq. (15) over a range (e.g., 10^-3 to 10^-1) and initial radii around the assumed value, keeping all other parameters fixed, and recompute the echo waveform. If the echo compression rate or the crossing time changes significantly (or if no crossing occurs), the claimed signature is parameter-dependent and the central conclusion requires a physical model for the surface response.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The central claim—that echoes are progressively compressed and then disappear as the event horizon engulfs the surface—is not a prediction of the model but an input. The surface trajectory dr_CO/dv = (1/2)(1 - 2m(v)/r_CO) - epsilon (Eq. 15) is prescribed with epsilon and r_CO(v0) unspecified; no equation of state or interior model determines it. Since the causal argument of Ref. [13] only sets an upper bound on the expansion rate, any sub-luminal trajectory is allowed, and the occurrence and timing of horizon crossing depend on the chosen epsilon and initial radius. Moreover, in Section IV the reflective boundary is 'replaced by the event horizon' at the crossing time; this hand-imposed switch guarantees that echoes cease after crossing, so the termination is tautological. Consequently, the observed compression and suppression are specific to the chosen trajectory, not a robust signature of accretion.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"The paper models an initially horizonless exotic compact object (ECO) whose exterior is described by the ingoing Vaidya spacetime with a tanh mass-accretion profile. The ECO surface is a timelike, perfectly reflecting boundary whose trajectory is prescribed by Eq. (15), and the event horizon is found by backward integrating the outgoing null geodesic equation. Scalar perturbations are evolved with a second-order characteristic double-null code. The key claim is that during accretion the echo train is progressively compressed and suppressed, and that once the event horizon overtakes the surface the echoes terminate and the waveform smoothly approaches the standard black-hole ringdown. The authors compare static and accreting ECO/BH waveforms and argue that the disappearance of echoes is a characteristic signature of horizon formation.","tokens_in":11245,"tokens_out":7164,"duration_ms":65161,"significance":"The causal inevitability of horizon formation for sufficiently compact accreting ECOs was established in Ref. [13]; the present paper attempts to translate this into a concrete gravitational-wave observable by simulating scalar ringdown through the ECO-to-BH transition. If the result is robust, it provides a useful proof-of-concept for dynamical echo searches and a framework for future gravitational/electromagnetic simulations. Strengths include the use of a standard Vaidya double-null construction, a physically motivated mass profile, and an explicit detection-feasibility estimate. However, the central numerical claim is not yet supported by convergence tests or a parameter study, and the echo termination follows largely from an assumed boundary-condition switch. The significance is therefore conditional on substantial revision.","major_comments":[{"comment":"No convergence or resolution study is reported. The characteristic scheme in Eq. (21) is second-order, but the paper gives no grid spacings, no Richardson extrapolation, and no comparison at different resolutions for the waveforms in Figs. 4 and 5. The late-time echo amplitudes in Fig. 5 are small (a factor of 4–5 below the primary ringdown), making them particularly sensitive to numerical dissipation and dispersion. Without an error estimate, the claimed progressive compression and suppression of echoes and the 'smooth transition' to BH ringdown are not quantitatively established. Please add a convergence test and report the grid parameters used.","section":"§IV and §V"},{"comment":"The termination of the echo train is built into the model setup, not derived from accretion dynamics. The surface trajectory, dr_CO/dv = (1/2)(1 - 2m(v)/r_CO) - epsilon, is prescribed with free epsilon and free initial radius, and Section IV states that the reflective boundary is replaced by the event horizon once the trajectories cross. Since an event horizon by definition prevents causal communication from the surface, the subsequent disappearance of echoes is a necessary consequence of this boundary switch. What is not a necessary consequence is the specific compression/suppression pattern, which depends on the chosen trajectory. The paper should either (i) scan a physically motivated range of epsilon and r_CO(v0) and show that the qualitative conclusion is unchanged, or (ii) explicitly frame the results as a proof-of-concept rather than as the 'robust' signature claimed in Section VI","section":"§III, Eq. (15) and §IV"},{"comment":"The numerical setup for the accreting ECO is under-specified. The text gives the mass-profile parameters (m1, m2, v1, rho), but does not state the initial surface radius r_CO(v0), the value of epsilon used in Eq. (15), the grid spacings (Δu, Δv), or the precise observer location for each panel of Fig. 5. Without these, the simulation is not reproducible and the role of the free parameters cannot be assessed. Please include a table listing all adopted parameters (for both static and accreting runs) and, ideally, the code/data availability.","section":"§III and §V"},{"comment":"The manuscript promises a study of quasinormal modes and a 'QNM spectrum', but no complex frequencies are ever extracted. Figures 4 and 5 show time-domain waveforms only, and the text makes qualitative statements about damping without quantifying the real