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We perform a joint Bayesian inference of neutron-star mass and radius constraints based on GW170817, observations of quiescent low-mass X-ray binaries (QLMXBs), photospheric radius expansion X-ray bursts (PREs), and X-ray timing observations of J0030+0451. With this data set, the form of the prior distribution still has an impact on the posterior mass-radius (MR) curves and equation of state (EOS), but this impact is smaller than recently obtained when considering QLMXBs alone. We analyze the consistency of the electromagnetic data by including an "intrinsic scattering" contribution to the uncertainties, and find only a slight broadening of the posteriors. This suggests that the gravitational-wave and electromagnetic observations of neutron-star structure are providing a consistent picture of the neutron-star mass-radius curve and the EOS., 6 pages, 4 figures and supplemental material

At supranuclear densities, explored in the core of neutron stars, a strong phase transition from hadronic matter to more exotic forms of matter might be present. To test this hypothesis, binary neutron-star mergers offer a unique possibility to probe matter at densities that we can not create in any existing terrestrial experiment. In this work, we show that, if present, strong phase transitions can have a measurable imprint on the binary neutron-star coalescence and the emitted gravitational-wave signal. We construct a new parameterization of the supranuclear equation of state that allows us to test for the existence of a strong phase transition and extract its characteristic properties purely from the gravitational-wave signal of the inspiraling neutron stars. We test our approach using a Bayesian inference study simulating 600 signals with three different equations of state and find that for current gravitational-wave detector networks already twelve events might be sufficient to verify the presence of a strong phase transition. Finally, we use our methodology to analyze GW170817 and GW190425, but do not find any indication that a strong phase transition is present at densities probed during the inspiral., 17 pages, 11 figures

In recent years, nuclear physics has benefited greatly from the development of powerful ab initio many-body methods and their combination with interactions from chiral effective field theory. With increasing computational power and continuous development of these methods, we are entering an era of precision nuclear physics. Indeed, uncertainties from nuclear Hamiltonians now dominate over uncertainties from many-body methods. This review summarizes the current status of, and future directions in, deriving and constraining nuclear Hamiltonians., 44 pages, 16 figures, Topical Review of ECT* workshop "New Ideas in Constraining Nuclear Forces", published version

Neutron stars are astrophysical objects of extremes, reaching the highest densities we can observe in the cosmos, and probing matter under conditions that cannot be recreated in terrestrial experiments. In August 2017, the first neutron-star merger has been observed, which provided compelling evidence that these events are an important site for r-process nucleosynthesis. Furthermore, the gravitational-wave signal of such events might shed light upon the nature of strongly interacting matter in the neutron-star core. To understand these remarkable events, reliable nuclear physics input is essential. In this contribution, I explain how to use chiral effective field theory and advanced many-body methods to provide a consistent and systematic approach to strongly inter- acting systems from nuclei to neutron stars with controlled theoretical uncertainties. I will discuss recent results for the equation of state relevant for the nuclear astrophysics of neutron stars and neutron-star mergers.

We investigate how a phase transition from neutron-star matter to spin-polarized neutron matter affects the equation of state and mass-radius relation of neutron stars. While general extension schemes for the equation of state allow for high pressures inside neutron stars, we find that a phase transition to spin-polarized neutron matter excludes extreme regimes. Hence, such a transition limits the maximum mass of neutron stars to lie below 2.6$-$$2.9 \, M_{\odot}$, depending on the microscopic nuclear forces used, while significantly larger masses could be reached without these constraints. These limits are in good agreement with recent constraints extracted from the neutron-star merger GW170817 and its electromagnetic counterpart. Assuming the description in terms of spin-polarized neutron matter to be valid in the center of neutron stars, we find that stars with a large spin-polarized domain in their core are ruled out by GW170817., 10 pages, 3 figures, published version

