A detailed description of the properties of dense matter in extreme conditions, as those within Neutron Star cores, is still an open problem, whose solution is hampered by both the lack of empirical data, and by the difficulties in developing a suitable theoretical framework for the microscopic nuclear dynamics in such regimes.

We report here the results of a study aimed at inferring the properties of the repulsive three-nucleon interaction, driving the stiffness of the equation of state at high densities, by performing bayesian inference on current and future astrophysical observations.

The dense matter equation of state (EoS), describing the state of matter under the extreme conditions found in neutron stars, is not accurately known. However, significant process has been made in recent years through the emergence of new observational avenues of neutron stars. Firstly, the X-ray timing telescope NICER has delivered two joint mass-radius measurements, for pulsars PSR J0030+0451 and PSR J0740+6620, using pulse profile modeling. Secondly, gravitational wave detections of binary neutron star (BNS) mergers allow for a measurement of the EoS-dependent tidal deformability, as demonstrated in the first detected BNS merger GW170817. Additionally, electromagnetic radiation from the subsequent ultraviolet-optical-infrared transient (the kilonova) originating from the ejected material in GW170817 further probes the binary system and the EoS. We demonstrate how the joint analysis of these multi-messenger observations of neutron stars significantly constrains the dense matter EoS. We then describe, in more detail, a framework to jointly analyse a gravitational wave signal and the accompanying kilonova light curves, focusing on possible future black hole–neutron star (BHNS) mergers. We highlight the potential for multimessenger BHNS to constrain the tidal deformability of the neutron star, thereby increasing our understanding of the dense matter EoS.

During the late stages of a neutron star binary inspiral finite-size effects come into play, with the tidal deformability of the supranuclear density matter leaving an imprint on the gravitational-wave signal. As demonstrated in the case of GW170817—the first direct detection of gravitational waves from a neutron star binary—this can lead to strong constraints on the neutron star equation of state. As detectors become more sensitive, effects which may have a smaller influence on the neutron star tidal deformability need to be taken into consideration. Dynamical effects, such as oscillation mode resonances triggered by the orbital motion, have been shown to contribute to the tidal deformability, especially close to the neutron star coalesence. We calculate the contribution of the various stellar oscillation modes to the tidal deformability and demonstrate the (anticipated) dominance of the fundamental mode, showing what the impact of the matter composition is on the tidal deformability.

The pore structure of vapour deposited ASW is poorly understood, despite its importance to fundamental processes such as grain chemistry, cooling of star forming regions, and planet formation. We studied structural changes of vapour deposited D2O on intra-molecular to 30 nm length scales at temperatures ranging from 18 to 180 K and observed enhanced mobility from 100 to 150 K. An Arrhenius type model describes the loss of surface area and porosity with a common set of kinetic parameters. The low activation energy (428 K) is commensurate with van der Waals forces between nm-scale substructures in the ice. Our findings imply that water porosity will always change with time, even at low temperatures.

For core-collapse and neutron star merger simulations, it is important to have adequate equations of state which describe dense and hot matter as realistically as possible. We present two newly constructed equations of state including the entire baryon octet, compatible with the main constraints coming from nuclear physics, both experimental and theoretical. One of the equations of state describes cold β-equilibrated neutron stars with a maximum mass of 2 Msun. Results obtained with the new equations of state are compared with the ones of DD2Y, the only existing equation of state containing the baryon octet and satisfying the above constraints. The main difference between our new equations of state and DD2Y is the harder symmetry energy of the latter. We show that the density dependence of the symmetry energy has a direct influence on the amount of strangeness inside hot and dense matter and, consequently, on thermodynamic quantities. We expect that these differences affect the evolution of a proto-neutron star or binary neutron star mergers. We propose also several parameterisations based on the DD2 and SFHo models calibrated to Lambda hypernuclei that satisfy the different constraints.

