Rotational evolution of young pulsars due to superfluid decoupling

Abstract

Pulsars are rotating neutron stars that are seen to slow down, and the spin-down rate is thought to be due to magnetic dipole radiation. This leads to a prediction for the braking index n, which is a combination of spin period and its first and second time derivatives. However, all observed values of n are below the predicted value of 3. Here we provide a simple model that can explain the rotational evolution of young pulsars, including the n=2.51 of the 958-year-old pulsar in the Crab nebula. The model is based on a decrease in effective moment of inertia due to an increase in the fraction of the stellar core that becomes superfluid as the star cools via neutrino emission. The results suggest that future large radio monitoring campaigns of pulsars will yield measurements of the neutron star mass, nuclear equation of state, and superfluid properties.