Braking Index of Isolated Pulsars II: A novel two-dipole model of pulsar magnetism

Abstract

The magnetic dipole radiation (MDR) model is currently the best approach we have to explain pulsar radiation. However a most characteristic parameter of the observed radiation, the braking index n${}_{\mathrm{obs}}$ shows deviations for all the eight best studied isolated pulsars, from the simple model prediction n${}_{\mathrm{dip}}$ = 3. The index depends upon the rotational frequency and its first and second time derivatives, but also on the assumption of that the magnetic dipole moment and inclination angle, and the moment of inertia of the pulsar are constant in time. In a recent paper [Phys. Rev. D 91, 063007 (2015)] we showed conclusively that changes in the moment of inertia with frequency alone, cannot explain the observed braking indices. Possible observational evidence for the magnetic dipole moment migrating away from the rotational axis at a rate $\dot{\alpha }$ $\sim$ 0.6${}^{\circ }$ per 100 years over the life time of the Crab pulsar has been recently suggested by Lyne et al. In this paper, we explore the MDR model with constant moment of inertia and magnetic dipole moment but variable inclination angle $\alpha$. We first discuss the effect of the variation of $\alpha$ on the observed braking indices and show they all can be understood. However, no explanation for the origin of the change in $\alpha$ is provided. After discussion of the possible source(s) of magnetism in pulsars we propose a simple mechanism for the change in $\alpha$ based on a toy model in which the magnetic structure in pulsars consists of two interacting dipoles. We show that such a system can explain the Crab observation and the measured braking indices.

arXiv commentPre-print: 17 pages, 4 figures

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• Explaining high-braking indice of magnetars SGR 0501+4516 and 1E 2259+586 using the double magnetic-dipole model
• Dense Matter in Neutron Stars with eXTP
• On the anomalously large extension of the Pulsar Wind Nebula HESS J1825-137
• Evolution of Spin Period and Magnetic Field of the Crab Pulsar: Decay of the Braking Index by the Particle Wind Flow Torque
• Utilizing cosmic-ray positron and electron observations to probe the averaged properties of Milky Way pulsars
• A short review of the pulsar magnetic inclination angles (II)
• Refinement of the timing-based estimator of pulsar magnetic fields
• Application of a two dipole model to PSR J1640-4631, a pulsar with an anomalous braking index
• Gamma-radiation sky maps from compact binaries