中子星的热演化和磁衰减

本研究了中子星的热演化，自转演化和磁场演化的相互影响，考虑了一个自洽模型，中子星因磁偶极辐射而自转减慢，在内部产生某些加热过程，中子星磁场通过壳层的欧姆耗散来衰减。结果表明，磁场衰减提高了加热过程的重要性，相反，加热效应减慢了磁衰减，因此可以得出，中子星的热，自转和磁场也许不是独立演化的，不仅如此，这些演化与初始条件有关，因此，人们也许可以从射电和Ｘ射线观测对脉冲星年龄，初始磁场和周期给出某些限     

小质量X射线双星中中子星自转演化的研究

为解释毫秒脉冲星自转周期的观测数据和理论结果之间的差异,采用数值分析的方法研究了小质量X射线双星中中子星的自转演化.在计算中,分别考虑了辐射压和中子星辐照引起的物质交流的不稳定性对系统的影响.结果如下:(1)吸积盘内的辐射压会使自转周期有小幅增加,中子星辐照导致的物质传输率的变化会缩短演化路径中自转减慢的阶段;(2)同时考虑辐射压和中子星辐照时在物质传输的高态阶段吸积会被辐射压抑制;(3)吸积的质量和快参数影响达到自转平衡的系统数目.     

磁化黑洞吸积盘的演化及放能效率

采用等效电路模型讨论了两种不同类型的磁场对黑洞的旋转能量和角动量的提取机制；Blandford-Znajek(BZ)过程和磁耦合过程，在研究磁化吸积盘中心黑洞自转参量演化特征的基础上，详细比较了纯吸积过程，BZ过程和磁耦合过程对黑洞吸积盘放能效率的贡献，结果表明，磁耦合过程是提取黑洞旋转能量重要的新机制，其放能效率与BZ过程几乎相等，在黑洞自转不是特别大的情况，纯吸积过程的放能效率高于BZ过程和磁耦合过程的放能效率，但是当黑洞自转接近极端Kerr黑洞的自转状态时，放能效率主要由BZ过程和磁耦合过程贡献。     

脉冲星磁场的演化

综述了前人对于单个脉冲星磁场的起源和演化的研究结果及其最新进展。脉冲星磁场的起源有多种模型，其所对应的初始磁场有两种位形：磁场束缚在核内和磁场束缚在壳层中。脉冲星的磁场如何演化，没有一致的结论。有各种观测证据可能直接或间接表明磁场的演化行为，如根据特征年龄和运动学年龄的差异可以推断出脉冲星磁场按指数规律衰减，而根据特征年龄与超新星遗迹年龄的差异或几颗年轻脉冲星的制动指数可以认为年轻脉冲星的磁场可能是增强的。脉冲星的样本合成研究（数值模拟）是研究脉冲星磁场演化的重要方法。模拟结果表明，假定脉冲星磁场按指数衰减，特征衰减时标必须为10^7yr或更长。而壳层中磁场的欧姆耗散模型数值计算显示脉冲星磁场演化行为因冷却模型和状态方程的取法不同而异，但最终无明显的衰减。由自转变慢诱导的脉冲星核内部磁场向壳层中扩散模型的计算表明脉冲星磁场的衰减只发生在10^7-10^8yr这段时间内，磁场衰减1－2个量级。     

中子星的磁场演化

中子星磁场的起源与演化问题一直没有满意解决。本文就近年的磁场观测统计分析和理论解释进行了系统介绍，并总结出中子星磁场演化的观测结论。     

反常X射线脉冲星的研究进展

反常X射线脉冲星（Anomalous X-ra Pulsars，简称AXP）是一类特殊的X射线源。与X射线脉冲星（通常处于大质量X射线双星系统中）相比，它们具有以下特征：X射线谱较软、光度低页稳定（≈10^27－10^29J.s^-1）、自转周期集中在10s左右稳定增长、迄今没有找到它们的光学、红外、射电的对应体、有一些可能戌超新星遗迹成协等。由观测到的自转周期变化可以确定它们的自转能损不足以提供有X射线辐射。解释AXP能源机制的理论模型目前主要有两大类：在吸积模型中，AXP被认为具有正常磁场强度（≈10^8T）的中子量，物质吸积提供X射线辐射原能源，并造成中子星的自转变化；另一种观点认为AXP是具有超强磁场（≈10^10-10^11T）的中子量（即磁星），其辐射能源来自它们巨大的磁或残余的热能，观测到的自转周期及其变化被归因子中子星的磁偶极辐射和物质抛射。两种模型各有优缺点，但目前看来观测事实对磁星模型较为有利。为了进一步明确AXP的性质，提供解释它们能源机制的线索，在介绍AXP的基本观测特征和理论解释的基础上，还将AXP与射电脉冲星、特强磁场射电脉冲量、射电宁静脉冲星侯选体及软γ射线复现源分别进行了比较。     

磁中子星巨耀发的观测和理论研究

软γ重复暴(soft gamma-ray repeater, SGR)被认为产生于磁中子星。发生在SGR上的巨耀发在短时标内释放出大于10^39J的巨大能量,被认为是宇宙中已知最强的能量释放过程之一,其剧烈程度仅次于超新星爆发和γ暴。详细介绍了几种磁球层理论模型,并重点介绍了以太阳爆发日冕物质抛射灾变理论为基础建立的磁中子星巨耀发的磁流体力学的半解析模型。在模型中,板块的转动或错位造成磁球层内磁场的扭缠,从而导致磁通量绳的形成和磁能缓慢的积累。当积累的能量超过阈值,系统就会失去平衡,然后产生爆发并释放能量。用该模型计算的SGR 1806-20, SGR 0526-66和SGR 1900+14这3次巨耀发的光变曲线都与观测基本相符。此外,有关磁中子星巨耀发的磁流体动力学的数值模拟工作也得到了全面的展开,通过求解各种初始条件和边界条件下的磁流体力学方程组,计算机的数值模拟可以得到磁中子星巨耀发过程中的磁场形态演化和内部精细结构。     

