The geometry of intermittent and magnetospheric state changes pulsars

¹ National Astronomical Research Institute of Thailand (Public Organization), 260 M.4, Donkaew, Maerim, Chiang Mai, 50180, Thailand
² Max Planck Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany
³ Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, University of Manchester, Manchester M13 9PL, UK
⁴ Dept. of Physics and Astronomy, University of British Columbia, 6224, Agricultural Road, Vancouver, B.C., V6T 1Z1, Canada

^⋆ Corresponding author: phrudth@gmail.com

Received: 28 February 2025
Accepted: 16 May 2025

Abstract

Context. Pulsars exhibit a variety of phenomena, including intermittency and magnetospheric state changes (MSC) – moding, nulling, and profile ν˙-correlation – which are thought to be related to changes in their magnetospheric plasma. The variation in pulsar emission patterns has been attributed to changes in the flow of magnetospheric plasma above the polar cap, affecting both radio emission and pulsar spin-down (ν˙).

Aims. This study aims to explore the link between these behaviours by investigating the geometry and spin-down characteristics of intermittent pulsars and MSC pulsars.

Methods. We estimated the magnetic inclination angles for intermittent pulsars and a sample of MSC pulsars using the rotating vector model and pulse-width-period correlations. A ‘plasma filling factor’ was introduced to extend the model proposed by Li et al. (2012b, ApJ, 746, 60) to MSC pulsars, with the aim of providing a unified framework for understanding these phenomena.

Results. Our results provide supporting evidence for a relationship between spin-down ratios and inclination angles that aligns with theoretical predictions. We determine the plasma filling factor explaining the observed spin-down variations for a number of pulsars. We also identify unusual emission height characteristics in PSR B1931+24 that suggest distinctive magnetospheric properties in intermittent pulsars.

Conclusions. This work establishes a quantitative link between intermittent and MSC pulsars through a simple unified model of magnetospheric plasma depletion. Future observations of additional pulsars with known spin-down variations will be crucial for refining this model and deepening our understanding of pulsar magnetospheres.