Electromagnetic and Radiative Properties of Neutron Star Magnetospheres

Abstract

Magnetospheres of neutron stars are commonly modeled as either devoid

of plasma in "vacuum'' models or filled with perfectly conducting plasma

with negligible inertia in "force-free'' models. While numerically

tractable, neither of these idealized limits can simultaneously

account for both the plasma currents and the accelerating electric

fields that are needed to explain the morphology and spectra of

high-energy emission from pulsars. In this work we improve upon these

models by considering the structure of magnetospheres filled with

resistive plasma. We formulate Ohm's Law in the minimal velocity

fluid frame and implement a time-dependent numerical code to construct a

family of resistive solutions that smoothly bridges the gap between

the vacuum and force-free magnetosphere solutions. We further apply

our method to create a self-consistent model for the recently

discovered intermittent pulsars that switch between two distinct

states: an "on'', radio-loud state, and an "off'', radio-quiet state

with lower spin-down luminosity. Essentially, we allow plasma to leak

off open field lines in the absence of pair production in the "off''

state, reproducing observed differences in spin-down rates. Next, we

examine models in which the high-energy emission from gamma-ray

pulsars comes from reconnecting current sheets and layers near and

beyond the light cylinder. The reconnected magnetic field provides a

reservoir of energy that heats particles and can power high-energy

synchrotron radiation. Emitting particles confined to the sheet

naturally result in a strong caustic on the skymap and double peaked

light curves for a broad range of observer angles. Interpulse bridge

emission likely arises from interior to the light cylinder, along last

open field lines that traverse the space between the polar caps and

the current sheet. Finally, we apply our code to solve for the

magnetospheric structure of merging neutron star binaries. We find

that the scaling of electromagnetic luminosity with orbital angular

velocity varies between the power 4 for nonspinning stars and the

power 1.5 for rapidly spinning millisecond pulsars near contact.

Our derived scalings and magnetospheres can be used to help understand

electromagnetic signatures from merging neutron stars to be observed

by Advanced LIGO.