Evidence for free precession in a pulsar

Pulsars are rotating neutron stars that produce lighthouse-like beams of ra dio emission from their magnetic poles. The observed pulse of emission enab les their rotation rates to be measured with great precision. For some youn g pulsars, this provides a means of studying the interior structure of neut ron stars. Most pulsars have stable pulse shapes, and slow down steadily (f or example, see ref. 20). Here we report the discovery of long-term, highly periodic and correlated variations in both the pulse shape and the rate of slow-down of the pulsar PSR B1828-11. The variations are best described as harmonically related sinusoids, with periods of approximately 1,000, 500 a nd 250 days, probably resulting from precession of the spin axis caused by an asymmetry in the shape of the pulsar. This is difficult to understand th eoretically, because torque-free precession of a solitary pulsar should be damped out by the vortices in its superfluid interior(1,2).