{"id":"34ccf85a-5b03-4736-a9b1-de9852c756dc","arxiv_id":"2411.19382","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":6.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":3,"one_line_summary":"A rotating iron core collapsing at about 45% of the Keplerian limit can shed enough mass to form the roughly 0.9 solar mass neutron star XMMU J1732, explaining its observed properties.","lead":"The paper proposes that the unusually light central object in the supernova remnant HESS J1731-347, XMMU J1732, is an ordinary neutron star born from the collapse of a rapidly spinning iron core that shed mass. The scenario ties together the object's low mass, warm surface, and a nearby post-AGB star, and if correct it would make XMMU J1732 the lightest neutron star known.","discovery_kind":"new_application","skeptic_critique":{"model":"deepseek-v4-flash","headline":"The mass-shedding threshold in Sec. 2.2 uses the uniformly rotating NS angular-momentum limit, but a core-collapse remnant is expected to be differentially rotating; this can substantially change the predicted surviving mass.","rationale":"The paper is transparent that it is advancing a scenario with a simplified hybrid model, and it explicitly calls for a full general-relativistic collapse calculation in the discussion. It also builds on established uniformly rotating NS sequences (Cipolletta et al. 2015) and a polytropic iron-core structure that matches detailed models, so the input physics is not arbitrary and deserves credit. Nevertheless, the single most load-bearing number is MNS≈0.91 Msun, produced by equating the initial cylindrical angular momentum with the uniformly rotating NS maximum angular momentum. The reader identified the initial uniform-rotation assumption and the chosen β=0.4 as the weak point; my concern is related but sharper: even granting the initial uniform rotation and β=0.4, cylindrical angular-momentum conservation drives the collapsing core toward differential rotation, and the mass-shedding threshold for a differentially rotating remnant is different. Because a differentially rotating star of the same mass can hold more angular momentum, the predicted surviving mass is likely higher, which would weaken the 'lightest neutron star' conclusion. This is a concrete, testable concern rather than a fatal flaw, and the existing CONDITIONAL verdict already captures the need for further verification. Hence I recommend no change to the reader's verdict.","tokens_in":14320,"tokens_out":11059,"duration_ms":103669,"concrete_test":"Recompute the Sec. 2.2 threshold using a differentially rotating equilibrium sequence instead of Eq. (3): for the same polytropic (or matching APR) EOS, take L(x) and M(x) from Eqs. (1)-(2), and solve L(xcrit)=Lmax_diff(M(xcrit)), where Lmax_diff is the maximum total angular momentum for a differentially rotating NS with the standard j-constant rotation law, obtained with a GR equilibrium code (e.g., RNS or Komatsu-Eriguchi-Hachisu). If the resulting M(xcrit) is significantly above 0.91 Msun, or if L(x) never exceeds Lmax_diff up to the full core, then the light-mass result in Sec. 2.2 is an artifact of the uniform-rotation assumption. If instead the masses agree within uncertainties, the concern is retired.","verdict_should_be":"UNCHANGED","load_bearing_attack":"In Sec. 2.2 the predicted final mass is fixed by the equality L(xcrit)=Lmax(M(xcrit)) (Eqs. 3 and 4), where Lmax≈0.7GM^2/c is explicitly taken from the general-relativistic uniformly rotating NS sequences of Cipolletta et al. (2015). For β=0.4 this gives M(xcrit)≈0.91 Msun and is the basis of conclusion (i). The load-bearing issue is that uniform rotation is a special slice of the equilibrium sequence, not the state expected after collapse. Cylindrical angular-momentum conservation (Eqs. 1-2) preserves specific angular momentum, and when a core contracts by a radius factor that depends on cylindrical radius, the resulting rotational profile is differential. A differentially rotating NS of a given gravitational mass can carry appreciably more total angular momentum than the uniformly rotating configuration at the mass-shedding limit; hence Eq. (4) is not the relevant threshold for mass shedding. If the remnant can hold L(xcrit) with a larger baryonic mass, less mass is shed and the surviving NS is heavier than 0.91 Msun, weakening the claim that