Multi-messenger observations of neutron star (NS) mergers have the potential to revolutionize nuclear astrophysics. They will improve our understanding of nucleosynthesis, provide insights about the equation of state (EOS) of strongly-interacting matter at high densities, and enable tests of the theory of gravity and of dark matter. Here, we focus on the EOS, where both gravitational waves (GWs) from neutron-star mergers and X-ray observations from space-based detectors such as NICER will provide more stringent constraints on the structure of neutron stars. Furthermore, recent advances in nuclear theory have enabled reliable calculations of the EOS at low densities using effective field theory based Hamiltonians and advanced techniques to solve the quantum many-body problem. In this paper, we address how the first observation of GWs from GW170817 can be combined with modern calculations of the EOS to extract useful insights about the EOS of matter encountered inside neutron stars. We analyze the impact of various uncertainties, the role of phase transitions in the NS core, and discuss how future observations will improve our understanding of dense matter., 18 pages, 10 figures, published version; Invited contribution prepared for the EPJ A Topical Issue "First joint gravitational wave and electromagnetic observations: Implications for nuclear and particle physics."

Gravitational waves from neutron-star mergers are expected to provide stringent constraints on the structure of neutron stars. At the same time, recent advances in nuclear theory have enabled reliable calculations of the low density equation of state using effective field theory based Hamiltonians and advanced techniques to solve the quantum many-body problem. In this paper, we address how the first observation of gravitational waves from GW170817 can be combined with modern calculations of the equation of state to extract useful insights about the equation of state of matter encountered inside neutron stars. We analyze the impact of various uncertainties and we show that the tidal deformability extracted from GW170817 is compatible, while less constraining, than modern nuclear physics knowledge., Comment: arXiv admin note: substantial text overlap with arXiv:1901.09874

In this White Paper we present the potential of the Enhanced X-ray Timing and Polarimetry (eXTP) mission for determining the nature of dense matter; neutron star cores host an extreme density regime which cannot be replicated in a terrestrial laboratory. The tightest statistical constraints on the dense matter equation of state will come from pulse profile modelling of accretion-powered pulsars, burst oscillation sources, and rotation-powered pulsars. Additional constraints will derive from spin measurements, burst spectra, and properties of the accretion flows in the vicinity of the neutron star. Under development by an international Consortium led by the Institute of High Energy Physics of the Chinese Academy of Science, the eXTP mission is expected to be launched in the mid 2020s., Comment: Accepted for publication on Sci. China Phys. Mech. Astron. (2019)

Interactions from chiral effective field theory have been successfully employed in a broad range of \textit{ab initio} calculations of nuclei and nuclear matter, but it has been observed that most results of few- and many-body calculations experience a substantial residual regulator and cutoff dependence. In this work, we investigate the behavior of local chiral potentials at leading order under variation of the cutoff scale for different local regulators. When varying the cutoff, we require that the resulting interaction produces no spurious bound states in the deuteron channel. We find that, for a particular choice of leading-order operators, nucleon-nucleon phase shifts and the deuteron ground-state energy converge to cutoff-independent plateaus, for all regulator functions we investigate. This observation may enable improved calculations with chiral Hamiltonians that also include three-nucleon interactions., 10 pages, 6 figures, published version

The correlation of the tidal polarizabilities $\Lambda_1$-$\Lambda_2$ for GW170817 is predicted by combining dense-matter equations of state (EOSs) that satisfy nuclear physics constraints with the chirp mass and mass asymmetry for this event. Our models are constrained by calculations of the neutron-matter EOS using chiral effective field theory Hamiltonians with reliable error estimates up to once or twice the nuclear saturation density. In the latter case, we find that GW170817 does not improve our understanding of the EOS. We contrast two distinct extrapolations to higher density: a minimal model (MM) which assumes that the EOS is a smooth function of density described by a Taylor expansion and a more general model parametrized by the speed of sound that admits phase transitions. This allows us to identify regions in the $\Lambda_1$-$\Lambda_2$ plots that could favor the existence of new phases of matter in neutron stars. We predict the combined tidal polarizability of the two neutron stars in GW170817 to be $80\le \tilde{\Lambda}\le 580$ ($280\le \tilde{\Lambda}\le 480$ for the MM), which is smaller than the range suggested by the LIGO-Virgo data analysis. Our analysis also shows that GW170817 requires a NS with $M=1.4M_\odot$ to have a radius $9.0, Comment: 6 pages, 4 figures, published version

