Methodology study of machine learning for the neutron star equation of state [CL]

We discuss a methodology of the machine learning to deduce the neutron star equation of state from a set of mass-radius observational data. We propose an efficient procedure to deal with a mapping from finite data points with observational errors onto an equation of state. We generate training data and optimize the neural network. Using independent validation data (mock observational data) we confirm that the equation of state is correctly reconstructed with precision surpassing observational errors.

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Y. Fujimoto, K. Fukushima and K. Murase
Tue, 21 Nov 17

Comments: 5 pages, 4 figures

The state of matter in simulations of core-collapse supernovae — Reflections and recent developments [HEAP]

In this review article we discuss selected developments regarding the role of the equation of state (EOS) in simulations of core-collapse supernovae. There are no first-principle calculations of the state of matter under supernova conditions since a wide range of conditions is covered, in terms of density, temperature and isospin asymmetry. Instead, model EOS are commonly employed in supernova studies. These can be divided into regimes with intrinsically different degrees of freedom: heavy nuclei at low temperatures, inhomogeneous nuclear matter where light and heavy nuclei coexist together with unbound nucleons, and the transition to homogeneous matter at high densities and temperatures. In this article we discuss each of these phases with particular view on their role in supernova simulations.

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T. Fischer, N. Bastian, D. Blaschke, et. al.
Tue, 21 Nov 17

Comments: 20 pages, 13 figures, special issue contribution to “The Physics of Neutron Stars” of PASA (Publications of the Astronomical Society of Australia)

Color superconductivity from the chiral quark-meson model [CL]

We study the two-flavor color superconductivity of low-temperature quark matter in the vicinity of chiral phase transition in the quark-meson model where the interactions between quarks are generated by pion and sigma exchanges. Starting from the Nambu-Gor’kov propagator in real-time formulation we obtain finite temperature (real axis) Eliashberg-type equations for the quark self-energies (gap functions) in terms of the in-medium spectral function of mesons. Exact numerical solutions of the coupled nonlinear integral equations for the real and imaginary parts of the gap function are obtained in the zero temperature limit using a model input spectral function. We find that these components of the gap display a complicated structure with the real part being strongly suppressed above $2\Delta_0$, where $\Delta_0$ is its on-shell value. We find $\Delta_0\simeq 40$ MeV close to the chiral phase transition.

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A. Sedrakian, R. Tripolt and J. Wambach
Tue, 21 Nov 17

Comments: 8 pages, 2 figures

Toward electrodynamics of unconventional phases of dilute nuclear matter [CL]

The phase diagram of isospin-asymmetrical nuclear matter may feature a number of unconventional phases, which include the translationally and rotationally symmetric, but isospin-asymmetrical BCS condensate, the current-carrying Larkin-Ovchinnikov-Fulde-Ferrell (LOFF) phase, and the heterogeneous phase-separated phase. Because the Cooper pairs of the condensate carry a single unit of charge, these phases are charged superconductors and respond to electromagnetic gauge fields by either forming domains (type-I superconductivity) or quantum vortices (type-II superconductivity). We evaluate the Ginzburg-Landau (GL) parameter across the phase diagram and find that the unconventional phases of isospin-asymmetrical nuclear matter are good type-II superconductors and should form Abrikosov vortices with twice the quantum of magnetic flux. We also find that the LOFF phase at the boundary of the transition to the type-I state, with the GL parameter being close to the critical value $1/\sqrt{2}$.

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A. Sedrakian and J. Clark
Tue, 21 Nov 17

Comments: Invited contribution presented by A. S. at the 19th International Conference on Recent Progress in Many-Body Theories, June 25-30, 2017, APCTP, Pohang, Korea, v1: 8 pages, 2 figs

Neutron skins and neutron stars in the multi-messenger era [CL]

The historical first detection of a binary neutron star merger by the LIGO-Virgo collaboration [B. P. Abbott {\sl et al.} Phys. Rev. Lett. 119, 161101 (2017)] is providing fundamental new insights into the astrophysical site for the $r$-process and on the nature of dense matter. A set of realistic models of the equation of state (EOS) that yield an accurate description of the properties of finite nuclei, support neutron stars of two solar masses, and provide a Lorentz covariant extrapolation to dense nuclear matter are used to confront its predictions against tidal polarizabilities extracted from the gravitational-wave data. Given the sensitivity of the gravitational-wave signal to the underlying EOS, limits on the tidal polarizabilities inferred from the observation translate into stringent constraints on the neutron-star radius. Based on these constraints, models that predict a stiff symmetry energy, and thus large stellar radii, can be ruled out. Indeed, under a particular binary-mass scenario, we deduce an upper limit on the radius of a $1.6\,M_{\odot}$ neutron star of $R_{\star}^{1.6}!<!13.25\,{\rm km}$. Given the sensitivity of the neutron-skin thickness of ${}^{208}$Pb to the symmetry energy, albeit at a lower density, we infer a corresponding upper limit of $R_{\rm skin}^{208}!\lesssim!0.25\,{\rm fm}$. However, if the upcoming PREX-II experiment measures a significantly thicker skin, this may be evidence of a softening of the symmetry energy at high densities—likely indicative of a phase transition in the interior of neutron stars.

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F. Fattoyev, J. Piekarewicz and C. Horowitz
Mon, 20 Nov 17

Comments: 6 pages, 4 figures

Reaction rates and transport in neutron stars [HEAP]

Understanding signals from neutron stars requires knowledge about the transport inside the star. We review the transport properties and the underlying reaction rates of dense hadronic and quark matter in the crust and the core of neutron stars and point out open problems and future directions.

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A. Schmitt and P. Shternin
Mon, 20 Nov 17

Comments: 71 pages; commissioned for the book “Physics and Astrophysics of Neutron Stars”, NewCompStar COST Action MP1304

Holographic compact stars meet gravitational wave constraints [HEAP]

We investigate a simple holographic model for cold and dense deconfined QCD matter consisting of three quark flavors. Varying the single free parameter of the model and utilizing a Chiral Effective Theory equation of state (EoS) for nuclear matter, we find four different types of compact stars: traditional neutron stars, strange quark stars, and two novel solutions we refer to as hybrid stars of the second and third kind (HS2 and HS3, respectively). The HS2s are composed of a nuclear matter core and a crust made of stable strange quark matter, while the HS3s have both a quark mantle and a nuclear crust on top of a nuclear matter core. For all types of stars constructed, we determine not only their mass-radius relations, but also tidal deformabilities, Love numbers, as well as moments of inertia and the mass distribution. We find that there exists a range of parameter values in our model, for which the novel hybrid stars have properties in very good agreement with all existing bounds on the macroscopic properties of compact stars. In particular, the tidal deformabilities of these solutions are smaller than those of ordinary neutron stars of the same mass, implying that they provide an excellent fit to the recent gravitational wave data GW170817 of LIGO and Virgo.

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E. Annala, C. Ecker, C. Hoyos, et. al.
Fri, 17 Nov 17

Comments: 7 pages, 5 figures