Thermal X‐Rays from Millisecond Pulsars: Constraining the Fundamental Properties of Neutron Stars

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Abstract
We model the X-ray properties of millisecond pulsars (MSPs) by considering hot-spot emission from a weakly magnetized neutron star (NS) covered by a hydrogen atmosphere. We investigate the limitations of using the thermal X-ray pulse profiles of MSPs to constrain the mass-to-radius (M/R) ratio of the NS. The accuracy is strongly dependent on the viewing angle and magnetic inclination, but is ultimately limited only by photon statistics. We demonstrate that valuable information regarding NSs can be extracted, even from data of fairly limited photon statistics through modeling of archival observations of the nearby isolated PSRs J0030+0451 and J2124–3358. The X-ray emission from these pulsars is consistent with the presence of an atmosphere and a dipolar field configuration. For both MSPs, the favorable geometry allows us to place limits on the allowed M/R of NSs. Assuming 1.4 M☉, the stellar radius is constrained to be R > 9.4 km and R > 7.8 km (68% confidence) for PSRs J0030+0451 and J2124–3358, respectively. We explore the prospects of using future observatories such as Constellation-X and XEUS to conduct X-ray-timing searches for MSPs not detectable at radio wavelengths due to unfavorable viewing geometry. We are also able to place strong constraints on the magnetic field evolution model proposed by Ruderman. The pulse profiles indicate that the magnetic field of an MSP does not have a tendency to align itself with the spin axis or migrate toward one of the spin poles during the low-mass X-ray binary phase.