No Arabic abstract
In this review we discuss self-consistent methods to calculate the global structure of strongly magnetised neutron stars within the general-relativistic framework. We outline why solutions in spherical symmetry cannot be applied to strongly magnetised compact stars, and elaborate on a consistent formalism to compute rotating magnetised neutron star models. We also discuss an application of the above full numerical solution for studying the influence of strong magnetic fields on the radius and crust thickness of magnetars. The above technique is also applied to construct a universal magnetic field profile inside the neutron star, that may be useful for studies in nuclear physics. The methodology developed here is particularly useful to interpret multi-messenger astrophysical data of strongly magnetised neutron stars.
We investigate the equation of state for a recently developed hybrid quark-meson-nucleon model under neutron star conditions of $beta-$equilibrium and charge neutrality. The model has the characteristic feature that at increasing baryon density chiral symmetry is restored in a first order transition within the hadronic phase by lifting the mass splitting between chiral partner states, before quark deconfinement takes place. Most important for this study are the nucleon (neutron, proton) and $N(1535)$ states. We present three sets for the two free parameters which result in compact star mass-radius relations in accordance with modern constraints on the mass from PSR~J0437-4715 and on the compactness from GW170817. We also consider the threshold for the direct URCA process for which a new relationship is given and suggest as an additional constraint on the parameter choice of the model that this process shall become operative at best for stars with masses above the range for binary radio pulsars, $M>1.4~M_odot$.
We investigated the structure of the low density regions of the inner crust of neutron stars using the Hartree-Fock-Bogoliubov (HFB) model to predict the proton content $Z$ of the nuclear clusters and, together with the lattice spacing, the proton content of the crust as a function of the total baryonic density $rho_b$. The exploration of the energy surface in the $(Z,rho_b)$ configuration space and the search for the local minima require thousands of calculations. Each of them implies an HFB calculation in a box with a large number of particles, thus making the whole process very demanding. In this work, we apply a statistical model based on a Gaussian Process Emulator that makes the exploration of the energy surface ten times faster. We also present a novel treatment of the HFB equations that leads to an uncertainty on the total energy of $approx 4$ keV per particle. Such a high precision is necessary to distinguish neighbour configurations around the energy minima.
Observations show that, at the beginning of their existence, neutron stars are accelerated briskly to velocities of up to $1000$ km/s. We discuss possible mechanisms contributing to these kicks in a systematic effective-field-theory framework. Anomalies of the underlying microscopic theory result in chiral transport terms in the hydrodynamic description, and we identify these as explanation for the drastic acceleration. In the presence of vorticity or a magnetic field, the chiral transport effects cause neutrino emission along the respective axes. In typical scenarios, the transport effect due to the magnetic field turns out to be strong enough to explain the kicks. Mixed gauge-gravitational anomalies enter in a distinct way, and we also discuss their implications.
We review the equation of state (EoS) models covering a large range of temperatures, baryon number densities and electron fractions presently available on the textsc{CompOSE} database. These models are intended to be directly usable within numerical simulations of core-collapse supernovae, binary neutron star mergers and proto-neutron star evolution. We discuss their compliance with existing constraints from astrophysical observations and nuclear data. For a selection of purely nucleonic models in reasonable agreement with the above constraints, after discussing the properties of cold matter, we review thermal properties for thermodynamic conditions relevant for core-collapse supernovae and binary neutron star mergers. We find that the latter are strongly influenced by the density dependence of the nucleon effective mass. The selected bunch of models is used to investigate the EoS dependence of hot star properties, where entropy per baryon and electron fraction profiles are inspired from proto-neutron star evolution. The $Gamma$-law analytical thermal EoS used in many simulations is found not to describe well these thermal properties of the EoS. However, it may offer a fair description of the structure of hot stars whenever thermal effects on the baryonic part are small, as shown here for proto-neutron stars starting from several seconds after bounce.
The so called hyperon puzzle, i.e. the difficulty to reconcile the measured masses of neutron stars (NSs) with the presence of hyperons in their interiors, is one of the hot topics in astrophysics which is stimulating copious experimental and theoretical research in hypernuclear physics. After illustrating the origin of the hyperon puzzle, I discuss some of its possible solutions, and particularly those related to the role of hyperonic two- and three-body interactions on the equation of state of dense matter. Afterward, I discuss a possibility to circumvent the hyperon puzzle allowing for the presence of strangeness in NSs in the form of deconfined strange quark matter, and thus considering the so called quark stars, i.e. hybrid stars or strange stars. Finally I discuss the astrophysical consequences of the possible conversion process of an hadronic star to a quark star.