{"id":"73c28b47-2539-46a6-a6b5-718bb3e98817","arxiv_id":"2608.11387","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":5.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":4,"one_line_summary":"For sub-solar neutron star and black hole mergers, CoCoA stochastic searches reach roughly 32 to 104 Mpc with current LIGO detectors and roughly 230 to 970 Mpc with Cosmic Explorer and Einstein Telescope.","lead":"This paper estimates how far current and future gravitational-wave detectors could see long, low-frequency chirps from mergers of sub-solar-mass neutron stars with black holes, using the CoCoA cross-correlation search algorithm. It finds such searches could reach very nearby events now and become systematic with next-generation detectors, a roadmap for testing the superkilonova hypothesis.","discovery_kind":"new_application","skeptic_critique":{"model":"deepseek-v4-flash","headline":"Eq. 2.3 is not derivable from Eq. 2.2: with f_SKN|coll = 10^-2 and R0,CCSN = 7e4 Gpc^-3 yr^-1, the upper bound should be 7e2, not 7, unless the paper silently redefines R_true_0,coll as the LLGRB rate.","rationale":"The reader identified the assumed branching fraction f_SKN|coll = 10^-2 as the key rate assumption. The stress-test sharpens this: even accepting that fiducial value, the printed upper bound R0,SKNe ≲ 7 Gpc^-3 yr^-1 does not follow from Eq. 2.2 and the quoted CCSN rate. Either the bound should be 7×10^2 (which would strengthen the detection roadmap and weaken the claim that several hundred Mpc are needed for confident yearly detection), or the effective branching fraction is 10^-4 rather than 10^-2 (which would weaken the roadmap). The paper cannot have it both ways without an explicit statement that R_true_0,coll in Eq. 2.2 is the LLGRB subset rate rather than the total collapsar rate. This is central because Section II uses the 7 Gpc^-3 yr^-1 bound to argue that horizons of order 470 Mpc are needed for a 95% one-year detection probability, and Section VII repeats that only next-generation detectors can reach such distances. If the correct bound is 700, then current-generation CoCoA horizons of roughly 32-104 Mpc already correspond to a non-negligible yearly detection probability under the fiducial branching fraction, changing the qualitative conclusion. If the correct bound is 7, then the rate argument depends on an unstated factor-of-100 reduction in the branching fraction. The CoCoA detectability estimates themselves, and the conclusion that CE/ET are needed to reach ~400 Mpc events, are based on waveform and sensitivity calculations that are not challenged by this concern. Therefore the reader's CONDITIONAL verdict remains appropriate: the detectability roadmap is plausible, but the rate-based interpretation must be corrected or explicitly redefined before the yearly detection statements are taken at face value.","tokens_in":18811,"tokens_out":11330,"duration_ms":104422,"concrete_test":"Recompute Eq. 2.3 from Eq. 2.2 using R_true_0,coll = R0,CCSN ≈ 7×10^4 Gpc^-3 yr^-1 and f_SKN|coll = 10^-2. If the product is 7×10^2 rather than 7, search the paper for any explicit redefinition of R_true_0,coll as the LLGRB rate; if none exists, revise Eq. 2.3 and rederive the 95% detection distances in Section II. The corrected distance scale settles whether the rate-based annual detection statements in Section VII survive.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The weakest point is not merely that f_SKN|coll is a fiducial prior; Eq. 2.3 is internally inconsistent with the printed definitions. Eq. 2.2 defines R0,SKNe = f_SKN|coll × R_true_0,coll, and the text immediately identifies the baseline rate as the CCSN rate R0,CCSN ≈ 7×10^4 Gpc^-3 yr^-1. Multiplying by the adopted f_SKN|coll = 10^-2 gives R0,SKNe ≈ 7×10^2 Gpc^-3 yr^-1, a factor of 100 larger than the claimed bound R0,SKNe ≲ 7 Gpc^-3 yr^-1. To obtain 7, R_true_0,coll would have to be roughly the LLGRB collapsar-jet rate ≈7×10^2 Gpc^-3 yr^-1, not the CCSN baseline used in Eq. 2.2; the paper does not state that redefinition. This is an internal arithmetic ambiguity, not just an unmeasured prior. Because the 95% detection horizons in Section II and the conclusion that a confident yearly detection is plausible only at several hundred Mpc scale with R^{-1/3}, correcting the rate by the implied factor of 100 shifts the required distance by a factor of about 4.6 in either direction: if R=700, current-generation horizons already give a credible yearly detection probability, whereas if the intended bound is 7, the quoted confidence statements depend on the fiducial f=10^-4 rather than the stated 10^-2. The CoCoA sensitivity calculation itself is not implicated; the rate interpretation of Table III is what becomes unreliable.