Pulsar Constraints on Neutron Star Structure and Equation of State

The sudden spin jumps, or glitches, commonly seen in isolated neutron stars are thought to represent angular momentum transfer between the crust and the liquid interior [1]. In this picture, as a neutron star’s crust spins down under magnetic torque, differential rotation develops between the stellar crust and a portion of the liquid interior. The more rapidly rotating component then acts as an angular momentum reservoir which occasionally exerts a spin-up torque on the crust as a consequence of an instability. The Vela pulsar, one of the most active glitching pulsars, typically undergoes fractional changes in rotation rate of ∼ 10−6 every three years on average [2]. With the Vela pulsar having exhibited 13 glitches, meaningful study of the statistical properties of these events is now possible. In this Letter we study the time distribution of Vela’s glitches and determine the average angular momentum transfer rate in Vela and in six other pulsars. We present evidence that glitches in Vela represent a self-regulating instability for which the star prepares over a waiting interval. We obtain a lower limit on the fraction of the star’s liquid interior responsible for glitches. Assuming that glitches are driven by the liquid residing in the inner crust, as in most glitch models, we show that Vela’s ‘radiation radius’ is R∞∼ 12 km for a mass of 1.4M . Future measurements of neutron star radii will check the universality of this constraint and hence test our understanding of neutron star structure and the origin of glitches.