Evolution of neutron stars in high-mass X-ray binaries

We consider the evolution of neutron stars during the X-ray phase of high-mass binaries. Calculations are performed assuming a crustal origin of the magnetic field. A strong wind from the companion can significantly influence the magnetic and spin behaviour of a neutron star even during the main-sequence life of the companion. In the course of evolution, the neutron star passes through four evolutionary phases ('isolated pulsar', propeller, wind accretion, and Roche lobe overflow). The model considered can naturally account for the observed magnetic fields and spin periods of neutron stars, as well as the existence of pulsating and non-pulsating X-ray sources in high-mass binaries. Calculations also predict the existence of a particular sort of high-mass binary with a secondary that fills its Roche lobe and a neutron star that does not accrete the overflowing matter because of fast spin.     

用中子星限制暗物质粒子散射截面

中子星可以通过重子物质和暗物质的相互作用吸积暗物质,且在一定条件下, 中子星吸积的暗物质粒子可以引发自引力塌缩形成小型黑洞, 生成的黑洞可能会进一步吞噬中子星.依据文献已有模型, 基于以上物理过程给出了在暗物质粒子不同质量下对暗物质粒子--中子的散射截面的限制.使用弱相互作用大质量粒子(Weakly Interacting Massive Particle, WIMP)模型, 并考虑暗物质粒子是玻色子的情形, 讨论了暗物质粒子有无自相互作用以及有无湮灭等条件下对限制暗物质参数的影响.既考虑了已发现的两个中子星系统来给出对暗物质参数空间的限制,也考虑了两个可能存在的年老中子星来预测未来观测可能对暗物质参数空间的限制.对于考虑玻色--爱因斯坦凝聚(Bose-Einstein Condensate, BEC)的玻色子暗物质, 在无自相互作用或有弱自相互作用, 无湮灭或有很小湮灭截面的条件下,中子星给出的间接观测对暗物质粒子-中子散射截面的限制的强度比XENON1T直接探测实验来得更强.未来, 如果在银心附近能观测到年老中子星, 其观测结果可以提升模型给出的对暗物质粒子--中子散射截面的限制, 从而帮助人们进一步理解暗物质.     

Cooling of quark stars in the colour superconductive phase: effect of photons from glueball decay

The cooling history of a quark star in the colour superconductive phase is investigated. Here we specifically focus on the two-flavour colour (2SC) phase where the novel process of photon generation via glueball (GLB) decay has already been investigated. The picture we present here can, in principle, be generalized to quark stars entering a superconductive phase where similar photon generation mechanisms are at play. As much as 10⁴⁵–10⁴⁷ erg of energy is provided by the GLB decay in the 2SC phase. The generated photons slowly diffuse out of the quark star, keeping it hot and radiating as a blackbody (with possibly a Wien spectrum in gamma-rays) for millions of years. We discuss hot radio-quiet isolated neutron stars in our picture (such as RX J185635–3754 and RX J0720.4–3125) and argue that their nearly blackbody spectra (with a few broad features) and their remarkably tiny hydrogen atmosphere are indications that these might be quark stars in the colour superconductive phase where some sort of photon generation mechanism (reminiscent of the GLB decay) has taken place. Fits to observed data of cooling compact stars favour models with superconductive gaps of  Δ_2SC∼ 15–35 MeV  and densities  ρ_2SC= (2.5–3.0) ×ρ_N  (ρ_N being the nuclear matter saturation density) for quark matter in the 2SC phase. If correct, our model combined with more observations of isolated compact stars could provide vital information to studies of quark matter and its exotic phases.     

Thermal evolution of rotating hybrid stars

As a neutron star spins down, the nuclear matter is continuously converted into quark matter due to the core density increase, and then latent heat is released. We have investigated the thermal evolution of neutron stars undergoing such deconfinement phase transition. We have taken into account the conversion in the frame of the general theory of relativity. The released energy has been estimated as a function of changed rate of deconfinement baryon number. The numerical solutions to the cooling equation are seen to be very different from those without the heating effect. The results show that neutron stars may be heated to higher temperatures which is well matched with pulsar's data despite the onset of fast cooling in neutron stars with quark matter cores. It is also found that the heating effect has a magnetic field strength dependence. This feature could be particularly interesting for high temperatures of low-field millisecond pulsars at a later stage. The high temperature could fit the observed temperature for PSR J0437−4715.     

