{
  "abstract": "X-ray pulse-profile modeling can measure neutron-star radii, but its physical interpretation remains limited by the largely phenomenological description of the surface hotspots. Here we connect the X-ray hotspots directly to the pulsar magnetospheric structure and jointly infer the mass, radius, and magnetic geometry of the massive millisecond pulsar PSR J0740+6620 from \\emph{NICER} and \\emph{XMM-Newton} observations. Using a GPU-accelerated implementation, we compare two low-dimensional magnetic configurations that can produce non-antipodal emission: a centered axisymmetric dipole--quadrupole and a shifted dipole. The shifted-dipole model is decisively favored under the adopted priors, with $Δ\\ln Z=5.65$ (Bayes factor $\\sim285$), and yields $M=2.059^{+0.068}_{-0.069}\\,M_{\\odot}$ and $R=13.28^{+1.47}_{-1.07}\\,\\mathrm{km}$, broadly consistent with previous phenomenological analyses, while requiring nearly symmetric heating of the two polar caps. Both the marginalized shifted-dipole posterior and the timing-compatible representative require an effective X-ray-emitting footprint much smaller than the nominal force-free polar cap calibrated for a centered dipole. This compact footprint is not reproduced by our exploratory photon-conversion estimates and instead points to missing physics in the mapping from magnetospheric currents to observable X-ray emission. Our results show that physically motivated hotspots can preserve the mass--radius inference while turning hotspot morphology into a probe of pulsar magnetospheric structure and emission physics.",
  "abstract_shown": true,
  "affiliations": [],
  "article_type": null,
  "arxiv": "2609.37928",
  "author_total": 3,
  "authors": [
    {
      "name": "Chun Huang"
    },
    {
      "name": "Tuomo Salmi"
    },
    {
      "name": "Alexander Y. Chen"
    }
  ],
  "bibtex": "https://research.draconitic.com/paper/a3940h0ngm5bw.bib",
  "cited_by": 0,
  "doi": "10.48550/arxiv.2609.37928",
  "fulltext_url": null,
  "id": "a3940h0ngm5bw",
  "issns": [],
  "landing_url": "https://arxiv.org/abs/2609.37928",
  "language": null,
  "licence": "LicenseRef-arXiv-nonexclusive-1.0",
  "licence_label": "arXiv licence",
  "licence_source": "article",
  "licence_url": "http://arxiv.org/licenses/nonexclusive-distrib/1.0/",
  "metadata_licence": "CC0-1.0",
  "native_subjects": [
    [
      "arxiv",
      "astro-ph.HE"
    ],
    [
      "arxiv",
      "astro-ph.SR"
    ],
    [
      "arxiv",
      "gr-qc"
    ],
    [
      "arxiv",
      "nucl-th"
    ],
    [
      "openalex",
      "3103|Pulsars and Gravitational Waves Research"
    ]
  ],
  "openalex": "W7214969949",
  "pdf_url": "https://arxiv.org/pdf/2609.37928",
  "pmcid": null,
  "pmid": null,
  "published": "2026-09-29",
  "publisher": null,
  "ris": "https://research.draconitic.com/paper/a3940h0ngm5bw.ris",
  "sources": [
    {
      "first_seen": "2026-10-30T12:25:31Z",
      "native_id": "2609.37928",
      "source": "arxiv",
      "url": "https://arxiv.org/abs/2609.37928"
    }
  ],
  "subjects": [
    {
      "field": "astronomy",
      "origin": "arxiv:astro-ph.HE",
      "subfield": "high-energy-astrophysics"
    },
    {
      "field": "astronomy",
      "origin": "arxiv:astro-ph.SR",
      "subfield": "stellar"
    },
    {
      "field": "physics",
      "origin": "arxiv:gr-qc",
      "subfield": "gravitation"
    },
    {
      "field": "physics",
      "origin": "arxiv:nucl-th",
      "subfield": "nuclear"
    }
  ],
  "title": "Joint Mass-Radius and Magnetic-Geometry Inference of PSR J0740+6620",
  "title_text": "Joint Mass-Radius and Magnetic-Geometry Inference of PSR J0740+6620",
  "type": "preprint",
  "updated_at": "2026-10-30T12:25:31Z",
  "url": "https://research.draconitic.com/paper/a3940h0ngm5bw",
  "venue": "arXiv",
  "versions": []
}