On the braking index of the unusual high-B rotation-powered pulsar PSR J1846-0258

TL;DR

This study measures the braking index of PSR J1846-0258 over seven years post-outburst, revealing a significant and unprecedented change that challenges existing pulsar spin-down models.

Contribution

It provides the first long-term measurement of the braking index after a magnetar-like outburst in a high-B pulsar, showing a substantial and sustained change.

Findings

Braking index decreased from 2.65 to 2.19 after outburst.

The change in braking index is statistically significant and long-lasting.

Results challenge current models of pulsar spin-down behavior.

Abstract

PSR J1846-0258 is an object which straddles the boundary between magnetars and rotation powered pulsars. Though behaving for many years as a rotation-powered pulsar, in 2006, it exhibited distinctly magnetar-like behavior - emitting several short hard X-ray bursts, and a flux increase. Here we report on 7 years of post-outburst timing observations of PSR J1846-0258 using the Rossi X-ray Timing Explorer and the Swift X-ray Telescope. We measure the braking index over the post-magnetar outburst period to be $n=2.19\pm0.03$. This represents a change of $\Delta n=-0.46\pm0.03$ or a 14.5$\;\sigma$ difference from the pre-outburst braking index of $n=2.65\pm0.01$, which itself was measured over a span of 6.5 yr. So large and long-lived a change to a pulsar braking index is unprecedented and poses a significant challenge to models of pulsar spin-down.