Hyperons in neutron stars

Generalized beta equilibrium involving nucleons, hyperons and isobars is examined for neutron star matter. The hyperons produce a considerable softening of the equation of state. It is shown that the observed masses of neutron stars can be used to settle a recent controversy concerning the nuclear compressibility. Compressibilities less than 200 MeV are incompatible with observed masses.     

Superfluidity in neutron stars

Arguments are presented to show that the BCS theory of superfluidity in its original form may not be applicable to neutron star matter over a wide range of density.     

Abnormal neutron stars

Quasi-classical field theory is used to predict abnormal behaviour of neutron matter in a model with neutrons, scalar mesons and vector mesons. We predict two stable families of neutron stars, a conventional one and an unconventional one with surface density in excess of 10¹⁵g cm^?3, roughly.     

Magnetic monopoles in neutron stars

The effects of monopole-antimonopole annihilation on a previously reported bound [1] on the product of the galactic flux of grand unified magnetic monopoles and the cross section for monopole catalyzed nucleon decay: (F_M/cm^?2s^?1sr^?1)(σ_ΔB/10^?2 cm²) ? 10^?22 are examined for several models of neutron star interiors. For neutron stars with superconducting interiors or large internal magnetic fields this bound is unaltered. In the unlikely event that old neutron stars are not superconducting and have internal magnetic fields B_int ? 10⁸ Gauss the effects of monopole-antimonopole annihilation relax the bound to (F_M/cm^?2 s^?1 sr^?1)(σ_ΔB/10^?27 cm^?2)² ? 10^?18. Magnetic monopoles may also have a significant effect on the structure of the interior magnetic field in neutron stars.     

X-rays from neutron stars

The basic theoretical ideas in the models of regularly pulsating X-ray sources are discussed, and put in relation to the observations. The topics covered include physics of the magnetosphere of an accreting neutron star, hydrodynamics of the accretion column, physical processes close to the surface of the neutron star such as proton-electron collisions, photon-electron interactions.     

The Zoo of neutron stars

In these lecture notes, I briefly discuss the present day situation and new discoveries in astrophysics of neutron stars focusing on isolated objects. The latter include soft gamma repeaters, anomalous x-ray pulsars, central compact objects in supernova remnants, the Magnificent Seven, and rotating radio transients. In the last part of the paper, I describe available tests of cooling curves of neutron stars and discuss different additional constraints that can help to confront theoretical calculations of cooling with observational data. The text was submitted by the author in English.     

Towards new tests of strong-field gravity with measurements of surface atomic line redshifts from neutron stars

In contrast to gravity in the weak-field regime, which has been subject to numerous experimental tests, gravity in the strong-field regime is largely unconstrained by observations. We show that gravity theories that pass solar system tests, but that diverge from general relativity in the strong-field regime, predict neutron stars with significantly different properties than their general relativistic counterparts. The range of redshfits of surface atomic lines predicted by such theories is significantly wider than the uncertainty introduced by our lack of knowledge of the equation of state of ultradense matter. Measurements of such lines with x-ray observatories can thus put new constraints on strong-field gravity.     

Internal structure of neutron stars

The equation of state and the structure and composition of neutron star matter are investigated in the density region 3.1 × 10¹¹−2 × 10¹⁵g/cm³. Below the density 3.1 × 10¹¹ g/cm³ the matter is a solid consisting of neutron-rich nuclei in a degenerate electron gas. At 3.1 × 10¹¹g/cm³ neutrons start to drip out of the nuclei; as the density increases, the lattice spacing continuously decreases while the geometrical size of the nuclei only slightly increases, until at about 15 × 10¹³g/cm³ the nuclei disappear by coalescing into a homogeneous liquid in an almost continuous phase transition. The maximum proton number per nucleus is 40, which is obtained between the densities 1−2.5 × 10¹³g/cm³; after that the proton number decreases until at the solid-to-liquid phase boundary it is about 20. In the liquid-core region, muons appear at the density 20.5 × 10¹³g/cm³.     

Compact stars in Eddington inspired gravity

A new, Eddington inspired theory of gravity was recently proposed by Ba?ados and Ferreira. It is equivalent to general relativity in vacuum, but differs from it inside matter. This viable, one-parameter theory was shown to avoid cosmological singularities and turns out to lead to many other exciting new features that we report here. First, for a positive coupling parameter, the field equations have a dramatic impact on the collapse of dust, and do not lead to singularities. We further find that the theory supports stable, compact pressureless stars made of perfect fluid, which provide interesting models of self-gravitating dark matter. Finally, we show that the mere existence of relativistic stars imposes a strong, near optimal constraint on the coupling parameter, which can even be improved by observations of the moment of inertia of the double pulsar.     

f-Mode instability in relativistic neutron stars

We present the first calculation of the basic properties of the f-mode instability in rapidly rotating relativistic neutron stars, adopting the Cowling approximation. By accounting for dissipation in neutron star matter, i.e., shear or bulk viscosity and superfluid mutual friction, we calculate the associated instability window. For our specific stellar model, a relativistic polytrope, we obtain a minimum gravitational growth time scale (for the dominant ?=m=4 mode) of the order of 10(3)-10(4) s near the Kepler frequency Ω(K) while the instability is active above ～0.92 Ω(K) and for temperatures ～(10(9)-2×10(10)) K, characteristic of newborn neutron stars.     

Strong hyperon-nucleon pairing in neutron stars

We explore the possibilities of hyperon-nucleon pairing, involving lambda or sigma- hyperons, using different Nijmegen hyperon-nucleon potentials. We find possible very large nsigma- gaps and estimate their relevance for neutron star physics.