We present quantum Monte Carlo calculations of light nuclei, neutron-$\alpha$ scattering, and neutron matter using local two- and three-nucleon ($3N$) interactions derived from chiral effective field theory up to next-to-next-to-leading order (N$^2$LO). The two undetermined $3N$ low-energy couplings are fit to the $^4$He binding energy and, for the first time, to the spin-orbit splitting in the neutron-$\alpha$ $P$-wave phase shifts. Furthermore, we investigate different choices of local $3N$-operator structures and find that chiral interactions at N$^2$LO are able to simultaneously reproduce the properties of $A=3,4,5$ systems and of neutron matter, in contrast to commonly used phenomenological $3N$ interactions., Comment: 5 pages, 3 figures, 1 table - updated version: small wording changes, one reference change

Chiral effective field theory (EFT) enables a systematic description of low-energy hadronic interactions with controlled theoretical uncertainties. For strongly interacting systems, quantum Monte Carlo (QMC) methods provide some of the most accurate solutions, but they require as input local potentials. We have recently constructed local chiral nucleon-nucleon (NN) interactions up to next-to-next-to-leading order (N$^2$LO). Chiral EFT naturally predicts consistent many-body forces. In this paper, we consider the leading chiral three-nucleon (3N) interactions in local form. These are included in auxiliary field diffusion Monte Carlo (AFDMC) simulations. We present results for the equation of state of neutron matter and for the energies and radii of neutron drops. In particular, we study the regulator dependence at the Hartree-Fock level and in AFDMC and find that present local regulators lead to less repulsion from 3N forces compared to the usual nonlocal regulators., 10 pages, 8 figures, 1 table, published version

We present details of the derivation of local chiral effective field theory interactions to next-to-next-to-leading order, and show results for nucleon-nucleon phase shifts and deuteron properties for these potentials. We then perform systematic auxiliary-field diffusion Monte Carlo calculations for neutron matter based on the developed local chiral potentials at different orders. This includes studies of the effects of the spectral-function regularization and of the local regulators. For all orders, we compare the quantum Monte Carlo results with perturbative many-body calculations and find excellent agreement for low cutoffs., 17 pages, 11 figures, published version, LA-UR-14-23894

We present the first quantum Monte Carlo (QMC) calculations with chiral effective field theory (EFT) interactions. To achieve this, we remove all sources of nonlocality, which hamper the inclusion in QMC calculations, in nuclear forces to next-to-next-to-leading order. We perform auxiliary-field diffusion Monte Carlo (AFDMC) calculations for the neutron matter energy up to saturation density based on local leading-order, next-to-leading order, and next-to-next-to-leading order nucleon-nucleon interactions. Our results exhibit a systematic order-by-order convergence in chiral EFT and provide nonperturbative benchmarks with theoretical uncertainties. For the softer interactions, perturbative calculations are in excellent agreement with the AFDMC results. This work paves the way for QMC calculations with systematic chiral EFT interactions for nuclei and nuclear matter, for testing the perturbativeness of different orders, and allows for matching to lattice QCD results by varying the pion mass., Comment: 6 pages, 3 figures, 1 table

The neutron-matter equation of state constrains the properties of many physical systems over a wide density range and can be studied systematically using chiral effective field theory (EFT). In chiral EFT, all many-body forces among neutrons are predicted to next-to-next-to-next-to-leading order (N3LO). We present details and additional results of the first complete N3LO calculation of the neutron-matter energy, which includes the subleading three-nucleon as well as the leading four-nucleon forces, and provides theoretical uncertainties. In addition, we discuss the impact of our results for astrophysics: for the supernova equation of state, the symmetry energy and its density derivative, and for the structure of neutron stars. Finally, we give a first estimate for the size of the N3LO many-body contributions to the energy of symmetric nuclear matter, which shows that their inclusion will be important in nuclear structure calculations., published version; 21 pages, 11 figures, 5 tables