Bayesian refinement of covariant energy density functionals

Abstract

The last five years have seen remarkable progress in our quest to determine the equation of state of neutron rich matter. Recent advances across the theoretical, experimental, and observational landscape have been incorporated in a Bayesian framework to refine existing covariant energy density functionals previously calibrated by the properties of finite nuclei. In particular, constraints on the maximum neutron star mass from pulsar timing, on stellar radii from the NICER mission, on tidal deformabilities from the LIGO-Virgo collaboration, and on the dynamics of pure neutron matter as predicted from chiral effective field theories, have resulted in significant refinements to the models, particularly to those predicting a stiff symmetry energy. Still, even after these improvements, we find challenging to reproduce simultaneously the neutron skin thickness of both 208 Pb and 48 Ca recently reported by the PREX/CREX collaboration.

Building an Equation of State Density Ladder

“…In this "re-calibration" of the models, input from χEFT, mass-radius measurements from NICER, and tidal deformability from LIGO-Virgo, were used in a Bayesian framework to provide updated sets of coupling constants. More information on this method can be found in [59]. Starting at the lowest rung of the density ladder, we indicated earlier that χEFT favors a fairly soft EOS for pure neutron matter, which according to Equation (3) suggests that the symmetry energy in the vicinity of saturation density is also soft.…”

“…In this section, we collect the theoretical, experimental, and observational information presented in the previous sections, and depicted on the EOS density ladder of Figure 1, to discuss the new set of energy density functionals introduced in Ref. [59] and its impact on the mass-radius relation.…”

“…Model Besides constraints from χEFT that, as we just saw, have a strong impact on the refinement of the functionals, especially in the case of FSUGold2, our previous work also incorporated astrophysical constraints on the EOS of neutron star matter from the tidal deformability of GW170817 extracted by the LIGO-Virgo collaboration [35], stellar masses and radii of two sources obtained by the NICER mission [48][49][50][51], and lower limits on the maximum mass of a neutron star obtained from long-time pulsar timing observations [52,53]. Indeed, all this information-together with constraints on the EOS of pure neutron matter predicted by χEFT-was included in the model refinement [59]. By including all this new information in a Bayesian inference approach, posterior distribution functions were obtained for the neutron star matter EOS, the resulting mass-radius relation, and the tidal deformability of a 1.4 M neutron star.…”

“…Moreover, the range of densities probed by each rung in the ladder overlaps with neighboring rungs, thereby providing consistency checks among the various methods. Following our recent work [59], in which previously calibrated covariant energy density functionals were refined by the plethora of new information, we have examined the predictions of the new models.…”

“…Figure 4. (a) Equation of state of neutron star matter as predicted by FSUGold2[69], FSUGarnet[70], along with the results post-refinement[59]. (b) Mass-radius relationship predictions from FSUGold2 and FSUGarnet are displayed with their respective colors, with the solid and dashed lines representing results before and after Bayesian refinement, respectively.…”