Possible evolution of the pulsar braking index from larger than three to about one

TL;DR

This paper models the evolution of pulsar braking index, showing it can start above 3 and decrease to about 1 as the pulsar's spin-down mechanism shifts from magneto-dipole radiation to particle wind dominance.

Contribution

It introduces a coupled evolution model of pulsar rotation and inclination angle, explaining the observed range of braking indices through evolutionary stages.

Findings

Braking index initially exceeds 3 during early evolution.

Braking index decreases to about 1 in later stages.

Evolutionary trend from Crab-like to Vela-like pulsars.

Abstract

The coupled evolution of pulsar rotation and inclination angle in the wind braking model is calculated. The oblique pulsar tends to align. The pulsar alignment will affect its spin-down behavior. As a pulsar evolves from the magneto-dipole radiation dominated case to the particle wind dominated case, the braking index will first increase and then decrease. In the early time, the braking index may be larger than 3. And during the following long time, the braking index will be always smaller than 3. The minimum braking index is about one. This can explain the existence of high braking index larger than 3, and low braking index of pulsars simultaneously. The pulsar braking index is expected to evolve from larger than three to about one. A general trend is that the pulsar braking index will evolve from the Crab-like case to the Vela-like case.