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The recently discovered rotationally powered pulsar PSR J1640-4631 is thefirst to have a braking index measured, with high enough precision, that isgreater than three. An inclined magnetic rotator in vacuum or plasma would besubject not only to spin-down but also to an alignment torque. The vacuum modelcan address the braking index only for an almost orthogonal rotator that isincompatible with the single peaked pulse profile. The magnetic dipole modelwith the corotating plasma predicts braking indices between $3-3.25$. We findthat the braking index of $3.15$ is consistent with two different inclinationangles, $18.5\pm 3$ degrees and $56 \pm 4$ degrees. The smaller angle ispreferred given the pulse profile has a single peak and the radio output of thesource is weak. We infer the change in the inclination angle to be at the rate$-0.23$ degrees per century, three times smaller in absolute value than therate recently observed for the Crab pulsar.

Inclination angle and braking index evolution of pulsars with plasma-filled magnetosphere: application to high braking index of PSR J1640-4631