ON THE BRAKING INDEX OF THE UNUSUAL HIGH-B ROTATION-POWERED PULSAR PSR J1846–0258

PSR J1846–0258 is an object that 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 ± 0.03. This represents a change of Δn = −0.46 ± 0.03 or a 14.5σ difference from the pre-outburst braking index of n = 2.65 ± 0.01, which itself was measured over a span of 6.5 years. A change to a pulsar braking index so large and long-lived is unprecedented and poses a significant challenge to models of pulsar spin-down