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

Alexander Philippov; Alexander Tchekhovskoy; Jason G. Li

doi:10.1093/mnras/stu591

paper

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

arXiv:1311.1513 · doi:10.1093/mnras/stu591

Abstract

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 $α$ between the magnetic and rotational axes. Although actual pulsar magnetospheres are plasma-filled, the time evolution of $α$ 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 ( $α = 0$ ). However, they approach the alignment in qualitatively different ways. Vacuum pulsars come into alignment exponentially fast, with $α \propto exp (- t / τ)$ and $τ \sim$ spindown timescale. In contrast, we find that plasma-filled pulsars align much more slowly, with $α \propto (t / τ)^{- 1/2}$ . We argue that the slow time evolution of obliquity of plasma-filled pulsars can potentially resolve several observational puzzles, including the origin of normal pulsars with periods of $\sim 1$ second, the evidence that oblique pulsars come into alignment over a timescale of $\sim 1 0^{7}$ years, and the observed deficit, relative to an isotropic obliquity distribution, of pulsars showing interpulse emission.

9 pages, 7 figures, MNRAS, in press

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

Abstract

References in corpus (7)

Cited by in corpus (86)