Coupled axisymmetric pulsar magnetospheres

Abstract

We present solutions of force-free pulsar magnetospheres coupled with a uniform external magnetic field aligned with the dipole magnetic moment of the pulsar. The inclusion of the uniform magnetic field has the following consequences: The equatorial current sheet is truncated to a finite length, the fraction of field lines that are open increases, and the open field lines are confined within a cylindrical surface instead of becoming radial. A strong external magnetic field antiparallel to the dipole allows for solutions where the pulsar magnetic field is fully enclosed within an ellipsoidal surface. Configurations of fully enclosed or confined magnetospheres may appear in a double neutron star (DNS) where one of the components is a slowly rotating, strongly magnetised pulsar and the other a weakly magnetised millisecond pulsar. Depending on the geometry, twisted field lines from the millisecond pulsar could form an asymmetric wind. More dramatic consequences could appear closer to merger: there, the neutron star with the weaker magnetic field may undergo a stage where it alternates between an open and a fully enclosed magnetosphere releasing up $10^{38}$erg. Under favourable combinations of magnetic fields DNSs can spend up to 10$^3$ yr in the coupled phase, implying a Galactic population of $0.02$ systems. Potential observational signatures of this interaction are discussed including the possibility of powering recurring Fast Radio Bursts (FRBs). We also note that the magnetic interaction cannot have any significant effect on the spin evolution of the two pulsars.