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Spheroidal force-free neutron star magnetospheres

Abridged. Fast rotating and self-gravitating astrophysical objects suffer strong deformations from centrifugal forces. If moreover they are magnetized, they generate an electromagnetic wave that is perturbed accordingly. When stellar objects are also surrounded by an ideal plasma, a magnetosphere is formed. We study the electromagnetic configuration of a force-free magnetosphere encompassing an ideal spheroidal rotating conductor as an inner boundary. We put special emphasize to millisecond period neutron star magnetospheres, those showing a significant oblate shape. Force-free solutions are computed by numerical integration of the time-dependent Maxwell equations in spheroidal coordinates. Relevant quantities such as the magnetic field structure, the spin down luminosity, the polar cap rims and the current density are shown. We find that the force-free magnetic field produced by spheroidal stars remains very similar to their spherical counterpart. However, the spin down luminosity slightly decreases with increasing oblateness or prolateness. Moreover the polar cap area increases and always mostly encompasses the equivalent spherical star polar cap rims. The polar cap current density is also drastically affected.