this CCO is the lightest NS. Angular-momentum losses during collapse (neutrino, gravitational-wave, magnetic) act in the same direction, allowing more mass to be retained, so 0.91 Msun is not a robust lower bound. The paper acknowledges the need for a full general-relativistic collapse calculation, but the numerical conclusion already rests on the unstated assumption that the newborn NS is uniformly rotating before the shedding threshold is set.","agreement_with_reader":"partial"},"referee_report":{"model":"deepseek-v4-flash","summary":"The manuscript proposes a formation scenario for the low-mass central compact object XMMU J1732 in HESS J1731–347, in which a 1.2 M_sun iron core rotating at about 40–45% of the Keplerian limit sheds roughly 0.3 M_sun during collapse through the mass-shedding instability, producing a neutron star of about 0.9 M_sun. The authors support this scenario with a polytropic collapse model using cylindrical angular momentum conservation, a magnetic-dipole spin-down estimate, thermal evolution simulations with the APR equation of state, and a binary disruption scenario involving the nearby post-AGB star IRAS 17287–3443. They conclude that XMMU J1732 is indeed a light neutron star formed in a tidally locked, ultra-stripped binary that was disrupted by the supernova.","tokens_in":14637,"tokens_out":8556,"duration_ms":72944,"significance":"If the mass-shedding mechanism is robust, the paper offers a plausible astrophysical channel for forming neutron stars with masses well below the canonical values, explaining the unusual CCO without invoking exotic matter. The binary scenario gives a concrete, testable set of parameters, including an orbital period, kick velocity limit, and projected separation. The thermal evolution calculation adds a consistency check on the ordinary-neutron-star hypothesis. The significance is reduced by the model's dependence on the assumed uniform rotation of the collapsing core and on a chosen light-element envelope fraction.","major_comments":[{"comment":"The mass-shedding threshold in Eqs. (3)–(4) uses the maximum angular momentum of uniformly rotating neutron stars from Cipolletta et al. (2015). However, conservation of specific angular momentum of cylindrical shells (Eqs. (1)–(2)) during collapse from a uniformly rotating iron core generically results in a differentially rotating newborn star, not a uniformly rotating one. Since a differentially rotating NS of a given gravitational mass can support more angular momentum than its uniformly rotating counterpart, the equality L(xcrit)=Lmax(xcrit) is not necessarily the relevant condition for mass shedding. If differential rotation persists, the surviving mass could be larger than 0.91 M_sun, weakening the conclusion that XMMU J1732 is the lightest neutron star. This is a load-bearing simplification because the final mass is directly set by this comparison. The authors should either justify the uniform-rotation assumption with a timescale estimate for angular momentum redistribution during collapse, or repeat the calculation using differentially rotating equilibrium sequences.","section":"Sec. 2.2, Eqs. (1)–(4)"},{"comment":"The cooling agreement with the observed temperature is obtained for a light-element envelope with ΔM/M=10^-7, which the text states is 'deliberate' and 'crucial' to match. The heavy-element envelope (Fig. 2) does not reproduce the observed point. Since the envelope fraction is a free parameter chosen to fit the data, the statement in conclusion (iii) that the concurrence 'is not merely coincidental' and that the model eliminates speculative hypotheses is too strong. The authors should explicitly identify ΔM/M as a fitted parameter in their model comparison and discuss how the assumed fallback accretion scenario predicts this value, or downgrade the cooling match to a conditional consistency check.","section":"Sec. 2.4 and Fig. 3; conclusion (iii)"},{"comment":"The final neutron-star mass is highly sensitive to the initial rotation parameter β; the paper selects β=0.4 as 'suitable for XMMU J1732' after the fact, and only a narrow range β≈0.3–0.45 produces low masses. Although Sec. 3.1 derives β≈0.445 from a tidally locked binary with a specific mass and radius, that derivation depends on assumed progenitor properties from Laplace et al. (2021). The authors should quantify the uncertainty