and imaginary parts of the frequency. If the focus is on the echo transition, the title and abstract should say so; if QNM frequencies are intended, a spectral extraction (e.g., Prony analysis or a frequency transform around the ringdown phase) is needed to support the claims.","section":"Title/Abstract and §II"}],"minor_comments":[{"comment":"The symbol epsilon is used with two different meanings: in Eq. (15) it is the trajectory deviation parameter, while in the static-ECO description r_CO = 2m0(1+epsilon) it is the compactness parameter. Please choose distinct notations.","section":"§V"},{"comment":"The initial pulse parameters in Eq. (22) are given (v_c=10, sigma=3, omega=0.25), but it should be stated explicitly whether the same pulse is used for the static ECO, accreting ECO, static BH, and accreting BH runs in Figs. 4 and 5.","section":"§IV"},{"comment":"The choice P(u) = -u/2 is stated but not derived. A brief explanation of how this fixes the double-null coordinate gauge would improve clarity.","section":"§II, Eq. (10)"},{"comment":"In the discussion around Eq. (16), the constant-accretion compactness bound is introduced, but it is not used further for the tanh profile. The connection between Eq. (16) and the chosen time-dependent parameters would be clearer if the maximum accretion rate from Eq. (18) were evaluated against the condition explicitly.","section":"§III"}],"recommendation":"major_revision","confidential_remarks":null},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Colleague,\n\nThe one-sentence take: this paper gives the first numerical look at echo evolution during an accretion-driven ECO-to-BH transition, and the broad physics is credible—echoes compress, decay, and stop once the horizon swallows the surface. But the signature they headline is not yet derived from accretion physics; it follows from a chosen surface trajectory, and the numerical reporting is too thin for a quantitative claim.\n\nWhat's new: combining the causal horizon-formation argument of Carballo-Rubio, Kumar, and Lu with the double-null Vaidya perturbation framework of Abdalla, Chirenti, and Saa. Prior work treats static ECO echoes and dynamical BH ringdown separately, so this fills a gap.\n\nWhat's good: the characteristic evolution is standard, the waveforms look physically sensible, and the detectability discussion is honest—they admit the echo SNR is ~O(1) for current stellar-mass events, so no one should mistake this for an imminent observable.\n\nSoft spots. Eq. (15) prescribes the surface trajectory dr_CO/dv = 1/2(1-2m/r) - epsilon, and neither epsilon nor the initial surface radius is stated in the text. The echo compression and the crossing time depend on that choice. The stress-test note is half right: the termination itself is not tautological—once the surface is inside the event horizon, it cannot send signals to infinity—but the shape of the echo train before crossing is put in by hand. A single parameter set is shown, with no scan over epsilon or initial compactness, so the claimed signature is not robustly characterized.\n\nAlso, no convergence tests, no grid resolution, no error bars, no code. Despite 'quasinormal' in the title, no QNM frequencies are ever extracted; it's waveform inspection only. All fixable, but real.\n\nBottom line: the central qualitative conclusion is very likely correct and worth refereeing, but the paper should not be accepted as is. A revision that specifies and scans the surface parameters, reports convergence, and tones down the QNM language would merit another look.","headline":"A plausible first numerical look at echo evolution across ECO-to-BH transition, but the headline signature is largely put in by hand and the numerics are underreported.","tokens_in":11635,"tokens_out":4384,"would_cite":true,"duration_ms":42074,"reading_group":"maybe","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":["83C57","83C35"],"pacs":["04.70.-s","04.30.-w"],"model":"deepseek-v4-flash","headline":"Accretion compresses and suppresses gravitational-wave echoes as an accreting exotic compact object forms a black hole, then the signal becomes ordinary ringdown.","keywords":["gravitational-wave echoes","exotic compact objects","Vaidya spacetime","horizon formation","quasinormal modes","accretion","double-null coordinates","black hole ringdown"],"falsifier":"An observation of a post-merger signal with persistent, equally spaced echoes of nearly constant amplitude for times much longer than the accretion timescale, in a system where accretion is known to be ongoing, would contradict the claim that accretion kills the echo train; alternatively, a numerical simulation using a physical equation of state for the ECO surface that keeps the surface outside the horizon during sustained accretion would falsify the horizon-formation premise.","tokens_in":10838,"feed_emoji":"🌌","tokens_out":4595,"duration_ms":38814,"temperature":0.7,"pith_summary":"The paper asks what happens to the gravitational-wave echoes of an exotic compact object (ECO) — a horizonless object whose reflective surface bounces radiation back and forth — when that object continuously accretes matter and eventually forms an event horizon. Working in the Vaidya spacetime, the authors evolve scalar perturbations numerically and find that the echo train is progressively compressed and damped while the horizon grows, and stops entirely at the moment the horizon overtakes the reflecting surface. After that crossing, the waveform smoothly joins the standard exponentially damped quasinormal ringdown of a black hole. The claim matters because it turns the disappearance of echoes into a possible