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.     

Possibility of a phase transition to a pion condensate in neutron stars

We consider the possibility that a pion condensate may arise in an infinite nuclear medium as a consequence of the modification of the pion propagator due to isobar-hole and nucleon-hole excitations, This does not seem likely in a system with N = Z at densities of less than 0.17 fm^?3, but does seem likely to occur at the densities encountered in the interior of neutron stars. We estimate a necessary neutron density of ? 0.38 fm^?3 and a condensate energy density of ? ? 0.25 MeV fm^?3.     

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.     

Conversion of dark matter axions to photons in magnetospheres of neutron stars

We propose a new method to detect observational appearance of dark matter axions. The method utilizes radio observations of neutron stars. It is based on the conversion of axions to photons in strong magnetic fields of neutron stars (the Primakoff effect). If the conversion occurs, the radio spectrum of the object has a very distinctive feature—a narrow spike at the frequency corresponding to the rest mass of the axion. For example, if the coupling constant of the photon-axion interaction is M = 10¹⁰ GeV, the density of dark matter axions is ρ = 10⁻²⁴ g cm⁻³ and the axion mass is 5 μeV; then the flux from a strongly magnetized (10¹⁴ G) neutron star at the distance 300 pc from the Sun is expected to be about few tenths of millijansky at a frequency of about 1200 MHz in a bandwidth of about 3 MHz. Close-by X-ray dim isolated neutron stars are proposed as good candidates to look for such radio emission. The article is published in the original.     

中子星内部结构

中子星是宇宙中一类极端致密的天体,其平均密度超过饱和核物质密度。对这类天体的研究,可以帮助人们了解极端条件下的物理性质,特别是深化关于引力和强力的认识。文章介绍了脉冲星和中子星的概念,并重点阐述了中子星内部结构的不同模型,以及如何通过最大质量和潮汐形变量等观测来检验这些模型。未来发现更多的双中子星并合或中子星黑洞并合事件,有望最终揭开中子星内部结构之谜。     

Slowly rotating neutron stars and hadronic stars in the chiral SU(3) quark mean-field model

The equations of state for neutron matter, strange and non-strange hadronic matter in the chiral SU(3) quark mean-field model are applied in the study of slowly rotating neutron stars and hadronic stars. The radius, mass, moment of inertia, and other physical quantities are carefully examined. The effect of the nucleon crust for the strange hadronic star is exhibited. Our results show that the rotation can increase the maximum mass of compact stars significantly. For a big enough mass of pulsars which cannot be explained as strange hadronic stars, theoretical approaches to increase the maximum mass are addressed.     

Evidence for heating of neutron stars by magnetic-field decay

We show the existence of a strong trend between neutron star (NS) surface temperature and the dipolar component of the magnetic field extending through three orders of field magnitude, a range that includes magnetars, radio-quiet isolated neutron stars, and many ordinary radio pulsars. We suggest that this trend can be explained by the decay of currents in the crust over a time scale of approximately 10(6) yr. We estimate the minimum temperature that a NS with a given magnetic field can reach in this interpretation.     

End point of the rp process on accreting neutron stars

We calculate the rapid proton ( rp) capture process of hydrogen burning on the surface of an accreting neutron star with an updated reaction network that extends up to Xe, far beyond previous work. In both steady-state nuclear burning appropriate for rapidly accreting neutron stars (such as the magnetic polar caps of accreting x-ray pulsars) and unstable burning of type I x-ray bursts, we find that the rp process ends in a closed SnSbTe cycle. This prevents the synthesis of elements heavier than Te and has important consequences for x-ray burst profiles, the composition of accreting neutron stars, and potentially galactic nucleosynthesis of light p nuclei.     

Bose-Einstein condensation of anti-kaons and neutron star twins

We investigate the role of Bose-Einstein condensation (BEC) of anti-kaons on the equation of state (EoS) and other properties of compact stars. In the framework of relativistic mean field model we determine the EoS for β-stable hyperon matter and compare it to the situation when anti-kaons condense in the system. We observe that anti-kaon condensates soften the EoS, thereby lowering the maximum mass of the stars. We also demonstrate that the presence of antikaon condensates in the high density core of compact stars may lead to a new mass sequence beyond white dwarf and neutron stars. The limiting mass of the new sequence stars is nearly equal to that of neutron star branch though they have distinctly different radii and compositions. They are called neutron star twins.     

Magnetic collapse of a neutron gas: Can magnetars indeed be formed?