in the final mass due to plausible variations in β and in the iron core mass, and discuss whether the required near-Keplerian rotation is physically expected for ultra-stripped binaries.","section":"Sec. 2.2 and conclusion (i)"}],"minor_comments":[{"comment":"The phrase 'testament to the robustness of the the underlying assumptions' contains a doubled 'the', and 'excelent' should be spelled 'excellent'.","section":"Conclusion (iii)"},{"comment":"The first reference entry 'a. Akmal, Pandharipande, V. R., & Ravenhall, D. G.' has an odd formatting; it should be 'Akmal, A., Pandharipande, V. R., & Ravenhall, D. G.'","section":"References"},{"comment":"There are two separate entries for Horvath et al. (2023) with overlapping content; these should be merged.","section":"References"},{"comment":"The superscript in M^2 is not rendered properly; the text shows 'M 2' instead of 'M^2', in both Eq. (3) and the Figure 1 caption.","section":"Eq. (3) and Fig. 1 caption"},{"comment":"The phrase 'The case of β=0.4 is suitable for XMMU J1732' is informal; it could be rephrased as 'β=0.4 yields a final mass within the 1σ observational uncertainty of XMMU J1732'.","section":"Sec. 2.2"}],"recommendation":"major_revision","confidential_remarks":"The paper presents an interesting but speculative scenario. The main technical concern is the use of the uniformly rotating angular-momentum limit for a collapse that inherently produces differential rotation; this affects the central mass-shedding claim. If the authors can address this issue with a sensitivity test and tone down the overstatements in the conclusions, the paper could be publishable in a journal of this scope. The absence of a direct prediction of β outside the chosen range is also a concern that should be handled carefully in revision."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"You should know this paper proposes that the central compact object XMMU J1732 in HESS J1731–347, with an inferred mass around 0.77 solar masses, is actually a light neutron star formed through mass-shedding during the collapse of a rotating iron core in a binary. The idea is new for this object: prior explanations invoked strange stars, hybrid stars, or dark-matter-admixed neutron stars. The paper assembles a full scenario—rotating core collapse, spin-down, cooling, and binary disruption with a kick—and the cooling simulations are real numerical work, not hand-waving. The binary parameters are tied to the Laplace et al. ultra-stripped star models, which is a nice touch.\n\nThe soft spots are real, though. The predicted mass of 0.91 solar masses comes directly from choosing the rotation parameter beta ≈ 0.4, which is justified after the fact as \"suitable\" for the observed object. The cooling match requires a light-element envelope fraction of 10^-7, also chosen to fit. More importantly, the mass-shedding threshold uses the maximum angular momentum of a uniformly rotating neutron star from Cipolletta et al. A collapsing iron core is not expected to be uniformly rotating; it will be differentially rotating, and a differentially rotating remnant can carry substantially more angular momentum for the same mass. That means less mass would be shed and the survivor could be heavier than 0.91. Angular momentum losses during collapse would push the same direction. The authors acknowledge the need for a full GR collapse simulation, but the numerical conclusion already rests on the uniform-rotation assumption.\n\nI don't think the scenario is dead, but the central claim is not established. The paper would be stronger if it presented the mass as a range under different rotation profiles, and if it addressed the differential-rotation issue head-on. As it stands, it is a plausible, well-written scenario with tuned parameters and no uncertainty propagation. The right audience is neutron-star formation theorists and CCO observers, and it is worth a serious referee because it makes a concrete, testable proposal for a puzzling object.