Slowly rotating anisotropic neutron stars in general relativity and scalar–tensor theory

Abstract

Some models (such as the Skyrme model, a low-energy effective field theory for QCD) suggest that the high-density matter prevailing in neutron star interiors may be significantly anisotropic. Anisotropy is known to affect the bulk properties of nonrotating neutron stars in General Relativity. In this paper we study the effects of anisotropy on slowly rotating stars in General Relativity. We also consider one of the most popular extensions of Einstein's theory, namely scalar-tensor theories allowing for spontaneous scalarization (a phase transition similar to spontaneous magnetization in ferromagnetic materials). Anisotropy affects the moment of inertia of neutron stars (a quantity that could potentially be measured in binary pulsar systems) in both theories. We find that the effects of scalarization increase (decrease) when the tangential pressure is bigger (smaller) than the radial pressure, and we present a simple criterion to determine the onset of scalarization by linearizing the scalar-field equation. Our calculations suggest that binary pulsar observations may constrain the degree of anisotropy or even, more optimistically, provide evidence for anisotropy in neutron star cores.

Gravitational fields with sources: From compact objects to black holes

“…Caio Macedo presented some recent work on slowly rotating anisotropic NSs in GR and in scalar-tensor theory 80 . Some models of matter at ultranuclear density suggest that the core of a NS may be significantly anisotropic.…”

Neutron stars in the Bogomol'nyi-Prasad-Sommerfield Skyrme model: Mean-field limit versus full field theory

“…On the contrary, the energy density ρ and pressure p are both metric dependent functions and, therefore, it is not expected that any unique EoS for all solution may exist. We remark that a similar geometry-dependent (so-called quasi-local) EoS has been used in an analysis of gravastars and neutron stars with anisotropic matter, see, e.g., [39] - [41]. However, for the assumed ansatz, which is natural for neutron stars, one can derive an on-shell EoS.…”

Scalarization of neutron stars with realistic equations of state

“…The phenomenon of spontaneous scalarization can lead to significant deviations of the basic neutron star properties from GR as demonstrated for static and slowly rotating neutron stars in [10,11,12,13,14,15,16,17,18]. Doneva and collaborators have extended these investigations to rapidly rotating neutron stars in STT [19,20,21,22], observing that the effect of scalarization is further enhanced.…”