Braking index of isolated pulsars: open questions and ways forward

TL;DR

This paper reviews the current understanding of pulsar braking indices, highlighting discrepancies between observed values and theoretical predictions, and discusses potential avenues for reconciling theory with observations in neutron star spin-down mechanisms.

Contribution

It provides a comprehensive review of the braking index problem in pulsars and explores new approaches to align theoretical models with observational data.

Findings

Observed braking indices are less than predicted by magnetic dipole radiation models.

Discrepancies suggest additional mechanisms may influence pulsar spin-down.

Future studies could incorporate alternative braking processes to improve theoretical models.

Abstract

Isolated pulsars are rotating neutron stars with accurately measured angular velocities $\Omega$, and their time derivatives which show unambiguously that the pulsars are slowing down. Although the exact mechanism of the spin-down is a question of debate, the commonly accepted view is that it arises either through emission of magnetic dipole radiation (MDR) from a rotating magnetized body, through emission of a relativistic particle wind, or via higher order magnetic multipole or gravitational quadrupole radiation. The calculated energy loss by a rotating pulsar is model dependent and leads to the power law $\dot{\Omega}$ = -K $\Omega^{\rm n}$ where $n$ is called the braking index. The theoretical value for braking index is $n = 1, 3, 5$ for wind, MDR, quadrupole radiation respectively. The accepted view is that pulsar braking is strongly dominated by MDR. Highly precise observations of isolated pulsars yield braking index values in the range $1 < n < 2.8$ which are consistently less than the value predicted from the MDR model. We discuss possible ways to bring theory closer to observation for the MDR, and also consider how the other mechanisms may play a role in future study of the braking index problem.