The melting of neutron stars' crystalline cores and gamma-ray bursts

Possible phase transitions in neutron star matter, particularly the melting of neutron stars' crystalline cores, are discussed. Such processes may explain the observed luminosity of pulsars. They are used also as a basis for an explanation of the origin of low energy gamma-ray bursts which have been intensively studied for the last few years. The authors discuss the structure of gamma-ray bursts and the possibility of obtaining from observational data some information on thermal evolution of neutron stars and dynamic processes in the pulsar crust.     

The effects of strong interaction on the observational discrimination between neutron and strange stars

Strange stars are compact objects similar to neutron stars composed of strange matter. This paper investigates the observational effects of the strong interaction between quarks. We believe: 1) that the conversion of a neutron star to a strange star is a large “period glitch” which is determined by the strong interaction; 2) that the strong interaction results in effective damping of oscillation of hot strange stars, which could be a new mechanism of driving supernova explosions; 3) that the strong interaction increases the difference in rotation between strange and neutron stars under high temperatures, making the minimum period for strange stars lower than that for neutron stars.     

Spin evolution of neutron stars in binary systems

I review our understanding of the evolution of the spin periods of neutron stars in binary stellar systems, from their birth as fast, spin-powered pulsars, through their middle life as accretion-powered pulsars, upto their recycling or “rebirth” as spin-powered pulsars with relatively low magnetic fields and fast rotation. I discuss how the new-born neutron star is spun down by electromagnetic and “propeller” torques, until accretion of matter from the companion star begins, and the neutron star becomes an accretion-powered X-ray pulsar. Detailed observations of massive radio pulsar binaries like PSR 1259-63 will yield valuable information about this phase of initial spindown. I indicate how the spin of the neutron star then evolves under accretion torques during the subsequent phase as an accretion-powered pulsar. Finally, I describe how the neutron star is spun up to short periods again during the subsequent phase of recycling, with the accompanying reduction in the stellar magnetic field, the origins of which are still not completely understood.     

Treatment of a static spherically symmetric matter distribution on the basis of the projective unified field theory

In order to confront the Projective Unified Field Theory with physical experience it is appropriate to apply this theory also to astrophysical objects, for mathematical reasons in particular to a static spherically symmetric matter distribution as a rough model for a star (e.g. neutron star or another exotic compact object). The problem mentioned is treated here as far as possible analytically in order to prepare the basis for the numerical approach.     

Exotic bulk viscosity and its influence on neutron star r-modes

We investigate the effect of exotic matter in particular, hyperon matter on neutron star properties such as equation of state (EoS), mass-radius relationship and bulk viscosity. Here we construct equations of state within the framework of a relativistic field theoretical model. As hyperons are produced abundantly in dense matter, hyperon–hyperon interaction becomes important and is included in this model. Hyperon–hyperon interaction gives rise to a softer EoS which results in a smaller maximum mass neutron star compared with the case without the interaction. Next we compute the coefficient of bulk viscosity and the corresponding damping time scale due to the non-leptonic weak process including Λ hyperons. Further, we investigate the role of the bulk viscosity on gravitational radiation driven r-mode instability in a neutron star of given mass and temperature and find that the instability is effectively suppressed.      

Neutron stars and the dense matter equation of state

Neutron stars provide a unique laboratory with which to study cold, dense matter. The observational quantities of primary astrophysics interest are the maximum mass and the typical radius of a neutron star. These quantities are related to the relative stiffness of neutron-rich matter at supernuclear densities and the density dependence of the nuclear symmetry energy near the nuclear saturation density. The measurements of these nuclear properties via nuclear systematics and structure, heavy-ion collisions and parity-violating electron scattering from neutron-rich nuclei, are discussed. Several new observations, including mass measurements of binary pulsars and a confirmed distance determination for a nearby cooling neutron star, will be summarized. Additionally addressed will be observations of thermal emissions from cooling neutron stars in globular clusters and thermonuclear explosions from accreting stars. It will be demonstrated how this astrophysical data is shedding light on the pressure-density relation of extremely dense matter.     

