Conrado Albertus

We describe the equation of state (EoS) of a warm ion plasma as obtained by performing microscopic many-body simulations using molecular dynamics computational techniques. Using the cold one-component plasma (OCP) composition in the neutron star (NS) outer crust assumed in U. Murarka et al. with a representative heavy nucleus for each density, we refine previous calculations. We include electron screening and modeling of ions as finite-size Gaussian distributions in the interaction potential, together with an efficient Ewald energy summation procedure. From this, the EoS relation P ( n _B , T ) is obtained as a function of baryonic number density n _B ∈ [7.48 × 10 ⁻¹⁰ , 2.09 × 10 ⁻⁴ ] fm ⁻³ and temperature k _B T ∈ [1, 5] MeV in the NS outer crust conditions. In order to improve the usability of our results, we provide tabulated data values along with a neural network parameterization available in the Zenodo repository; see DOI: 10.5281/zenodo.15348712 . We find that even at moderate temperatures thermal effects of ions are key in the higher-density region closer to the inner crust, when described using a thermal effective parameterization based on the thermal adiabatic index Γ _th . We compare our results with other EoSs in the literature and provide a critical discussion.

We consider the early binary neutron star inspiral phase as a scenario to probe environmental axion–photon resonant conversion. For this we approximately model the merger site electromagnetic fields as the superposition of two rotating dipolar stellar magnetic fields at the 1000 km scale when both magnetospheres are not largely distorted. We capture the time-sliced near-zone magnetospheric geometry relevant for axion–photon mixing. Plasma effects are incorporated through an effective Goldreich–Julian charge density, used to determine the effective plasma frequency and the location of resonant conversion surfaces. Our results show that axion–photon resonant conversion in binary magnetospheres mostly occurs on extended peanut-shaped surfaces whose global geometry evolves as the binary inspiral evolves. The resulting signal is intrinsically narrowband, with photon frequencies set by the axion mass, and its amplitude is slowly modulated by the inspiral through the changing resonant geometry. We illustrate our findings with benchmark values g _aγ = 10 ⁻¹² GeV ⁻¹ and ρ _a = 10 ²³ GeV cm ⁻³ . As an optimistic benchmark, we consider a Galactic source at D = 10 kpc and compare it with a realistic GW170817-like event at D = 40 Mpc. The corresponding D ⁻² suppression reduces the flux by ∼10 ⁻⁸ , placing extragalactic signals below the sensitivity of current and planned radio facilities. Our work provides a theoretical and phenomenological framework showing that binary neutron star magnetospheres may support extended resonant axion–photon conversion regions whose geometry and emission correlate with the GW frequency.

Einstein Telescope (ET) is the European project for a gravitational-wave (GW) observatory of third-generation. In this paper we present a comprehensive discussion of its science objectives, providing state-of-the-art predictions for the capabilities of ET in both geometries currently under consideration, a single-site triangular configuration or two L-shaped detectors. We discuss the impact that ET will have on domains as broad and diverse as fundamental physics, cosmology, early Universe, astrophysics of compact objects, physics of matter in extreme conditions, and dynamics of stellar collapse. We discuss how the study of extreme astrophysical events will be enhanced by multi-messenger observations. We highlight the ET synergies with ground-based and space-borne GW observatories, including multi-band investigations of the same sources, improved parameter estimation, and complementary information on astrophysical or cosmological mechanisms obtained combining observations from different frequency bands. We present advancements in waveform modeling dedicated to third-generation observatories, along with open tools developed within the ET Collaboration for assessing the scientific potentials of different detector configurations. We finally discuss the data analysis challenges posed by third-generation observatories, which will enable access to large populations of sources and provide unprecedented precision.

We present microscopic Molecular Dynamics simulations including the efficient Ewald sum procedure to study warm and dense stellar plasmas consisting of finite-size ion charges immerse in a relativistic neutralizing electron gas. For densities typical of Supernova matter and crust in a proto-neutron star, we select a representative single ion composition and obtain the virialized equation of state (vEoS). We scrutinize the finite-size and screening corrections to the Coulomb potential appearing in the virial coefficients B₂, B₃ and B₄ as a function of temperature. In addition, we study the thermal heat capacity at constant volume, C_V, and the generalized Mayer’s relation i.e. the difference C_P − C_V with C_P being the heat capacity at constant pressure, obtaining clear features signaling the onset of the liquid-gas phase transition. Our findings show that microscopic simulations reproduce the discontinuity in C_V, whose value lies between that of idealized gas and crystallized configurations. We study the pressure isotherms marking the boundary of the metastable region before the gaseous transition takes place. The resulting vEoS displays a behaviour where effective virial coefficients include extra density dependence showing a generalized density-temperature form. As an application we parametrize pressure as a function of density and temperature under the form of an artificial neural network showing the potential of machine learning for future regression analysis in more refined multicomponent approaches. This is of interest to size the importance of these corrections in the liquid-gas phase transition in warm and dense plasma phases contributing to the cooling behaviour of early Supernova phases and proto-neutron stars.