Luminosity Evolution of Gamma-ray Pulsars

TL;DR

This paper models the evolution of gamma-ray emission from pulsars by analyzing how the outer-magnetospheric accelerator's structure changes with age, influenced by the neutron star's surface composition, affecting luminosity and emission energy.

Contribution

It introduces a model linking pulsar age, envelope composition, and gamma-ray luminosity evolution, providing explanations for observed luminosity bounds and spectral cutoff energies.

Findings

Gamma-ray luminosity remains constant for light-element envelopes in early pulsar life.

Heavy-element envelopes lead to a rapid decrease in gamma-ray luminosity with age.

The primary curvature emission cutoff energy is below several GeV due to pair polarization effects.

Abstract

We investigate the electrodynamic structure of a pulsar outer-magnetospheric particle accelerator and the resultant gamma-ray emission. By considering the condition for the accelerator to be self-sustained, we derive how the trans-magnetic-field thickness of the accelerator evolves with the pulsar age. It is found that the thickness is small but increases steadily if the neutron-star envelope is contaminated by sufficient light elements. For such a light element envelope, the gamma-ray luminosity of the accelerator is kept approximately constant as a function of age in the initial ten thousand years, forming the lower bound of the observed distribution of the gamma-ray luminosity of rotation-powered pulsars. If the envelope consists of only heavy elements, on the other hand, the thickness is greater but increases less rapidly than what a light element envelope has. For such a heavy element envelope, the gamma-ray luminosity decreases relatively rapidly, forming the upper bound of the observed distribution. The gamma-ray luminosity of a general pulsar resides between these two extreme cases, reflecting the envelope composition and the magnetic inclination angle with respect to the rotation axis. The cutoff energy of the primary curvature emission is regulated below several GeV even for young pulsars, because the gap thickness, and hence the acceleration electric field is suppressed by the polarization of the produced pairs.