On the internal structure of stars

A new model of the internal structure of certain types of celestial bodies is proposed. It is based on the concept that some neutron stars might have been formed earlier than all other type of stars, at an early stage of expansion of the universe, directly from continuous cosmic matter. Under such conditions, a neutron star after forming becomes an efficient center for the accretion of cosmic plasma. The plasma streams falling onto the neutron star carry magnetic fields with them that are created in the process (by thermoelectric currents and the dynamo process) and pack the fields tightly around the star. After a certain time, an extended and strongly magnetized plasma layer is formed around the neutron star. As a result, a stellar configuration is formed with an outer layer, mass, radius, and luminosity similar to those of an ordinary star. In the magnetized part of such a configuration, the gravitational attraction of the masses is compensated for by a magnetic pressure gradient, while the plasma is confifned by the magnetic field itself. Numerical estimates corroborate the possibility that magnetized stars exist. The radii and masses of the magnetized spheres of such stars are considerably less than the radii and masses of the corresponding configurations, so in observations they should not differ from ordinary stars: the outer layers (intermediate layer, photosphere, and chromosphere) of the magnetized configuration are the same as for an ordinary star. Structural differences may appear in the inner regions, however, involving magnetic activity and neutrino luminosity, for example.     

Maximum mass of a hot neutron star with a quark core

We have considered a hot neutron star with a quark core,a mixed phase of quark-hadron matter,and a hadronic matter crust and have determined the equation of state of the hadronic phase and the quark phase.We have then found the equation of state of the mixed phase under the Gibbs conditions.Finally,we have computed the structure of a hot neutron star with a quark core and compared our results with those of the neutron star without a quark core.For the quark matter calculations,we have used the MIT bag model...     

A General Relativistic Calculation of Boundary Layer and Disc Luminosity for Accreting Non-magnetic Neutron Stars in Rapid Rotation

We calculate the disc and boundary layer luminosities for accreting rapidly rotating neutron stars with low magnetic fields in a fully general relativistic manner. Rotation increases the disc luminosity and decreases the boundary layer luminosity. A rapid rotation of the neutron star substantially modifies these quantities as compared with the static limit. For a neutron star rotating close to the centrifugal mass shed limit, the total luminosity has contribution only from the extended disc. For such maximal rotation rates, we find that well before the maximum stable gravitational mass configuration is reached, there exists a limiting central density, for which particles in the innermost stable orbit will be more tightly bound than those at the surface of the neutron star. We also calculate the angular velocity profiles of particles in Keplerian orbits around the rapidly rotating neutron star. The results are illustrated for a representative set of equation of state models of neutron star matter.     

The evolution of the magnetic fields of neutron stars

Observational evidence, and theoretical models of the magnetic field evolution of neutron stars is discussed. Observational data indicates that the magnetic field of a neutron star decays significantly only if it has been a member of a close interacting binary. Theoretically, the magnetic field evolution has been related to the processing of a neutron star in a binary system through the spin evolution of the neutron star, and also through the accretion of matter on the neutron star surface. I describe two specific models, one in which magnetic flux is expelled from the superconducting core during spin-down, via a copuling between Abrikosov fluxoids and Onsager-Feynman vortices; and another in which the compression and heating of the stellar crust by the accreted mass drastically reduces the ohmic decay time scale of a magnetic field configuration confined entirely to the crust. General remarks about the behaviour of the crustal field under ohmic diffusion are also made.     

Magnetic and spin evolution of neutron stars in close binaries

The evolution of neutron stars in close binary systems with a low-mass companion is considered, assuming the magnetic field to be confined within the solid crust. We adopt the standard scenario for the evolution in a close binary system, in which the neutron star passes through four evolutionary phases ('isolated pulsar'–'propeller'– accretion from the wind of a companion – accretion resulting from Roche-lobe overflow). Calculations have been performed for a great variety of parameters characterizing the properties of both the neutron star and the low-mass companion. We find that neutron stars with more or less standard magnetic field and spin period that are processed in low-mass binaries can evolve to low-field rapidly rotating pulsars. Even if the main-sequence life of a companion is as long as 10¹⁰ yr, the neutron star can maintain a relatively strong magnetic field to the end of the accretion phase. The model that is considered can account well for the origin of millisecond pulsars.     