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"Prompted by the tentative association between the sub-threshold GW candidate S250818k and the Type IIb supernova SN 2025ulz, this paper investigates whether long-duration, non-standard GW chirps from hierarchical mergers involving sub-solar-mass neutron stars and lower-mass-gap black holes could be detected with the Cross-Correlation Algorithm (CoCoA). The paper derives a superkilonova rate bound from a fiducial branching fraction, constructs approximate analytic and LAL IMRPhenomD waveforms, computes single-trial matched-filter and CoCoA stochastic horizon distances for O4 and next-generation detector sensitivities, and estimates sky-localization areas for O4, O5, A#, and XG networks. The main conclusion is that EM-triggered CoCoA searches are worthwhile in current-generation data for nearby events at ~40 Mpc, while reaching the ~400 Mpc distance of SN 2025ulz requires next-generation detectors; this conclusion is explicitly acknowledged to depend on an unmeasured superkilonova branching fraction.","tokens_in":19228,"tokens_out":20622,"duration_ms":177590,"significance":"Assuming the rate ambiguity is resolved, the paper is a well-scoped, transparent detectability roadmap. Its strengths are that all sensitivity inputs come from public ASD curves, the CoCoA formalism is taken from a peer-reviewed derivation, the analytic waveforms are cross-checked against IMRPhenomD, and the choices are conservative in several places (neglect of tidal deformability, single-trial horizons, Hanning-window treatment of spectral leakage). The comparison between matched-filter and CoCoA reach and the network-localization curves are useful for planning EM-triggered and GW-triggered searches. The rate-based statements are the fragile part: they scale as the cube root of the assumed rate, so the Section VII claim that a confident yearly detection requires hundreds of Mpc is only as strong as the fiducial branching fraction.","major_comments":[{"comment":"The inequality R0,SKNe ≲ 7 Gpc−3 yr−1 does not follow from the definitions printed immediately above it. Equation (2.2) and the surrounding text treat R0,CCSN ≈ 7×10^4 Gpc−3 yr−1 as the baseline collapsar rate, so fSKN|coll = 10−2 yields R0,SKNe ≈ 7×10^2 Gpc−3 yr−1; the value 7 is obtained only if R_true_0,coll is identified with the LLGRB/collapsar rate ≈7×10^2 Gpc−3 yr−1, a redefinition that the paper does not state. Because the 95% detection distances in Section II and the \"≲85% probability\" in Section VI A scale as R^{−1/3}, this factor-of-100 ambiguity changes the required distance by about 4.6 and changes the Section VII conclusion about whether current-generation horizons are sufficient. Please define R_true_0,coll explicitly and correct Eq. (2.3) or the baseline rate.","section":"Section II, Eq. (2.3)"},{"comment":"The paper calls R0,SKNe ≲ 7 Gpc−3 yr−1 an upper bound and uses it to compute detection probabilities, but Section II states that fSKN|coll = 10−2 is a fiducial benchmark rather than an empirically measured rate. The manuscript should consistently label Eq. (2.3), the 470 Mpc and 218 Mpc horizons in Section II, and the ~85% figure in Section VI A as conditional on that benchmark, and should state explicitly how these numbers scale if fSKN|coll differs. This is not merely a wording issue, because the central conclusion in Section VII about the value of current-generation versus next-generation detectors is driven by this rate input.","section":"Sections II and VI A"}],"minor_comments":[{"comment":"The unit '(Gpc)' for the O4 CoCoA stochastic horizon column appears to be a typographical error; the values (32.2, 53.7, ...) are used in the text as megaparsecs, and the comparison with the ~40 Mpc events in Section VI A confirms that the intended unit is Mpc.","section":"Table III, header row"},{"comment":"The last approximation in Eq. (3.2) assumes m1 ≫ m2, but for the outer binary the mass ratio can be as large as ~0.3 (e.g., MBH = 3 M⊙, Mrem = 0.8 M⊙); please state the range of validity or use the exact Peters formula.","section":"Section III A, Eq. (3.2)"},{"comment":"The claimed robustness of CoCoA to waveform deviations from the superkilonova scenario is not demonstrated with mismodeled or eccentric injections; the paper should either add a caveat that the quoted horizons assume the quasi-circular time-frequency tracks are accurate, or include a quantitative robustness test.","section":"Sections IV A and VII"},{"comment":"The individual factors ffrag, f≥2NS|frag, fpair|≥2NS, and fmerge|pair are not assigned fiducial values; since only their product is used, the paper should state that the decomposition is schematic or provide the assumed values.","section":"Section II, Eq. (2.1)"},{"comment":"References [4] and [5] appear to be the same arXiv identifier (2605.05444) and should be merged or corrected.","section":"References [4] and [5]"},{"comment":"The '≲85% probability' estimate assumes 100% search efficiency out to 400 Mpc and does not fold in the EM selection effects inherent to an EM-triggered strategy; please state this explicitly or introduce an efficiency factor.","section":"Section VI A"}],"recommendation":"major_revision","confidential_remarks":"The principal issue is the rate arithmetic in Section II; once the authors clarify whether R_true_0,coll is the CCSN rate or the LLGRB/collapsar rate, the quantitative conclusions can be assessed properly. I see no problem with novelty or overlap; the manuscript is appropriate for astro-ph.HE, and the sensitivity estimates are otherwise transparent and reproducible."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"The short version: this is a solid, honest feasibility study that takes a known search method (CoCoA) and applies it, for the first time, to sub-solar NS-BH superkilonova chirps. The new numbers—Table III horizons and Figure 5 localization areas—are useful for planning LVK and XG searches. The paper is transparent about its conservative assumptions: single-trial horizons, ideal Gaussian noise, optimal orientation, no tidal deformability. The calculations trace cleanly to published equations and public ASDs.\n\nThe soft spot is the rate arithmetic in Section II. Equation 2.3 quotes R0,SKNe ≲ 7 Gpc^-3 yr^-1, which only works if R_true_0,coll is the engine-driven collapsar (LLGRB-like) rate of ~7×10^2, not the CCSN baseline of ~7×10^4 introduced earlier. A reader plugging the CCSN rate into Eq. 2.2 gets 7×10^2, a factor of 100 larger. I think the authors intend the LLGRB identification, but they don't say so. The correction matters because the yearly detection probability scales as R^{-1/3}; a factor of 100 moves the required distance by ~4.6. This should be fixed before publication. The rate is also a fiducial benchmark, not a measured constraint, which the authors acknowledge.\n\nThe other criticisms—no shipped code, single-trial versus full trials—are real but minor, and mostly acknowledged in the text. The waveform models are approximate but checked against IMRPhenomD.\n\nThis is a paper for GW search strategists and EM follow-up planners. It deserves peer review; the sensitivity results are solid, and the rate ambiguity is fixable with a clear definition. I'd bring it to a reading group if the conversation is about search strategies for speculative sources, but it's not a must-read for the broader community.","headline":"A useful, honest feasibility study of CoCoA for sub-solar NS chirps; the rate arithmetic in Eq. 2.3 needs a clearer definition of R_true_0,coll, but the sensitivity results hold up.","tokens_in":19744,"tokens_out":5808,"would_cite":true,"duration_ms":44834,"reading_group":"maybe","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":[],"model":"deepseek-v4-flash","headline":"This paper argues that electromagnetic-triggered CoCoA searches could detect long-duration sub-solar neutron-star merger chirps in current detectors out to tens of megaparsecs.","keywords":["gravitational waves","sub-solar-mass neutron stars","superkilonova","cross-correlation algorithm","long-duration gravitational-wave transients","lower mass gap","electromagnetic-triggered searches","core-collapse supernovae"],"falsifier":"A concrete test would be to run a CoCoA electromagnetic-triggered stochastic search over the sky position of SN 2025ulz and the merger-time