Unraveling Pulsar Magnetospheres: Challenges in Particle-in-Cell(PIC) Simulations and Theory

Pulsars, the spinning remnants of massive stars, act as cosmic lighthouses, emitting beams of radiation that reveal the extreme physics of neutron stars. Yet, our understanding of their magnetospheres—the magnetic environments powering these beams—remains incomplete, limiting our understanding of pulsar variability. A reference ideal force-free magnetosphere for an oblique rotator is still lacking.

The most recent global Particle-in-Cell (PIC) simulations have raised several questions. Notably, these simulations yield separatrix regions between open and closed field lines that exhibit a significant thickness beyond the simulation resolution, and a multi-layered internal structure. This is intriguing because the same simulations are able to generate thin current sheets in the equator. Moreover, their closed-line regions terminate well inside the light cylinder, with regions of strong electromagnetic dissipation beyond their tips, something that was not expected by Goldreich and Julian. Interestingly, the shape of the tips resembles a pointed Y, thus deviating from the theoretically predicted T shape.

We do not understand the physical origin of these effects in PIC simulations. Unfortunately, it is in general hard to differentiate between artificial (numerical) and true (physical) features in the published solutions. The resolution of current global PIC simulations is inadequate by several orders of magnitude to properly model the microphysics of the equatorial current sheet. In order to generate pulsar light curves and spectra that may be compared with observations, the simulation results are extrapolated by several orders of magnitude. Unfortunately, there is no agreement among different research groups on the particular method of extrapolation, and as a result, there is still no generally accepted understanding of the physical origin of the high-energy radiation from pulsars.

In this Collection, we propose to discuss all of the above open issues in 5 main categories: numerical methods, the reference 3D ideal force-free solution, pair production, magnetospheric dissipation, high-energy radiation.

Keywords: pulsar magnetospheres, numerical methods, relativistic particle acceleration, relativistic current sheets, high-energy radiation (gamma-rays)