Quadrupole Moments of Rotating Neutron Stars

Numerical models of rotating neutron stars are constructed for four equationsof state using the computer code RNS written by Stergioulas. For five selectedvalues of the star's gravitational mass (in the interval between 1.0 and 1.8solar masses) and for each equation of state, the star's angular momentum isvaried from J=0 to the Keplerian limit J=J_{max}. For each neutron-starconfiguration we compute Q, the quadrupole moment of the mass distribution. Weshow that for given values of M and J, |Q| increases with the stiffness of theequation of state. For fixed mass and equation of state, the dependence on J iswell reproduced with a simple quadratic fit, Q \simeq - aJ^2/M c^2, where c isthe speed of light, and a is a parameter of order unity depending on the massand the equation of state.Comment: ReVTeX, 7 pages, 5 figures, additional material, and references adde