window of S250818k in existing two-detector data; a null detection would directly bound the nearby superkilonova rate and weaken the claim that ~40 Mpc events are accessible.","tokens_in":18655,"feed_emoji":"🌊","tokens_out":12186,"duration_ms":96151,"temperature":0.7,"pith_summary":"This paper asks whether long-duration gravitational-wave chirps from 'superkilonova' mergers, in which sub-solar-mass neutron stars formed in a collapsar disk spiral into a lower-mass-gap black hole, can be detected when the waveforms are too uncertain for standard matched-filter templates. The authors argue that electromagnetic-triggered searches using the cross-correlation algorithm CoCoA can reach such signals in current-generation two-detector data out to roughly 32–104 Mpc, comparable to the ~40 Mpc distances of the nearest known supernova/GRB-associated explosions. They further show that only next-generation detectors reach the ~400 Mpc distance of SN 2025ulz and can localize the source tightly enough for gravitational-wave-triggered follow-up. A rate estimate anchored on an assumed 1% superkilonova branching fraction yields an upper bound of about 7 Gpc⁻³ yr⁻¹, implying that a confident yearly detection requires horizons of several hundred megaparsecs.","feed_headline":"Sub-solar neutron-star chirps may be audible to 100 Mpc today","feed_subtitle":"EM-triggered cross-correlation searches can probe nearby superkilonova mergers now; next-generation detectors reach 400 Mpc.","key_machinery":"The load-bearing object is CoCoA, the Cross-Correlation Algorithm, a triggered search that cross-correlates short Fourier transforms (SFTs) of detector data along modeled time–frequency tracks. This paper uses CoCoA in its stochastic limit, where only SFT pairs from different detectors at the same time are correlated, trading sensitivity for robustness against waveform mismatch. The detection condition is that the signal's RMS amplitude at the detector, computed from approximate analytical in-spiral waveforms for the outer neutron-star–black-hole binary, must exceed a threshold set by noise spectral densities, SFT duration (9 ms here), false-alarm and false-dismissal probabilities, and antenna-pattern factors. The rate argument is carried by a branching-fraction identity that multiplies the fraction of collapsars that fragment, form at least two surviving sub-solar neutron stars, bind them into a binary, and merge that binary before disk dispersal; with a fiducial 1% product, the well-measured local core-collapse supernova rate bounds the superkilonova rate at about 7 Gpc⁻³ yr⁻¹.","core_discovery":"The central claim is that CoCoA in its stochastic limit can detect the long-duration chirps predicted in the superkilonova scenario, provided the search is electromagnetic-triggered and the source is nearby. For black-hole masses of 3–5 solar masses and neutron-star remnant masses of 0.1–0.5 solar masses, single-trial CoCoA horizons in current-generation detector noise range from about 32 Mpc to 104 Mpc, covering the ~40 Mpc distances of the nearest known gravitational-wave/GRB-related explosions; the same waveforms would be visible to roughly 230–970 Mpc with a next-generation network of planned 40 km and 15 km detectors. These numbers imply that superkilonova searches in current data are worth doing only as targeted, electromagnetic-triggered campaigns, while the population as a whole becomes accessible only to next-generation detectors, which also shrink 90% localization areas by at least two orders of magnitude compared with current networks. The paper is careful to present this as a detectability roadmap for an unconfirmed hypothesis, not as evidence that superkilonovae exist.","pith_inferences":["The same CoCoA horizon machinery transfers to any long-duration quasi-periodic gravitational-wave signal with uncertain phase evolution; the 9 ms SFT and stochastic limit make the quoted reaches conservative relative to matched filtering by a factor of several.","If upcoming wide-field infrared surveys identify superkilonova-like transients with accurate sky positions, the electromagnetic-triggered strategy becomes the fastest route to detection because timing uncertainty, not sky localization, is then the dominant sensitivity loss.","A direct archival test would be to run a CoCoA electromagnetic-triggered search over the SN 2025ulz sky