A relativistic degenerate neutron gas in equilibrium with a background of electrons and protons in a magnetic field exerts its pressure anisotropically, having a smaller value perpendicular to than along the magnetic field. For critical fields the magnetic pressure may produce the vanishing of the equatorial pressure of the neutron gas. Taking this as a model for neutron stars, the outcome could be a transverse collapse of the star. This fixes a limit to the fields to be observable in stable neutron star pulsars as a function of their density. The final structure left over after the implosion might be a mixed phase of nucleons and a meson condensate, a strange star, or a highly distorted black hole or black ”cigar”, but not a magnetar, if viewed as a superstrongly magnetized neutron star. However, we do not exclude the possibility of superstrong magnetic fields arising in supernova explosions which lead directly to strange stars. In other words, if any magnetars exist, they cannot be neutron stars. Received: 25 November 2002 / Revised version: 25 February 2003 / Published online: 5 May 2003     

Influence of strong magnetic field on β decay in the crusts of neutron stars

βdecay in the strong magnetic field of the crusts of neutron stars is analysed by an improved method. The reactions ⁶⁷ Ni(β^-)⁶⁷ Cu and ⁶² Mn\beta ^-62 Fe are investigated as examples. The results show that a weak magnetic field has little effect on βdecay but a strong magnetic field (B>10¹²G) increases β decay rates obviously. The conclusion derived may be crucial to the research of late evolution of neutron stars and nucleosynthesis in r-process.     

Role of hyperons and pions in neutron stars and supernova

Neutron stars are studied in the framework of nuclear relativistic field theory. Hyperons and pions significantly soften the equation of state of neutron star matter at moderate and high density. We conjecture that they are responsible for the softening that is found to be crucial to the bounce scenario in supernova calculations. Hyperons reduce the limiting mass of neutron stars predicted by theory by one half solar mass or more, which is a large effect compared to the range in which theories of matter predict this limit to fall.     

Neutron star magnetism

It is shown that

1. an appreciable change of magnetic moment of a neutron star cannot occur via ohmic dissipation
2. pulsars provide evidence for large internal magnetic fields in main sequence stars. If pulsars are born from stars with masses exceeding 3 ?_⊙ the internal field must be of the order of 10³-10⁴ Gauss while if they derived from less massive urstars 10² Gauss are sufficient to give rise to a magnetic moment ofM～10³⁰ Gauss cm³.

     

3PF2 neutron superfluidity in neutron stars and three-body force effect

We investigate the ^3PF2 neutron superfluidity in H-stable neutron star matter and neutron stars by using the BCS theory and the Brueckner-Hartree-Fock approach. We adopt the Argonne V18 potential supplemented with a microscopic three-body force as the realistic nucleon-nucleon interaction. We have concentrated on studying the threebody force effect on the ^3PF2 neutron pairing gap. It is found that the three-body force effect is to enhance remarkably the ^3PF2 neutron superfluidity in neutron star matter and neutron stars.     

Pion condensation and the Meisner effect in superdense neutron stars

The pion condensation in superdense neutron stars is at the origin of a Meisner effect with a very small London penetration depth.     

Axion flux from nascent neutron stars

The emission rate of the invisible axion due to electron-related processes is evaluated in detail at high temperatures (T∼20 MeV) and at high densities (ρ∼10¹⁴g cm⁻³), relevant to the supernova explosion. If one properly treats relativistic kinematics, no useful bound on the Peccei-Quinn symmetry breaking scale (f_A) is obtained from SN1987A, contrary to the recent argument by others. A brief comment on the bound from the neutron-neutron axion bremsstrahlung process is also made.     

Effect of superstrong magnetic field on electron screening at the crusts of neutron stars

The influences of electron screening (ES) and electron energy correction (EEC) are investigated by superstrong magnetic field (SMF). We also discuss in detail the discrepant factor between our results and those of Fushiki, Gudmundsson and Pethick (FGP) in SMF. The results show that SMF has only a slight effect on ES when B < 109 T on the surfaces of most neutron stars. Whereas for some magnetars, SMF influence ES greatly when B > 109 T . For instance, due to SMF the ES potential may be increased about 23.6% and the EEC may be increased about 4 orders of magnitude at ρ/μe = 1.0 × 106 mol/cm3 and T9 = 1. On the other hand, the discrepant factor shows that our results are in good agreement with FGP’s when B < 109 T . But the difference will be increased with increasing SMF.     

双中子星的星族合成研究

双中子星是天体物理中重要的研究对象,对于许多研究领域都有重要作用。2017年LIGO/Virgo发现第一个双中子星并合事件GW170817,开启了用引力波探测发现双中子星的序幕。文章主要介绍了双中子星的双星演化通道,以及双中子星星族合成的一些性质。此外,还介绍了双星演化中的一些不确定性以及形成GW170817双中子星的双星演化通道。