NICER constraints on low density matching and high density continuation in neutron star equations of state

Abstract

We investigate whether current astrophysical data constrain not only the high-density continuation of the neutron star equation of state, but also the low density matching procedure itself. To this end, we compare two low density branches propagated through a common high density extension and confront them with direct NICER mass-radius posteriors, a lower bound on the maximum mass and an effective constraint on ${\Lambda }_{1.4}$. We find that the observable predictions of the two branches remain strongly overlapping, while the NICER-informed posterior still induces a nontrivial constraint on the matching parameters. Current data therefore constrain primarily the shared continuation above $n_1$, but also indirectly restrict the low density matching sector.

arXiv comment15 pages, 5 figures