Neutron stars with hyperon cores: stellar radii and equation of state near nuclear density

Abstract

Context. The existence of 2 M pulsars puts very strong constraints on the equation of state (EOS) of neutron stars (NSs) with hyperon cores, which can be satisfied only by special models of hadronic matter. The radius-mass relation for these models is sufficiently specific that it could be subjected to an observational test with future X-ray observatories. Aims. We want to study the impact of the presence of hyperon cores on the radius-mass relation for NS. We aim to find out how, and for which particular stellar mass range, a specific relation R(M), where M is the gravitational mass, and R is the circumferential radius, is associated with the presence of a hyperon core. Methods. We consider a set of 14 theoretical EOS of dense matter, based on the relativistic mean-field approximation, allowing for the presence of hyperons in NSs. We also discuss a recent EOS based on non-relativistic G-matrix theory yielding NSs with hyperonic cores and M > 2 M . We seek correlations between R(M) and the stiffness of the EOS below the hyperon threshold needed to pass the 2 M test. Results. For NS masses 1.0 < M/M < 1.6, we get R > 13 km, because of a very stiff pre-hyperon segment of the EOS. At nuclear density (n 0 = 0.16 fm −3 ), the pressure is significantly higher than a robust upper bound obtained recently using chiral effective field theory. Conclusions. If massive NSs do have a sizable hyperon core, then according to current models the radii for M = 1.0−1.6 M are necessarily >13 km. If, on the contrary, a NS with a radius R (obs) < 12 km is observed in this mass domain, then sizable hyperon cores in NSs, as we model them now, are ruled out. Future X-ray missions with <5% precision for a simultaneous M and R measurement will have the potential to solve the problem with observations of NSs. Irrespective of this observational test, present EOS allowing for hyperons that fulfill condition M max > 2 M yield a pressure at nuclear density that is too high relative to up-to-date microscopic calculations of this quantity.

Rotating neutron stars with exotic cores: masses, radii, stability

“…11). It should be stressed that both properties are valid for M < 0.9 M (stat) max [11]. The results reviewed above can be corroborated quantitatively by theoretical considerations.…”

“…R ∞ can also be constrained by the modelling of the shape of the X-ray pulses observed from rotation-powered radio millisecond pulsars (RP-MSP) in particular if their mass is known from radio observations. Figure 1 shows the most recent constraints on the radius R 1.4 of a 1.4M NS obtained for various types of sources (see details in [11]). The constraints QXT-1 and RP-MSP being mutually exclusive, so far no consensus on R 1.4 can be reached.…”

Neutron stars: From the inner crust to the core with the (extended) Nambu–Jona-Lasinio model

“…However, in [67], it was shown that in order to prevent the EOS from violating causality, the radius should satisfy R 1.4 10.7 km, if it is imposed that the EOS also describes a 2 M star. In [45], taking experimental constraints and causality restrictions for large maximum masses, the 1.4 M star radii were constrained to be within the interval 12.1 ± 1.1 km (see [68]). …”

Hyperons in neutron star matter within relativistic mean-field models

“…A summary and discussion of different observational radius determinations can be found in [22]. In addition, for a rotating star due to its deformation there is no unambiguous relation between the observed quantity and the radii determined theoretically.…”