\n\nFor peer review, I would send it out, with the expectation of major revision focusing on the rotation profile and on presenting the free parameters as a studied range rather than fitted values.","headline":"A plausible but parameter-tuned scenario for the lightest neutron star, with the central mass-shedding calculation resting on a uniform-rotation assumption that may not hold after core collapse.","tokens_in":15168,"tokens_out":2862,"would_cite":false,"duration_ms":25962,"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":"The paper claims that XMMU J1732 in HESS J1731–347 is a genuine low-mass neutron star of about 0.9 solar masses, formed by the collapse of a rapidly rotating iron core that sheds mass.","keywords":["central compact object","neutron star formation","core-collapse supernova","mass shedding","rotating iron core","XMMU J1732","HESS J1731-347","binary disruption"],"falsifier":"Run a full general-relativistic simulation of the collapse of a 1.2 solar mass iron core with uniform rotation at β = 0.4 and a realistic equation of state: if the resulting neutron star mass is not close to 0.9 solar masses, or if the shed material does not escape, the central claim is refuted.","tokens_in":14111,"feed_emoji":"🔄","tokens_out":6714,"duration_ms":49165,"temperature":0.7,"pith_summary":"The paper argues that the central compact object XMMU J1732 in the supernova remnant HESS J1731–347 is not exotic matter but an ordinary neutron star of roughly 0.9 solar masses, making it the lightest neutron star ever observed. The formation path is the gravitational collapse of a 1.2 solar mass iron core that spins at 40–45% of the breakup limit, with angular momentum conservation causing about 0.3 solar masses to be shed during collapse. The paper then ties this to a binary scenario: the progenitor was an ultra-stripped core tidally locked to a post-AGB companion, and the supernova disruption explains the observed 0.3 pc offset. It also shows that standard cooling with a carbon-rich atmosphere reproduces the observed temperature and age. A sympathetic reader would care because this offers a way to form light neutron stars without invoking quark matter or dark matter.","feed_headline":"A spinning core collapse can make the lightest neutron star","feed_subtitle":"Angular momentum conservation sheds 0.3 solar masses, explaining the puzzling XMMU J1732.","key_machinery":"The key machinery is the cylindrical angular-momentum conservation condition for rotating core collapse. The iron core is modeled as a polytropic sphere, and for each cylindrical coordinate x the enclosed mass and angular momentum are computed (Eqs. 1–2). The collapse proceeds only for material that satisfies L(x) ≤ Lmax, where Lmax ≈ 0.7 G $M^{2}$/c is the maximum angular momentum a uniformly rotating neutron star can hold (Eq. 3). The equality defines a critical radius xcrit; matter inside falls into the neutron star, matter outside is shed. This turns the observed mass of XMMU J1732 into a constraint on the pre-collapse rotation parameter β.","core_discovery":"The central claim is a mass-shedding mechanism that resolves the low-mass neutron star puzzle. By requiring the collapsing iron core's angular momentum within each cylindrical radius to stay below the maximum angular momentum of a uniformly rotating relativistic neutron star, the paper shows that a core of 1.2 solar masses rotating at β = 0.4 of the Keplerian limit produces a newborn neutron star of about 0.91 solar masses, shedding 0.24–0.3 solar masses in the process. The resultant object spins down under magnetic dipole braking to a modest rotation rate, and its thermal evolution with a light-element (carbon-rich) envelope matches the observed surface temperature of about 2×$10^{6}$ K at the estimated age of 4.5–10 kyr. The paper concludes that XMMU J1732 is indeed a light neutron star of about 0.9 solar masses formed by rotation-assisted mass shedding.","pith_inferences":["The same cylindrical mass-shedding argument may apply to other low-mass compact objects, potentially predicting a population of light neutron stars produced by tidally locked binary progenitors.","If fallback accretion of the shed material is significant, the final mass could be closer to the observed 0.77 solar masses, and the carbon-rich surface could be a natural result of accretion; this can be tested with detailed fallback simulations.","A full general-relativistic collapse simulation with realistic microphysics would quantitatively test whether the shed mass escapes or is partly accreted, refining the expected final mass and spin.","The scenario predicts a specific pre-SN binary with an orbital period of about 1.43 days and no Roche-lobe overflow; binary