Neutron star spin up is the name given to the increase in rotational speed over time first noted in Cen X-3 and Her X-1 but now observed in other X-ray pulsars. In the case of Cen X-3, the pulse period is decreasing over a timescale of 3400 years (defined as P / P ˙ ˙ , where P is the rotation period and P ˙ ˙ is the rate of change in the rotation period).