Joint Mass-Radius and Magnetic-Geometry Inference of PSR J0740+6620
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 Δln Z=5.65 (Bayes factor ∼285), and yields M=2.059+0.068−0.069 M⊙ and R=13.28+1.47−1.07 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.
Related
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BibTeX
@misc{huang2026joint,
title = {{Joint Mass-Radius and Magnetic-Geometry Inference of PSR J0740+6620}},
author = {Chun Huang and Tuomo Salmi and Alexander Y. Chen},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.37928},
eprint = {2609.37928},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.37928},
}
RIS
TY - UNPB
TI - Joint Mass-Radius and Magnetic-Geometry Inference of PSR J0740+6620
AU - Chun Huang
AU - Tuomo Salmi
AU - Alexander Y. Chen
PY - 2026
DA - 2026/09/29
PB - arXiv
DO - 10.48550/arxiv.2609.37928
UR - https://arxiv.org/abs/2609.37928
AB - 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 Δln Z=5.65 (Bayes factor ∼285), and yields M=2.059^+0.068_−0.069 M_⊙ and R=13.28^+1.47_−1.07 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.
ER -