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Researchers analyzed X-ray observations of the massive millisecond pulsar PSR J0740+6620 to simultaneously determine its mass, radius, and magnetic field geometry by directly connecting surface hotspots to the magnetospheric structure. They found that a shifted-dipole magnetic configuration is strongly preferred over a centered dipole-quadrupole model, yielding a mass of 2.06 solar masses and radius of 13.28 kilometers. The analysis reveals that the X-ray-emitting region is much smaller than theoretical predictions suggest, indicating missing physics in current models of how magnetospheric currents produce observable X-ray emission.
Why it matters
This work improves constraints on the neutron star equation of state, which is crucial for understanding the behavior of matter at extreme densities. The methodology demonstrates that X-ray pulse profile modeling can simultaneously probe both neutron star structure and magnetospheric physics, potentially leading to better understanding of pulsar emission mechanisms.
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⚠️ Preprint – Noch nicht peer-reviewed
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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 $Deltaln Z=5.65$ (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.
Source: Joint Mass-Radius and Magnetic-Geometry Inference of PSR J0740+6620