observational marker of actual horizon formation, and because it bridges two phenomena usually treated separately: ECO echoes and black-hole ringdown.","feed_headline":"Accretion compresses and kills gravitational-wave echoes","feed_subtitle":"Echoes fade and vanish as the horizon engulfs the surface, leaving ordinary black-hole ringdown.","key_machinery":"The central object is the double-null Vaidya geometry, which describes a spherically symmetric spacetime sourced by null radiation and allows the background itself to be time-dependent. Two trajectories drive the calculation: the ECO surface, evolved causally along the timelike relation dr_CO/dv = (1/2)(1 - 2m(v)/r_CO) - epsilon, and the event horizon, found by integrating the outgoing null geodesic equation backward from the final Schwarzschild state. The wave evolution uses a characteristic integration scheme for the scalar wave equation on that background, with the effective potential built from the instantaneous mass and areal radius.","core_discovery":"The paper's central discovery is that the dynamical transition from a horizonless accreting compact object to a black hole is imprinted in the gravitational-wave signal in a specific way: during accretion the echo peaks arrive closer together and with reduced amplitude, and once the growing event horizon crosses the object's surface, the reflecting boundary ceases to communicate with the exterior and the echo sequence terminates, with the waveform evolving seamlessly into ordinary quasinormal ringdown. This is established numerically by evolving linear scalar perturbations on an ingoing Vaidya background cast in double-null coordinates, with a reflecting boundary at the timelike ECO surface","pith_inferences":["If the ECO surface is not perfectly reflecting, or if it expands faster than the prescribed trajectory, the echo compression and termination time would shift; the qualitative claim of echo suppression might still hold, but the precise waveform would depend on the interior model.","The same mechanism likely applies to gravitational and electromagnetic perturbations, so the disappearing-echo signature could be searched for in broadband gravitational-wave data rather than only in scalar toy models.","The amplitude ratio between echoes and the primary ringdown (roughly 1/4 to 1/5 in the scalar case) suggests that even optimistic stellar-mass events give echo signal-to-noise of order unity in current detectors, making this a target for next-generation instruments.","A testable extension would be to check whether the echo compression rate tracks the instantaneous accretion rate—or the horizon growth rate—by extracting the time-dependent echo spacing from the waveform."],"forward_implications":["If accretion is realistic, a sufficiently compact ECO cannot remain horizonless for long; echoes should be a transient phase, not a persistent feature.","The compression and suppression of echoes during accretion provides a time-dependent signature that distinguishes a collapsing ECO from a static ECO or a black hole.","The smooth transition to standard ringdown means searches for echoes should not expect a sharp cutoff; the echo signal fades continuously as the horizon grows.","Detecting the disappearance of echoes in a post-merger signal could serve as evidence that a horizon formed during the observation.","The framework extends stationary analyses, so future low-frequency detectors like LISA could probe accretion-driven horizon formation in supermassive mergers."],"fun_headline_variants":["Accretion kills gravitational-wave echoes as horizon forms","Echoes vanish as accretion forms event horizon","How accretion snuffs out gravitational-wave echoes","Accretion compresses then silences black-hole echoes","Echoes die as horizon swallows reflecting surface"],"cache_read_input_tokens":2304,"weakest_assumption_plain":"The simulation depends on the hand-prescribed surface trajectory dr_CO/dv = (1/2)(1 - 2m(v)/r_CO) - epsilon and on the assumption that the surface remains perfectly reflecting until the instant the event horizon crosses it; if the true surface responds to accretion differently, absorbs partially, or is not at the prescribed radius, the echo compression and termination would change or vanish.","fun_headline_variants_meta":{"raw":{"variants":["Accretion kills gravitational-wave echoes as horizon forms","Echoes vanish as accretion forms event horizon","How accretion snuffs out gravitational-wave echoes","Accretion compresses then silences black-hole echoes","Echoes die as horizon swallows reflecting surface"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000106,"raw_usage":{"total_tokens":818,"prompt_tokens":625,"completion_tokens":193,"prompt_tokens_details":{"cached_tokens":256},"prompt_cache_hit_tokens":256,"prompt_cache_miss_tokens":369,"completion_tokens_details":{"reasoning_tokens":119}},"tokens_in":369,"tokens_out":193,"duration_ms":2687,"temperature":1.0,"reasoning_tokens":119,"cache_read_input_tokens":256,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-01T13:56:56.506144+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"An observation of a post-merger signal with persistent, equally spaced echoes of nearly constant amplitude for times much longer than the accretion timescale, in a system where accretion is known to be ongoing, would contradict the claim that accretion kills the echo train; alternatively, a numerical simulation using a physical equation of state for the ECO surface that keeps the surface outside the horizon during sustained accretion would falsify the horizon-formation premise.","supporting_citations":[],"review_version":1}