Non‐linear hydrodynamical evolution of rotating relativistic stars: numerical methods and code tests

We present numerical hydrodynamical evolutions of rapidly rotating relativistic stars, using an axisymmetric, non‐linear relativistic hydrodynamics code. We use four different high‐resolution shock‐capturing (HRSC) finite‐difference schemes (based on approximate Riemann solvers) and compare their accuracy in preserving uniformly rotating stationary initial configurations in long‐term evolutions. Among these four schemes, we find that the third‐order piecewise parabolic method scheme is superior in maintaining the initial rotation law in long‐term evolutions, especially near the surface of the star. It is further shown that HRSC schemes are suitable for the evolution of perturbed neutron stars and for the accurate identification (via Fourier transforms) of normal modes of oscillation. This is demonstrated for radial and quadrupolar pulsations in the non‐rotating limit, where we find good agreement with frequencies obtained with a linear perturbation code. The code can be used for studying small‐amplitude or non‐linear pulsations of differentially rotating neutron stars, while our present results serve as testbed computations for three‐dimensional general‐relativistic evolution codes.