The matter can be modeled to be a perfect fluid because observations of pulsar glitches have shown that the departures from a perfect fluid equilibrium (due to the presence of a solid crust) are of order 10–5 (see ). The temperature of a cold neutron star does not affect its bulk properties and can be assumed to be 0 K, because its thermal energy () is much smaller than Fermi energies of the interior ( 60 MeV). One can then use a zero-temperature, barotropic equation of state (EOS) to describe the matter:[101, 102, 78] . In recent work, Shapiro  suggests that magnetic braking of differential rotation by Alfvén waves could be the most effective damping mechanism, acting on short timescales of the order of minutes.
Within roughly a year after its formation, the temperature of a neutron star becomes less than 109 K and its outer core is expected to become superfluid (see  and references therein). Rotation causes superfluid neutrons to form an array of quantized vortices, with an intervortex spacing of2 s–1. On scales much larger than the intervortex spacing, e.g., on the order of 1 cm, the fluid motions can be averaged and the rotation can be considered to be uniform . With such an assumption, the error in computing the metric is of order
The above arguments show that the bulk properties of an isolated rotating relativistic star can be modeled accurately by a uniformly rotating, zero-temperature perfect fluid. Effects of differential rotation and of finite temperature need only be considered during the first year (or less) after the formation of a relativistic star.
© Max Planck Society and the author(s)