High-energy radiation from the pulsar Equatorial Current Sheet

Abstract

Pulsars emit beams of radiation that reveal the extreme physics of neutron star magnetospheres. Yet, their understanding remains incomplete. Recent global Particle-in-Cell (PIC) simulations have raised several questions that led us to question their validity and their extrapolation to realistic particle Lorentz factors, electric and magnetic fields. We want to generate realistic sky maps of high-energy radiation from first principles. We propose a novel method to study the Equatorial Current Sheet (ECS) where most of the particle acceleration and the high-energy radiation is expected to originate. We first determine its shape and external magnetic field with a steady-state ideal force-free solution. Then, we consider the extra electric and magnetic field components that develop when dissipation is considered. Finally, we study the particle acceleration and radiation that is due to these extra field components for realistic field and particle parameters. We generate realistic sky maps of high-energy radiation and compare them with those obtained via PIC simulations. These sky maps may also be closely reproduced using the ECS of the split-monopole solution beyond the light cylinder. The ECS is probably stabilized by the normal magnetic field component that is due to the global magnetospheric reconnection. Our method helps us better understand the origin of the pulsed high-energy radiation in the pulsar magnetosphere.

Figures (4)

• Figure 1: Sketch of the Equatorial Current Sheet (ECS) along its thickness. Left: Ideal Harris-type current sheet with perfect field reversal accross it and no dissipation. Right: Dissipative current sheet with non-zero extra electric and magnetic field components $\delta E$ and $\delta B$ respectively in its interior. The normal magnetic field component $\delta B$ is absent in local reconnection simulations that start from a Harris-type current sheet. Wherever $\delta E>\delta B$, there is particle acceleration, thus also dissipation in the current sheet. In both cases, the ECS lies inside a steady-state corotating ideal force-free (FFE) magnetosphere.
• Figure 2: Cross section of the current sheet of our numerical solution (black line) along the plane containing the magnetic and rotation axes. For comparison, the current sheet of the split monopole is also shown (red line). Dashed line: light cylinder.
• Figure 3: High-energy sky maps and sample light curves for pulsar inclination $\lambda=20^\circ$. Phase zero corresponds to the arrival of the radio pulse emitted from the stellar magnetic poles. White dashed line: radial radiation from the ECS of a central split monopole from the stellar surface and beyond. Red line: radial radiation from the ECS of a central split monopole from the light cylinder and beyond. We observe that in all cases except for (3), the radiation extends to latitudes higher than $\pm\lambda$. We also expect that since the ECS consists mostly of positrons, the above right plots in the top 2 subplots that correspond to the electrons are expected to be much weaker (but not negligible) than the left ones that correspond to the positrons.
• Figure 4: Similar to Figure \ref{['skymaps']} for the split monopole solution of section 3 at inclination $\lambda=20^\circ$. The light curves are very similar to the ones produced with our numerical solution. They are also narrower (more caustic-like). We see the clear shift from the red to the white line when 'free' particles in the ECS start emitting from the stellar surface.