position and the S250818k merger-time window in existing two-detector data; a null result would directly bound the nearby superkilonova rate."],"forward_implications":["Electromagnetic-triggered CoCoA searches in current-generation detector data could already probe the nearest superkilonova-like events, since the reach in Table III overlaps the ~40 Mpc distances of known nearby GRB/SN explosions.","A planned sensitivity upgrade (A#) would extend current-generation reach to roughly 150 Mpc, the volume where transient surveys already classify several stripped-envelope supernovae per year, making follow-up more systematic.","Next-generation detectors are required both to reach ~400 Mpc events like SN 2025ulz and to provide small enough sky localizations for gravitational-wave-triggered searches to be practical.","Under the fiducial 1% branching fraction, the superkilonova rate upper bound of roughly 7 Gpc⁻³ yr⁻¹ implies that yearly confident detection is plausible only at horizons of several hundred megaparsecs."],"supporting_citations":[{"why":"It supplies the superkilonova scenario and the analytical in-spiral waveform approximations used for signal modeling.","marker":"[34]"},{"why":"It defines CoCoA and its stochastic-limit detection statistic, which sets the horizon distance estimates.","marker":"[38]"},{"why":"It provides the CoCoA electromagnetic-triggered search framework and quantifies the timing-uncertainty sensitivity loss.","marker":"[90]"},{"why":"It simulates collapsar-disk fragmentation into hierarchical sub-solar mergers, motivating the waveform complexity.","marker":"[104]"},{"why":"It shows how mass transfer in eccentric black-hole–neutron-star mergers can create sub-solar compact objects.","marker":"[106]"},{"why":"It analyzes the S250818k–SN 2025ulz association and supplies the ~400 Mpc distance target.","marker":"[57]"},{"why":"It constrains the fraction of engine-powered broad-lined Type Ic supernovae used in the rate upper bound.","marker":"[77]"},{"why":"It provides the local core-collapse supernova rate density that anchors the superkilonova rate estimate.","marker":"[87]"}],"fun_headline_variants":["CoCoA can hear sub-solar neutron-star chirps if EM-triggered to 100 Mpc","Sub-solar neutron-star chirps: EM-triggered CoCoA reaches 100 Mpc now","Next-gen detectors boost superkilonova chirp detection to ~1 Gpc","Superkilonova signals: CoCoA searches from EM alerts for sub-solar mergers","EM-triggered CoCoA could reveal sub-solar neutron-star chirps nearby"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The whole rate-and-reach interpretation rests on an unmeasured assumption, the idea that about one in a hundred collapsing massive stars produces a superkilonova; if the true fraction is far smaller, the quoted detection probabilities and the conclusion that a confident yearly detection is plausible at several hundred megaparsecs collapse.","fun_headline_variants_meta":{"raw":{"variants":["CoCoA can hear sub-solar neutron-star chirps if EM-triggered to 100 Mpc","Sub-solar neutron-star chirps: EM-triggered CoCoA reaches 100 Mpc now","Next-gen detectors boost superkilonova chirp detection to ~1 Gpc","Superkilonova signals: CoCoA searches from EM alerts for sub-solar mergers","EM-triggered CoCoA could reveal sub-solar neutron-star chirps nearby"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000432,"raw_usage":{"total_tokens":2319,"prompt_tokens":1178,"completion_tokens":1141,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":794,"completion_tokens_details":{"reasoning_tokens":1018}},"tokens_in":794,"tokens_out":1141,"duration_ms":9048,"temperature":1.0,"reasoning_tokens":1018,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-15T14:12:06.862698+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"A concrete test would be to run a CoCoA electromagnetic-triggered stochastic search over the sky position of SN 2025ulz and the merger-time window of S250818k in existing two-detector data; a null detection would directly bound the nearby superkilonova rate and weaken the claim that ~40 Mpc events are accessible.","supporting_citations":[{"cited_title":"2017, ApJ, 848, L13, doi:10.3847/ 2041-8213/aa920c","cited_arxiv_id":null,"evidence_quote":"It simulates collapsar-disk fragmentation into hierarchical sub-solar mergers, motivating the waveform complexity."}],"review_version":1}