Physics-motivated Models of Pulsar X-Ray Hotspots: Off-center Dipole Configurations

Recently, it was proposed that an off-center dipole magnetic configuration, together with a nontrivial 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. 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 the case of an aligned dipole, the difference can be bigger in the case of an off-center dipole. Finally, we apply Markov Chain Monte Carlo 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.