Neutron star equation of state and uncertainty on the radius determination

Abstract

The new generation of X-ray telescopes will provide simultaneous measurements of the radius and mass of a number of neutron stars (NSs), potentially constraining their equation of state (EoS). However the use of non-unified EoSs, that are based on different nuclear models for the crust and core, can introduce an uncertainty on the NS radius of the order of the precision expected from these instruments. Two solutions to this problem are presented. First a large number of unified EoSs and second an approximate approach to the NS crust that allows to obtain with a very large precision the relation between the NS mass and radius with no need to use an EoS for the crust. Correlations between the NS radius and nuclear parameters, which are properties of the EoS that can be indirectly measured in laboratory, are discussed. These open the possibility of constraining the NS radius with experiments on Earth. Finally, the appearance of hyperons in the core of NSs is examined. They are included in the EoS models consistently with available measurements on the hyperon properties. Because of the scarcity of these experimental data one can not exclude that hyperons appear at high densities in NSs in spite of the observations of massive NSs.