The Role of Baryon Structure in Neutron Stars

Abstract

The current state of neutron star mass measurements is depicted in Fig. 1. We see that the only measurements of heavy neutron stars with reasonably small errors are those of PSR J1614-2230 [29] and PSR J0348+0432 [2], along with PSR J0740+6620 [30] which is the newest member of the heavy neutron star club. Nevertheless, the uncertainties on the aforementioned pulsars still make them all comparable with each other within two standard deviations. Therefore, a conservative lower estimate for the maximum mass of neutron stars is that of PSR J1614-2230, namely, 1.908±0.016M . RadiiOther than their masses, the radii of neutron stars are extremely sensitive to different models for the equations of state. However, the experimental constraints on radii are less numerous and until recently the systematic errors were quite large. The first new source of a constraint came on August 17th 2017, with the first gravitational wave measurement [4] of the merger of two neutron stars. This yielded the result that the radii of the neutron stars involved in the merger both lie within R = 11.9 +1.4 −1.4 , at the 90% confidence level.More recently, the NICER mission, with its X-ray telescope on the space station, obtained results [5,37] for PSR J0030+0451. The collaboration applied different analyses on the data and obtained values of (all at 68% credibility) 12.71 +1.14 −1.19 km and mass 1.34 +0.15 −0.16 M in Ref.[5] and 13.02 +1.24 −1.06 km in Ref.[37] with the mass estimated at 1.44 +0.15 −0.14 M . Another recent measurement by the NICER collaboration was that of pulsar PSR J0740+6620 which by virtue of being much heavier, is of considerable interest to nuclear physicists. They measured a radius of 12.39 +1.3 −0.98 km for a mass of 2.07 ± 0.06M [38]. Modelling Dense MatterHistorically, studies of the properties of nuclear matter were primarily aimed at understanding finite nuclei. They were based primarily on using two-body and three-body potentials with standard techniques in quantum many-body theory, such as Brueckner-Hartree-Fock (BHF) [39], Bethe-Brueckner-Goldstone (BBG) [40]. The two-body potentials were derived from phenomenological fits to nucleonnucleon scattering [41]. The quantum Monte Carlo variational approach has had considerable success with light nuclei [42], with parameters of the phenomenological three-body force tuned to nuclear data. In recent years there has been a great deal of activity using chiral effective field theory [43,44,45,46,47,48], with its systematic expansion in powers of momentum. This approach builds in the constraints of chiral symmetry within a Lagrangian theory built upon pion and nucleon degrees of freedom, plus counterterms. With a similar number of parameters to those needed for phenomenological potentials (typically 25-30), it yields a good description of nucleon-nucleon scattering data. Within the same framework, one also has a systematic expansion of a three-nucleon force, with counterterms again tuned to nuclear data. Once again one finds excellent agreement with data for light nuc...

Exotic Cores with and without Dark-Matter Admixtures in Compact Stars

“…The advent of detecting gravitational waves from merging neutron stars, the related multimessenger astrophysics [1][2][3][4][5][6][7][8] and the improving mass-radius determinations of neutron stars, in particular by NICER data [9][10][11], stimulated a wealth of activities. Besides mass and radius, moments of inertia and tidal deformarbilities become experimentally accessible and can be confronted with theoretical models [12][13][14][15][16][17][18]. The baseline of the latter ones is provided by non-rotating, spherically symmetric cold dense matter configurations.…”