Realistic exact solution for the exterior field of a rotating neutron star

Abstract

A new six-parametric, axisymmetric, and asymptotically flat exact solution of Einstein-Maxwell field equations having reflection symmetry is presented. It has arbitrary physical parameters of mass, angular momentum, mass-quadrupole moment, current octupole moment, electric charge, and magnetic dipole, so it can represent the exterior field of a rotating, deformed, magnetized, and charged object; some properties of the closed-form analytic solution such as its multipolar structure, electromagnetic fields, and singularities are also presented. In the vacuum case, this analytic solution is matched to some numerical interior solutions representing neutron stars, calculated by Berti and Stergioulas [E. Berti and N. Stergioulas, Mon. Not. R. Astron. Soc. 350, 1416 (2004)], imposing that the multipole moments be the same. As an independent test of accuracy of the solution to describe exterior fields of neutron stars, we present an extensive comparison of the radii of innermost stable circular orbits (ISCOs) obtained from the Berti and Stergioulas numerical solutions, the Kerr solution [R. P. Kerr, Phys. Rev. Lett. 11, 237 (1963)], the Hartle and Thorne solution [J. B. Hartle and K. S. Thorne, Astrophys. J. 153, 807 (1968)], an analytic series expansion derived by Shibata and Sasaki [M. Shibata and M. Sasaki, Phys. Rev. D 58, 104011 (1998)], and our exact solution. We found that radii of ISCOs from our solution fits better than others with realistic numerical interior solutions.