Ab-initio QCD Calculations Impact the Inference of the Neutron-star-matter Equation of State

Abstract

We demonstrate that ab-initio calculations in QCD at high densities offer significant and nontrivial information about the equation of state of matter in the cores of neutron stars, going beyond that which is obtainable from current astrophysical observations. We do so by extrapolating the equation of state to neutron-star densities using a Gaussian process and conditioning it sequentially with astrophysical observations and QCD input. Using our recent work, imposing the latter does not require an extrapolation to asymptotically high density. We find the QCD input to be complementary to the astrophysical observations, offering strong additional constraints at the highest densities reached in the cores of neutron stars; with the QCD input, the equation of state is no longer prior dominated at any density. The QCD input reduces the pressure and speed of sound at high densities, and it predicts that binary collisions of equal-mass neutron stars will produce a black hole with greater than 95% (68%) credence for masses M ≥ 1.38M ⊙ (M ≥ 1.25M ⊙). We provide a Python implementation of the QCD likelihood function so that it can be conveniently used within other inference setups.

On the Maximum Mass and Oblateness of Rotating Neutron Stars with Generic Equations of State

“…The median values, together with the 95% error bars, are reported in Table 1. Overall, our results compare well with the corresponding slices calculated by Annala et al (2023), as well as with the results of Gorda et al (2023a), obtained with a Gaussian process regression technique (Landry & Essick 2019).…”

“…In particular, the lower panels in these figures show that the distributions of radii for 1.4 M e models vary only modestly whether or not the EOS ensemble is restricted to being compatible with the results of pQCD. On the other hand, the contours of the distribution close to the critical limit clearly show that the constraint has a non-negligible impact on TOVs (which was already shown in Gorda et al 2023a), as well as on rapidly rotating stars, which we show here for the first time.…”

“…So far, the identification and study of quasi-universal relations has often been limited by the relatively small number of available models. At the same time, a growing number of recent works have explored the possibility of building very large sets of EOSs that satisfy all known theoretical and observational constraints and cover the physically allowed space of EOSs (Landry & Essick 2019;Annala et al 2020) and of related quantities, such as the sound speed (Altiparmak et al 2022;Gorda et al 2023a; or the conformal anomaly (Annala et al 2023;Brandes et al 2023;Marczenko et al 2023). These EOSs are built using either generic piecewise polytropes (see, e.g., Read et al 2009;Kurkela et al 2014;Lattimer & Steiner 2014;Tews et al 2018aTews et al , 2018bMost et al 2018;Steiner et al 2018), a parameterization of the sound speed (see, e.g., Annala et al 2020;Altiparmak et al 2022; or nonparametric Gaussian process regression (see, e.g., Landry & Essick 2019;Gorda et al 2023aGorda et al , 2023b.…”

What Can We Learn about the Unstable Equation-of-state Branch from Neutron Star Mergers?

“…Komoltsev & Kurkela (2022) showed that pQCD calculations can also constrain the EOS at lower densities by imposing causality and thermodynamic consistency of the EOS with high-density pQCD calculations. Using this approach, Gorda et al (2023a) outlined that pQCD impacts the EoS of stable NSs while Somasundaram et al (2023) found that the integral pQCD constraints have a marginal impact on the NS EoS selection once nuclear physics and astrophysical sources of information are accounted for. However, pQCD impacts the EOS in the unstable branch above n TOV (see Figure 1).…”

Core States of Neutron Stars from Anatomizing Their Scaled Structure Equations

“…To illustrate some impacts of the constraining band on the NS central EOS inferred above, the inset of Figure 2 compares our result (shown as the magenta crossed band) with predictions/ constraints on the dense NS matter EOS from several other approaches: (1) gray shaded area: a combination (Annala et al 2018) of low-density nuclear theories, high-density perturbative QCD (pQCD) theories, and the tidal deformabilities from GW170817 (Abbott et al 2018); (2) yellow solid line: pQCD with constraint of the speed of sound at very high densities (Komoltsev & Kurkela 2022); (3) red dashed-dotted line: similar physical inputs as (1), combining a Gaussian Process (GP) inference algorithm (Gorda et al 2023); (4) pink vertical hatched area: consistent construction of EOS ensembles consistent with nuclear theories, pQCD, astronomical observations, and continuous speed of sound (Altiparmak et al 2022); (5) black dashed line: nonparametric inference via GP adopting similar astronomical observations, together with; (6) green horizontal hatched area: the prediction on the central pressure density of PSR J0740+6620 (Legred et al 2021); and (7) blue solid line: the constraint from an earlier model analysis of the NICER data (Miller et al 2021). Our constraining band on the central NS EOS directly from the NICER data, using the mass and radius scalings without using any specific input EOS model, is mostly consistent with, but much narrower than, the NS EOS bands from the previous studies listed above.…”