On the Spin-Down of Intermittent Pulsars

TL;DR

This paper presents self-consistent numerical models of intermittent pulsar magnetospheres, explaining the observed differences in spin-down rates between their 'on' and 'off' states through plasma supply variations.

Contribution

The study introduces a novel numerical approach modeling both states of intermittent pulsars, linking plasma supply disruption to changes in spin-down rates and pulsar inclination angles.

Findings

Spin-down ratio between states ranges from 1.2 to 2.9.

'Off' state modeled as nearly vacuum on open lines, force-free on closed lines.

Range of inclination angles consistent with observed spin-down differences.

Abstract

Magnetospheres of pulsars are thought to be filled with plasma, and variations in plasma supply can affect both pulsar emission properties and spin-down rates. A number of recently discovered "intermittent" pulsars switch between two distinct states: an "on", radio-loud state, and an "off", radio-quiet state. Spin-down rates in the two states differ by a large factor, $\sim 1.5-2.5$, which is not easily understood in the context of current models. In this Letter we present self-consistent numerical solutions of "on" and "off" states of intermittent pulsar magnetospheres. We model the "on" state as a nearly ideal force-free magnetosphere with abundant magnetospheric plasma supply. The lack of radio emission in the "off" state is associated with plasma supply disruption that results in lower plasma density on the open field lines. We model the "off" state using nearly vacuum conditions on the open field lines and nearly ideal force-free conditions on the closed field lines, where plasma can remain trapped even in the absence of pair production. The toroidal advection of plasma in the closed zone in the "off" state causes spin-downs that are a factor of $\sim 2$ higher than vacuum values, and we naturally obtain a range of spin-down ratios between the "on" and "off" states, $\sim 1.2-2.9$, which corresponds to a likely range of pulsar inclination angles of $30{-}90^\circ$. We consider the implications of our model to a number of poorly understood but possibly related pulsar phenomena, including nulling, timing noise, and rotating radio transients.