Abstract

Several important properties of rotation-powered millisecond pulsars (MSPs), such as their mass-radius ratio, equation of state, and magnetic field topology, can be inferred from precise observations and modeling of their X-ray light curves. In the present study, we model the thermal X-ray signals originating in MSPs, all the way from numerically solving the surrounding magnetospheres up to the ray tracing of the emitted photons and the final computation of their light curves and spectra. Our modeled X-ray signals are then compared against very accurate NICER observations of four target pulsars: PSR J0437–4715, PSR J1231−1411, PSR J2124−3358, and PSR J0030+0451. We find very good simultaneous fits for the light curve and spectral distribution in all these pulsars.

The magnetosphere is solved by performing general relativistic force-free simulations of a rotating neutron star (NS) endowed with a simple centered dipolar magnetic field, for many different stellar compactness and pulsar misalignments. From these solutions, we derive an emissivity map over the star's surface, which is based on the electric currents in the magnetosphere. In particular, the emission regions (ERs) are determined in this model by spacelike four-currents that reach the NS. We show that this assumption, together with the gravitational curvature on the force-free simulations, leads to non-standard ERs facing the closed zone of the pulsar, in addition to other ERs within the polar caps. The combined X-ray signals from these two kinds of ERs (both antipodal) allow approximating the non-trivial interpulses found in all the target MSPs light curves.