Physics of Pulsar Magnetospheres

Abstract

The Fermi Gamma-ray Space Telescope greatly increased the number of known pulsars in the gamma-ray band. It was discovered that pulsars often emit a significant fraction of their spin-down energy in gamma-rays, offering a new powerful diagnostic tool of neutron star magnetic field structure. The modeling of gamma-ray data suggests that in order to interpret the observations one needs to understand the field geometry and the plasma state in the emission region. In recent years, significant progress has been achieved in understanding the magnetospheric structure in the limit of abundant plasma supply, the so-called force-free limit. I used force-free solutions to study the time evolution of the pulsar's obliquity angle. I showed that pulsar rotation and magnetic axes evolve toward alignment as a power-law in time, in broad agreement with observations.

Despite the success of the force-free approach for understanding the global field geometry, the location of the emission region and radiation mechanism remained a mystery. To address this from first principles, in this thesis I developed kinetic three dimensional simulations of pulsar magnetospheres which include the physics of plasma production and particle acceleration. I found that the state of the magnetosphere crucially depends on the way charges are introduced into the magnetosphere. In simulations with self-consistent pair production I found several solutions with qualitatively different plasma distribution in the open field line region depending on the inclination of the pulsar. In a surprising finding, I discovered that GR effects help to establish efficient particle acceleration and pair production near the surface of low obliquity pulsars. Finally, I performed GR kinetic simulations of oblique rotators and showed that pulsar high-energy emission is dominated by synchrotron radiation emitted by particles that are energized by magnetic reconnection in the equatorial current sheet. The computed lightcurves and spectra of high-energy emission reproduce the observed morphology of gamma-ray pulsars.