Ab initio pulsar magnetosphere: Three-dimensional particle-in-cell simulations of oblique pulsars

Abstract

We present“first-principles”relativistic particle-in-cell simulations of the oblique pulsar magnetosphere with pairformation. The magnetosphere starts to form with particles extracted from the surface of the neutron star. Theseparticles are accelerated by surface electricfields and emit photons capable of producing electron–positron pairs.We inject secondary pairs at the locations of primary energetic particles whose energy exceeds the threshold forpair formation. Wefind solutions that are close to the ideal force-free magnetosphere with theY-point and currentsheet. Solutions with obliquities⩽40°do not show pair production in the openfield line region because the localcurrent density along the magneticfield is below the Goldreich–Julian value. The bulk outflow in these solutions ischarge-separated, and pair formation happens in the current sheet and return current layer only. Solutions withhigher inclinations show pair production in the openfield line region, with high multiplicity of the bulkflow andthe size of the pair-producing region increasing with inclination. We observe the spin-down of the star to becomparable to MHD model predictions. The magnetic dissipation in the current sheet ranges between 20% for thealigned rotator and 3% for the orthogonal rotator. Our results suggest that for low obliquity neutron stars withsuppressed pair formation at the light cylinder, the presence of phenomena related to pair activity in the bulk of thepolar region, e.g., radio emission, may crucially depend on the physics beyond our simplified model, such as theeffects of curved spacetime or multipolar surfacefields.