Neutron star retention and millisecond pulsar production in globular clusters

We investigate the conditions by which neutron star retention in globular clusters is favoured. We find that neutron stars formed in massive binaries are far more likely to be retained. Such binaries are likely to then evolve into contact before encountering other stars, possibly producing a single neutron star after a common envelope phase. A large fraction of the single neutron stars in globular clusters are then likely to exchange into binaries containing moderate-mass main-sequence stars, replacing the lower-mass components of the original systems. These binaries will become intermediate-mass X-ray binaries (IMXBs), once the moderate-mass star evolves off the main sequence, as mass is transferred on to the neutron star, possibly spinning it up in the process. Such systems may be responsible for the population of millisecond pulsars (MSPs) that has been observed in globular clusters. Additionally, the period of mass-transfer (and thus X-ray visibility) in the vast majority of such systems will have occurred 5–10 Gyr ago, thus explaining the observed relative paucity of X-ray binaries today, given the MSP population.     

Small-scale Gradients and Large-scale Diffusion of Charged Particles in the Heliospheric Magnetic Field

We present a simple spin-evolution model that predicts that rapidly rotating accreting neutron stars will be confined mainly to a narrow range of spin frequencies: P=1.5-5 ms. This is in agreement with current observations of neutron stars in both the low-mass X-ray binaries and the millisecond radio pulsars. The main ingredients in the model are (1) the instability of r-modes above a critical spin rate, (2) the thermal runaway that is due to the heat released as viscous damping mechanisms counteract the r-mode growth, and (3) a revised estimate of the strength of the dissipation that is due to the presence of a viscous boundary layer at the base of the crust in an old and relatively cold neutron star. We discuss the gravitational waves that are radiated during the brief r-mode-driven spin-down phase. We also briefly touch on how the new estimates affect the predicted initial spin periods of hot young neutron stars.     

Merging strangeon stars

The state of supranuclear matter in compact stars remains puzzling, and it is argued that pulsars could be strangeon stars. What would happen if binary strangeon stars merge? This kind of merger could result in the formation of a hyper-massive strangeon star, accompanied by bursts of gravitational waves and electromagnetic radiation(and even a strangeon kilonova explained in the paper). The tidal polarizability of binary strangeon stars is different from that of binary neutron stars, because a strangeon star is self-bound on the surface by the fundamental strong force while a neutron star by the gravity, and their equations of state are different. Our calculation shows that the tidal polarizability of merging binary strangeon stars is favored by GW170817. Three kinds of kilonovae(i.e., of neutron, quark and strangeon) are discussed, and the light curve of the kilonova AT 2017 gfo following GW170817 could be explained by considering the decaying strangeon nuggets and remnant star spin-down. Additionally,the energy ejected to the fireball around the nascent remnant strangeon star, being manifested as a gamma-ray burst, is calculated. It is found that, after a prompt burst, an X-ray plateau could follow in a timescale of 10~2-10~3 s. Certainly, the results could be tested also by further observational synergies between gravitational wave detectors(e.g., Advanced LIGO) and X-ray telescopes(e.g., the Chinese HXMT satellite and e XTP mission), and especially if the detected gravitational wave form is checked by peculiar equations of state provided by the numerical relativistical simulation.     

早期中子星和夸克物质

夸克禁闭的解除与夸克物质的存在一直是物理学家极感兴趣的问题。尽管理论上已指出在超高温或超高密的条件下可以有夸克物质存在,但是由于地面实验室的条件所限,目前还不能通过实验证实这一点.宇宙中被观测到的中子星(例如crab和Vela脉冲星)的中心密度大于4倍的核物质密度,其中心温度也可以达到10~8—10~9K,于是人们希     

Signal of quark deconfinement in thermal evolution neutron stars with deconfinement heating

As neutron stars spin-down and contract, the deconfinement phase transition can continue to occur, resulting in energy release (so-called deconfinement heating) in case of the first-order phase transition. The thermal evolution of neutron stars is investigated to combine phase transition and the related energy release self-consistently. We find that the appearance of deconfinement heating during spin-down result in not only the cooling delay but also the increase of surface temperature of stars. For stars characterized by intermediate and weak magnetic field strength, a period of increasing surface temperature could exist. Especially, a sharp jump in surface temperature can be produced as soon as quark matter appears in the core of stars with a weak magnetic field. We think that this may serve as evidence for the existence of deconfinement quark matter. The results show that deconfinement heating facilitates the emergence of such characteristic signature during the thermal evolution process of neutron stars.