Physics motivated models of pulsar X-ray hotspots: off-center dipole configurations

TL;DR

This paper develops a physics-based model of pulsar X-ray hotspots considering off-center dipole magnetic fields and current distributions, successfully fitting observed light curves and highlighting the impact of return currents.

Contribution

It introduces a first-principles calculation of current and temperature distributions for off-center dipoles, improving the modeling of pulsar X-ray emission.

Findings

Model can fit observed X-ray light curves of PSR J0030+0451 and PSR J0437--4715.

Volumetric return current effects are more significant in off-center dipole configurations.

The approach enhances understanding of magnetic field geometry's influence on pulsar emission.

Abstract

Recently, it was proposed that an off-center dipole magnetic configuration, together with a non-trivial temperature profile, may be the best model to explain the X-ray light curve of PSR J0030+0451 observed by the Neutron Star Interior Composition Explorer (\emph{NICER}). Using a theoretical model for the electric current density in a force-free pulsar magnetosphere, we compute from first principles the distribution of electric current over the polar cap associated with an off-center magnetic dipole. We then use a simple prescription to compute the resulting temperature distribution, which allows us to derive the observed X-ray light curve. We investigate the role of the volumetric return current region in the polar cap and find that although it does not make a big difference in an aligned dipole case, the difference can be bigger in the case of an off-center dipole. Finally, we apply Markov Chain Monte Carlo (MCMC) fitting to the X-ray light curves of pulsars PSR J0030+0451 and PSR J0437--4715 with and without the volumetric return current, and find that our model can reasonably recover the observed X-ray light curves.