A High Braking Index for a Pulsar

TL;DR

This paper reports a precise measurement of an unusually high braking index for a young pulsar, suggesting alternative spin-down mechanisms beyond magnetic dipole radiation, with implications for pulsar physics.

Contribution

The study provides the first reliable measurement of a braking index greater than 3 for PSR J1640-4631, indicating new physical processes in pulsar spin-down.

Findings

Braking index measured as 3.15+/-0.03

High braking index not due to timing noise

Constraints on pulsar flux at 1.4 GHz

Abstract

We present a phase-coherent timing solution for PSR J1640-4631, a young 206 ms pulsar using X-ray timing observations taken with NuSTAR. Over this timing campaign, we have measured the braking index of PSR J1640-4631 to be n = 3.15+/-0.03. Using a series of simulations, we argue that this unusually high braking index is not due to timing noise, but is intrinsic to the pulsar's spin-down. We cannot, however, rule out contamination due to an unseen glitch recovery, although the recovery timescale would have to be longer than most yet observed. If this braking index is eventually proven to be stable, it demonstrates that pulsar braking indices greater than 3 are allowed in nature, hence other physical mechanisms such as mass or magnetic quadrupoles are important in pulsar spin-down. We also present a 3-sigma upper limit on the pulsed flux at 1.4 GHz of 0.018 mJy.