We adopt two- and three-body nuclear forces derived at the next-to-next-to-leading-order in the framework of effective chiral perturbation theory to calculate the equation of state of β-stable neutron star matter using the Brueckner–Hartree–Fock many-body approach. We use the recent optimized chiral two-body nuclear interaction at next-to-next-to-leading-order derived by Ekström et al. and two different parametrizations of the three-body next-to-next-to-leading-order interaction: the first one is fixed to reproduce the saturation point of symmetric nuclear matter while the second one is fixed to reproduce the binding energies of light atomic nuclei. We show that in the second case the properties of nuclear matter are not well determined whereas in the first case various empirical nuclear matter properties around the saturation density are well reproduced. We finally calculate various neutron star properties and in particular the mass-radius and mass-central density relations. We find that the adopted interactions based on a fully microscopic framework, are able to provide an equation of state which is consistent with the present data of measured neutron star masses.

. We analyzed a 40-year set of multicolor photometry and a 15-year set of synoptic monitoring of SS 433 along with fragmentary spectral and radio data. This system contains a neutron star and an A3–A7 I giant. The system is found to be either close, in contact, or it has a common envelope from time to time. The A-type giant is now in transition to the dynamical mass transfer.

Matter in neutron star cores reaches extremely high densities, forming states of matter that cannot be generated in the laboratory. The Equation of State (EOS) of the matter links to macroscopic observables, such as mass M and radius R, via the stellar structure equations. A promising technique for measuring M and R exploits hotspots (burst oscillations) that form on the stellar surface when material accreted from a companion star undergoes a thermonuclear explosion. As the star rotates, the hotspot gives rise to a pulsation, and relativistic effects encode information about M and R into the pulse profile. However the burst oscillation mechanism remains unknown, introducing uncertainty when inferring the EOS. I review the progress that we are making towards cracking this long-standing problem, and establishing burst oscillations as a robust tool for measuring M and R. This is a major goal for future large area X-ray telescopes.

A superconductor of paired protons is thought to form in the core of neutron stars soon after their birth. Minimum energy conditions suggest that magnetic flux is expelled from the superconducting region due to the Meissner effect, such that the neutron star core retains or is largely devoid of magnetic fields for some nuclear equation of state and proton pairing models. We show via neutron star cooling simulations that the superconducting region expands faster than flux is expected to be expelled because cooling timescales are much shorter than timescales of magnetic field diffusion. Thus magnetic fields remain in the bulk of the neutron star core for at least 106 − 107yr. We estimate the size of flux free regions at 107yr to be ≲ 100m for a magnetic field of 1011G and possibly smaller for stronger field strengths.

In this work, one intends to computationally simulate and investigate, via thermochemical calculations, how the chemical environment influences some molecular properties, such as IR spectra and absorption cross section, of individual species embedded in the solid phase employing the Polarized Continuum Model (PCM) approach. The trial molecules used here to check these effects are CO, CO2 and H2O. The solid phase (bulk ice) is simulated using different dielectric constant values representing different types of astrophysical ice at PCM approach. The effect of temperature is also investigated since it is known it affects the dielectric constant of the solvent medium.

GD 358 is the brightest (mv=13.7) and best studied helium atmosphere white dwarf pulsator. We present an analysis based on over 1000 hours of observations spanning 2007-2014 as well as archival data going back to 1982. From the complete data set, we identify a total of 27 independent frequencies and fit 14 of them as m=0 modes in our asteroseismic analysis. We add GD358 to a set of helium atmosphere white dwarfs fitted with similar models. With this consistent set, we can see a trend in the thickness of the pure helium layer that are quantitatively consistent with time-dependent diffusion calculations.

High mass and low mass stars follow a similar evolution until the inert core phase that follows the end of the core helium burning stage. In particular, one common phase of stellar evolution is the alpha capture reaction that turns carbon into oxygen in the core. We can obtain constraints on this reaction rate by studying the remnants of low mass stars, as this is the ultimate reaction that occurs in their core. We also present results that allow us to test the time dependent calculations of diffusion in dense interiors.