population synthesis could assess how common such configurations are."],"forward_implications":["If correct, XMMU J1732's mass of roughly 0.9 solar masses is a real neutron star mass, making it the lightest neutron star known.","The measured mass and radius of XMMU J1732 would then constrain the nuclear equation of state in the low-density regime.","Rotation-assisted mass shedding becomes a viable formation channel for low-mass neutron stars in binary systems.","The binary disruption scenario predicts a kick velocity of up to 670 km/s, consistent with the observed 0.3 pc projected separation at the remnant age.","Standard neutron star cooling with a carbon-rich atmosphere can explain the object's thermal emission, so exotic alternatives are not needed for this source."],"supporting_citations":[{"why":"Provides the measured mass and radius of XMMU J1732, the observational baseline the model must reproduce.","marker":"Doroshenko et al. 2022"},{"why":"Identifies the post-AGB companion IRAS 17287–3443, estimates the SNR age, and gives the dust shell mass used in the binary scenario.","marker":"Doroshenko et al. 2016"},{"why":"Supplies the uniformly rotating iron core model, including the moment-of-inertia factor and the Keplerian angular velocity formula.","marker":"Boshkayev et al. 2013"},{"why":"Provides general relativistic relations for uniformly rotating neutron stars, including the mass correction and maximum angular momentum Lmax ≈ 0.7 GM^2/c.","marker":"Cipolletta et al. 2015"},{"why":"Gives ultra-stripped core evolution models with a 4.23 solar mass stripped star and 1.27 solar mass iron core, matching the inferred progenitor parameters.","marker":"Laplace et al. 2021"},{"why":"Establishes the standard lower limit of about 1.17 solar masses for neutron stars from core collapse, the threshold this paper challenges.","marker":"Suwa et al. 2018"},{"why":"Supplies the binary disruption criterion (ejected mass exceeding half the total mass) used to argue the supernova disrupted the binary and to estimate the kick velocity.","marker":"Hills 1983"}],"fun_headline_variants":["Rotation sheds mass, births lightest neutron star","Rotating core collapse yields 0.9-solar-mass neutron star","Spinning core collapse solves lightest neutron star puzzle","Mass-shedding collapse explains XMMU J1732's low mass","Angular momentum loss forms the lightest known neutron star"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The whole mass-shedding calculation rests on the assumption that the iron core rotates uniformly at 40–45% of the Keplerian limit and conserves angular momentum in cylindrical shells during collapse; if the rotation is differential or angular momentum is redistributed or lost, the predicted surviving mass changes.","fun_headline_variants_meta":{"raw":{"variants":["Rotation sheds mass, births lightest neutron star","Rotating core collapse yields 0.9-solar-mass neutron star","Spinning core collapse solves lightest neutron star puzzle","Mass-shedding collapse explains XMMU J1732's low mass","Angular momentum loss forms the lightest known neutron star"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.00069,"raw_usage":{"total_tokens":3225,"prompt_tokens":1143,"completion_tokens":2082,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":759,"completion_tokens_details":{"reasoning_tokens":2007}},"tokens_in":759,"tokens_out":2082,"duration_ms":12229,"temperature":1.0,"reasoning_tokens":2007,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-12T10:13:01.313544+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Run a full general-relativistic simulation of the collapse of a 1.2 solar mass iron core with uniform rotation at β = 0.4 and a realistic equation of state: if the resulting neutron star mass is not close to 0.9 solar masses, or if the shed material does not escape, the central claim is refuted.","supporting_citations":[{"cited_title":"2016, , 458, 2565, 10.1093/mnras/stw499","cited_arxiv_id":null,"evidence_quote":"Identifies the post-AGB companion IRAS 17287–3443, estimates the SNR age, and gives the dust shell mass used in the binary scenario."},{"cited_title":"A., Ruffini , R., & Siutsou , I","cited_arxiv_id":null,"evidence_quote":"Supplies the uniformly rotating iron core model, including the moment-of-inertia factor and the Keplerian angular velocity formula."}],"review_version":1}