{"paper":{"title":"Which Neutron Stars Reach the Stiffening Regime? Multimessenger Constraints on Core Sound Speed and Stellar-Mass Thresholds","license":"http://arxiv.org/licenses/nonexclusive-distrib/1.0/","headline":"Multimessenger data show neutron star core stiffening typically begins near 1.6 solar masses.","cross_cats":["astro-ph.SR","hep-ph","nucl-th"],"primary_cat":"astro-ph.HE","authors_text":"Nicol\\'as Viaux, Sebasti\\'an Mendizabal","submitted_at":"2026-04-08T19:37:25Z","abstract_excerpt":"We present a concise multimessenger inference of the neutron-star core sound-speed profile using GW170817 and three \\textit{NICER} mass--radius posteriors (PSR J0030$+$0451, PSR J0740$+$6620, and PSR J0437$-$4715). The main result is not only a preference for intermediate-density stiffening within smooth equation-of-state families, but a translation of that inference into the stellar masses that access the relevant density regime. In the baseline smooth peaked family, the posterior probability that $c_s^2 > 1/3$ at $3.5\\,n_{\\rm sat}$ is $85.4\\,\\%$, while equal-prior averaging over peaked, mono"},"claims":{"count":4,"items":[{"kind":"strongest_claim","text":"Posterior-resampled exact Tolman-Oppenheimer-Volkoff solutions show that the onset density of the inferred stiffening is typically reached near 1.6 M_⊙, whereas the peak region is accessed only near 2.1 M_⊙. A J0740-like 2.07 M_⊙ pulsar reaches the onset in 91% of posterior draws but the peak in only 46%.","source":"verdict.strongest_claim","status":"machine_extracted","claim_id":"C1","attestation":"unclaimed"},{"kind":"weakest_assumption","text":"That the four chosen families of equations of state (smooth peaked, monotonic, piecewise, transition-capable) together span the relevant physics without systematic bias from missing degrees of freedom or phase transitions not captured by the parameterization.","source":"verdict.weakest_assumption","status":"machine_extracted","claim_id":"C2","attestation":"unclaimed"},{"kind":"one_line_summary","text":"Multimessenger data favor core stiffening that typically starts near 1.6 solar masses and reaches its peak near 2.1 solar masses, with a 2.07-solar-mass pulsar accessing the onset in 91% of draws but the peak in only 46%.","source":"verdict.one_line_summary","status":"machine_extracted","claim_id":"C3","attestation":"unclaimed"},{"kind":"headline","text":"Multimessenger data show neutron star core stiffening typically begins near 1.6 solar masses.","source":"verdict.pith_extraction.headline","status":"machine_extracted","claim_id":"C4","attestation":"unclaimed"}],"snapshot_sha256":"42d2bde0ee006af7c2e012c8df129e81c9687699da58dfe8e98f289506313c91"},"source":{"id":"2604.07542","kind":"arxiv","version":1},"verdict":{"id":"98465f1f-3f02-4d03-8ebf-f8792fbf9551","model_set":{"reader":"grok-4.3"},"created_at":"2026-05-10T17:10:51.404019Z","strongest_claim":"Posterior-resampled exact Tolman-Oppenheimer-Volkoff solutions show that the onset density of the inferred stiffening is typically reached near 1.6 M_⊙, whereas the peak region is accessed only near 2.1 M_⊙. A J0740-like 2.07 M_⊙ pulsar reaches the onset in 91% of posterior draws but the peak in only 46%.","one_line_summary":"Multimessenger data favor core stiffening that typically starts near 1.6 solar masses and reaches its peak near 2.1 solar masses, with a 2.07-solar-mass pulsar accessing the onset in 91% of draws but the peak in only 46%.","pipeline_version":"pith-pipeline@v0.9.0","weakest_assumption":"That the four chosen families of equations of state (smooth peaked, monotonic, piecewise, transition-capable) together span the relevant physics without systematic bias from missing degrees of freedom or phase transitions not captured by the parameterization.","pith_extraction_headline":"Multimessenger data show neutron star core stiffening typically begins near 1.6 solar masses."},"integrity":{"clean":true,"summary":{"advisory":0,"critical":0,"by_detector":{},"informational":0},"endpoint":"/pith/2604.07542/integrity.json","findings":[],"available":true,"detectors_run":[],"snapshot_sha256":"c28c3603d3b5d939e8dc4c7e95fa8dfce3d595e45f758748cecf8e644a296938"},"references":{"count":19,"sample":[{"doi":"","year":2010,"title":"A. Kurkela, P. Romatschke, and A. Vuorinen, Cold Quark Matter, Phys. Rev. D81, 105021 (2010)","work_id":"d03e377d-3bda-486c-af0c-96b43cdeb6f7","ref_index":1,"cited_arxiv_id":"","is_internal_anchor":false},{"doi":"","year":2014,"title":"E. S. Fraga, A. Kurkela, and A. Vuorinen, Interacting quark matter equation of state for compact stars, Astro- phys. J. Lett.781, L25 (2014)","work_id":"e3c2c03f-9004-40e2-87f9-8240f53251dc","ref_index":2,"cited_arxiv_id":"","is_internal_anchor":false},{"doi":"","year":2021,"title":"T. Gorda, A. Kurkela, R. Paatelainen, S. S¨ appi, and A. Vuorinen, Soft Interactions in Cold Quark Matter, Phys. Rev. Lett.127, 162003 (2021)","work_id":"de5a7c73-baac-4f73-8310-f44d7f67235c","ref_index":3,"cited_arxiv_id":"","is_internal_anchor":false},{"doi":"","year":2018,"title":"E. Annala, T. Gorda, A. Kurkela, and A. Vuorinen, Gravitational-Wave Constraints on the Neutron-Star- Matter Equation of State, Phys. Rev. Lett.120, 172703 (2018)","work_id":"b7857e4e-0899-4560-bef4-e15cc016f057","ref_index":4,"cited_arxiv_id":"","is_internal_anchor":false},{"doi":"","year":2023,"title":"E. Annala, T. Gorda, J. Hirvonen, A. Kurkela, J. N¨ attil¨ a, and A. Vuorinen, Strongly interacting matter exhibits deconfined behavior in neutron stars, Nat. Commun.14, 8451 (2023)","work_id":"021618c2-57d4-41b7-a7ce-e08b9b28373f","ref_index":5,"cited_arxiv_id":"","is_internal_anchor":false}],"resolved_work":19,"snapshot_sha256":"376cec8dd5c69a5b822d330b796347485ab43559bc3ad9db0441d0ef05f15d1e","internal_anchors":0},"formal_canon":{"evidence_count":0,"snapshot_sha256":"258153158e38e3291e3d48162225fcdb2d5a3ed65a07baac614ab91432fd4f57"},"author_claims":{"count":0,"strong_count":0,"snapshot_sha256":"258153158e38e3291e3d48162225fcdb2d5a3ed65a07baac614ab91432fd4f57"},"builder_version":"pith-number-builder-2026-05-17-v1"}