Evolution of non-spherical pulsars with plasma-filled magnetospheres

Abstract

Pulsars are famous for their rotational stability. Most of them steadily spin down and display a highly repetitive pulse shape. But some pulsars experience timing irregularities such as nulling, intermittency, mode changing and timing noise. As changes in the pulse shape are often correlated with timing irregularities, precession is a possible cause of these phenomena. Whereas pulsar magnetospheres are filled with plasma, most pulsar precession studies were carried out within the vacuum approximation and neglected the effects of magnetospheric currents and charges. Recent numerical simulations of plasma-filled pulsar magnetospheres provide us with a detailed quantitative description of magnetospheric torques exerted on the pulsar surface. In this paper, we present the study of neutron star evolution using these new torque expressions. We show that they lead to (1) much slower long-term evolution of pulsar parameters and (2) much less extreme solutions for these parameters than the vacuum magnetosphere models. To facilitate the interpretation of observed pulsar timing residuals, we derive an analytic model that (1) describes the time evolution of non-spherical pulsars and (2) translates the observed pulsar timing residuals into the geometrical parameters of the pulsar. We apply this model to two pulsars with very different temporal behaviours. For the pulsar B1828-11, we demonstrate that the timing residual curves allow two pulsar geometries: one with stellar deformation pointing along the magnetic axis and one along the rotational axis. For the Crab pulsar, we use the model show that the recent observation of its magnetic and rotational axes moving away from each other can be explained by precession.