Mode switching and oscillations in PSR B1828-11

The young pulsar PSR B1828􀀀11 has long been known to show correlated shape and spindown changes with timescales of roughly 500 and 250 days, perhaps associated with largescale magnetospheric switching. Here we present multi-hour observations with the Parkes and Green Bank Telescopes at multiple phases across the 500-day cycle and show that the pulsar undergoes mode-changing between two stable, extreme profile states. The fraction of time spent in each profile state naturally accounts for the observed overall "shape parameter" (defined to be 0 for wide profiles and 1 for narrow ones); this and the variable rate of the mode transitions are directly related to the spin-down changes. We observe that the mode transition rate could plausibly function as an additional parameter governing the chaotic behaviour in this object which was proposed earlier by Seymour & Lorimer. Free precession is not needed to account for the variations.