Alexander A. Philippov

Particle acceleration in axisymmetric pulsar current sheets

The equatorial current sheet in pulsar magnetospheres is often regarded as an ideal site for particle acceleration via relativistic reconnection. Using 2D spherical particle-in-cell simulations, we investigate particle acceleration in the axisymmetric pulsar magnetosphere as a function of the injected plasma multiplicity and magnetization. We observe a clear transition from a highly charge-separated magnetosphere for low plasma injection with little current and spin-down power, to a nearly force-free solution for high plasma multiplicity characterized by a prominent equatorial current sheet and high spin-down power. We find significant magnetic dissipation in the current sheet, up to 30% within 5 light-cylinder radii in the high-multiplicity regime. The simulations unambiguously demonstrate that the dissipated Poynting flux is efficiently channeled to the particles in the sheet, close to the Y-point within about 1-2 light cylinder radii from the star. The mean particle energy in the sheet is given by the upstream plasma magnetization at the light cylinder. The study of particle orbits shows that all energetic particles originate from the boundary layer between the open and the closed field lines. Energetic positrons always stream outward, while high-energy electrons precipitate back towards the star through the sheet and along the separatrices, which may result in auroral-like emission. Our results suggest that the current sheet and the separatrices may be the main source of high-energy radiation in young pulsars.

Modelling high-energy pulsar light curves from first principles

Current models of gamma-ray lightcurves in pulsars suffer from large uncertainties on the precise location of particle acceleration and radiation. Here, we present an attempt to alleviate these difficulties by solving for the electromagnetic structure of the oblique magnetosphere, particle acceleration, and the emission of radiation self-consistently, using 3D spherical particle-in-cell simulations. We find that the low-energy radiation is synchro-curvature radiation from the polar-cap regions within the light cylinder. In contrast, the high-energy emission is synchrotron radiation that originates exclusively from the Y-point and the equatorial current sheet where relativistic magnetic reconnection accelerates particles. In most cases, synthetic high-energy lightcurves contain two peaks that form when the current sheet sweeps across the observers line of sight. We find clear evidence of caustics in the emission pattern from the current sheet. High-obliquity solutions can present up to two additional secondary peaks from energetic particles in the wind region accelerated by the reconnection-induced flow near the current sheet. The high-energy radiative efficiency depends sensitively on the viewing angle, and decreases with increasing pulsar inclination. The high-energy emission is concentrated in the equatorial regions where most of the pulsar spindown is released and dissipated. These results have important implications for the interpretation of gamma-ray pulsar data.

Time evolution of pulsar obliquity angle from 3D simulations of magnetospheres

The rotational period of isolated pulsars increases over time due to the extraction of angular momentum by electromagnetic torques. These torques also change the obliquity angle

Three-dimensional analytical description of magnetized winds from oblique pulsars

\alpha

AB INITIO PULSAR MAGNETOSPHERE: THE ROLE OF GENERAL RELATIVITY

between the magnetic and rotational axes. Although actual pulsar magnetospheres are plasma-filled, the time evolution of

PULSAR MAGNETOSPHERES: BEYOND THE FLAT SPACETIME DIPOLE

\alpha

Evolution of non-spherical pulsars with plasma-filled magnetospheres

has mostly been studied for vacuum pulsar magnetospheres. In this work, we self-consistently account for the plasma effects for the first time by analysing the results of time-dependent 3D force-free and magnetohydrodynamic simulations of pulsar magnetospheres. We show that if a neutron star is spherically symmetric and is embedded with a dipolar magnetic moment, the pulsar evolves so as to minimise its spin-down luminosity: both vacuum and plasma-filled pulsars evolve toward the aligned configuration (

Analysis of Spin-Orbit Misalignment in the Eclipsing Binary DI Herculis

\alpha=0

Ab-initio Pulsar Magnetosphere: Particle Acceleration in Oblique Rotators and High-energy Emission Modeling

). However, they approach the alignment in qualitatively different ways. Vacuum pulsars come into alignment exponentially fast, with

Polarized synchrotron emission from the equatorial current sheet in gamma-ray pulsars

\alpha \propto \exp(-t/\tau)