Based on the free–free absorption (FFA) model of gigahertz peaked spectrum (GPS) sources, we explain both Faraday rotation asymmetry and opacity asymmetry consistently between two components of a GPS quasar 2134+004. The FFA model assumes dense plasma around the central core to produce FFA, and the difference of path lengths in the plasma toward each component could cause these asymmetries. The component that is closer to the observer has a shorter path length, and consequently, smaller opacity of FFA and lower Faraday rotation. In a simple case, the ratio of Faraday rotation between two components is a function of the ratio of path length, and is the same as the ratio of opacity. Then these two ratios are shown to be essentially the same by our observations. We could thus distinguish between near-side and far-side components by the asymmetries.

Type IIn supernovae have bright optical emission and high bolometric luminosities. Due to their high mass loss, their are expected to have dense circumstellar interaction, thus produce bright radio and X-ray emission. We aim to carry out systematic study to understand their circumstellar interaction, mass loss properties. Here, I provide specific examples of two Type IIn supernovae, 2006jd and 2010jl.

We examine to what extent the inferred surface temperature of magnetars in quiescence can constrain the presence of a superfluid in the neutron star core and the role of magnetic field decay in the core. By performing detailed simulations of neutron star cooling, we show that extremely strong heating from field decay in the core cannot produce the high observed surface temperatures nor delay the onset of neutron superfluidity in the core. We find that it is not possible to conclude that magnetar cores are in a non-superfluid state purely from high surface temperatures. We find that neutron superfluidity in the core occurs less than a few hundred years after neutron star formation for core fields < 1016 G. Thus all known neutron stars, including magnetars, without a core containing exotic particles, should have a core of superfluid neutrons and superconducting protons.

Using the solutions of the gap equations of the magnetic-color-flavor-locked (MCFL) phase of paired quark matter in a magnetic field, and taking into consideration the separation between the longitudinal and transverse pressures due to the field-induced breaking of the spatial rotational symmetry, the equation of state (EoS) of the MCFL phase is self-consistently determined. Implications for stellar models of magnetized (self-bound) strange stars and hybrid (MCFL core) stars are discussed.

Among the many different classes of stellar objects, neutron stars provide a unique environment where we can test (at the same time) our understanding of matter with extreme density, temperature, and magnetic field. In particular, the properties of matter under the influence of magnetic fields and the role of electromagnetism in physical processes are key areas of research in physics. However, despite decades of research, our limited knowledge on the physics of strong magnetic fields is clear: we only need to note that the strongest steady magnetic field achieved in terrestrial labs is some millions of Gauss, only thousands of times stronger than a common refrigerator magnet. In this general context, I will review here the state of the art of our research on the most magnetic objects in the Universe, a small sample of neutron stars called magnetars. The study of the large high-energy emission, and the flares from these strongly magnetized (~ 1015 Gauss) neutron stars is providing crucial information about the physics involved at these extremes conditions, and favoring us with many unexpected surprises.

The modelling of X-ray pulse profiles from accreting millisecond pulsars is a way to infer masses and radii of neutron stars. We briefly describe how a pulse shape encodes information on the mass and radius, but also depends on other parameters such as hot spot location and observer viewing angle. A numerical model that we have developed is then described. The model includes light bending, time-delay effects, and Doppler effects for photons. The model accounts for oblateness of the neutron star, caused by the rapid rotation, and for scattered light from the surface of the accretion disk. The millisecond pulsar SAX J1808-3658 has multiple observations taken during different outbursts. The observed pulse shapes vary greatly, and it is a challenging test to fit the different observations. Some of the latest results are given.

I present recent results studying flare emission in magnetars. Strong quasi-periodic oscillations observed in the tail of giant magnetar flares are frequently interpreted as evidence for global seismic oscillations. I demonstrate that such a global oscillation is not directly observable in the lightcurve. New work suggests the amplitude for the strongest QPO stays nearly constant in the rotation phases where it is observed, which I argue suggests it is produced by an additional emission process from the star.