Hierarchy of Universal Relations for Neutron Stars in Terms of Multipole Moments

Abstract

Recent studies of the analytical and numerical models of neutron stars suggest that their exterior field can be described by only four arbitrary parameters of the 2-soliton solution of Einstein's equations. Assuming that this is the case, we show that there exists an infinite hierarchy of the universal relations for neutron stars in terms of multipole moments that arises as a series of the degeneration conditions for generic soliton solutions. Our analysis of the simplest of these relations shows that the no-hair conjecture for neutron stars proposed by Yagi {\it et al.} fails to be verified by the perfect fluid models, but we argue that the conjecture could still be true for the models involving anisotropic fluid.