脉冲星磁衰减制动力矩对两成分模型自旋的长期减速(理论研究)

主要研究磁衰减对具有两成分模型的脉冲星自转减速的作用。利用分析方法研究了两成分模型的脉冲星在磁衰减制动力矩作用下,两成分的自转角速度对具有两成分的蟹状星云脉冲星(PSR0531+21, Crab)和船帆座脉冲星(PSR0833-45, Vela)在磁衰减作用下的数值计算。结果表明:两个脉冲星的自转角速度逐年减慢,脉冲星PSR0531+21每年减速-0.171 0 rad/s,而脉冲星PSR0833-45每年减速-0.007 1 rad/s。最后讨论了文中得到的结果并给出在两成分模型中存在磁衰减的结论。     

中子星的第二类磁场

以往普遍认为,中子星具有的磁场是衰减性的,衰减时标约为(5—9)×10~6年。近年发现了另一种不衰减或衰减极慢的磁场,称之为第二类磁场。本文将对中子星的这种第二类磁场的研究现状作比较系统的介绍和述评。非常可能,这种磁场相当普遍地存在于各类中子星。脉冲星、X射线双星以及γ射线爆源,都可能存在着这种第二类磁场。     

探究双脉冲星候选体PSR J1906+0746的物理性质

PSR J1906+0746是2006年发现的一颗自旋周期为144 ms的射电脉冲星,由于探测到的是第2颗形成的非自转加速伴星,主星和伴星组成的系统极有可能成为另一对双脉冲星.从PSR J1906+0746的基本物理量出发,针对性地对比研究双中子星和脉冲双星(包括中子星-白矮星)的磁场-周期关系,大致得出他们的演化路径;其次通过双中子星的形成机制以及双星形成过程中的物质损失,假定非自转加速的伴星形成于电子俘获,得出伴星前身星的质量约为1.57 M⊙,在形成双中子星系统过程中损失了约0.23 M⊙.这可能是由于电子俘获能量较低,不能抗衡伴星的引力束缚能,最终抛出物质没有逃逸,形成了一个椭圆环,成功解释了在X射线波段观测到的围绕着PSR J1906+0746的环结构.     

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.     

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.     

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.     

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.     

On the torque exerted by a warped,magnetically threaded accretion disk

Most astrophysical accretion disks are likely to be warped.In X-ray binaries,the spin evolution of an accreting neutron star is critically dependent on the interaction between the neutron star magnetic field and the accretion disk.There have been extensive investigations on the accretion torque exerted by a coplanar disk that is magnetically threaded by the magnetic field lines from the neutron stars,but relevant works on warped/tilted accretion disks are still lacking.In this paper we develop a simplified twocomponent model,in which the disk is comprised of an inner coplanar part and an outer,tilted part.Based on standard assumption on the formation and evolution of the toroidal magnetic field component,we derive the dimensionless torque and show that a warped/titled disk is more likely to spin up the neutron star compared with a coplanar disk.We also discuss the possible influence of various initial parameters on the torque.     

Magnetic fields of coalescing neutron stars and the luminosity function of short gamma-ray bursts

Coalescing binary neutron stars are the most promising candidates for detection by gravitational-wave detectors and are considered to be most promising for explaining the phenomenon of short gamma-ray bursts. The magnetic fields of neutron stars during their coalescence can produce a number of interesting observational manifestations and can affect significantly the shape of the gravitationalwave signal. In this paper, we model the distribution of magnetic fields in coalescing neutron stars by the population synthesis method using various assumptions about the initial parameters of the neutron stars and the evolution laws of their magnetic fields. We discuss possible electromagnetic phenomena preceding the coalescence of magnetized neutron stars and the effect of magnetic field energy on the shape of the gravitational-wave signal during the coalescence. For a log-normal (Gaussian in logarithm) distribution of the initialmagnetic fields consistent with the observations of radio pulsars, the distribution inmagnetic field energy during the coalescence is shown to describe adequately the observed luminosity function of short gamma-ray bursts under various assumptions about the pattern of field evolution and initial parameters of neutron stars.     

Thermal Evolution of Neutron Stars

We present results from simulations of protoneutron star thermal evolution using neutrino opacities that are consistently calculated with the equation of state. When hyperons are allowed to appear in the system, we obtain metastable configurations that after the deleptonization stage become unstable. Concerning the evolution of old neutron stars, we present the results of our investigation on the effect of the Joule heating due to magnetic field dissipation. We conclude that this mechanism can be efficient in maintaining the surface temperature of the star above 3 × 10⁴ - 10⁵ K during a very long time (≥ 100 Myr), comparable with the decay time of the magnetic field. This revised version was published online in July 2006 with corrections to the Cover Date.     

Heating and cooling of magnetars with accreted envelopes

We study the thermal structure and evolution of magnetars as cooling neutron stars with a phenomenological heat source in an internal layer. We focus on the effect of magnetized (   B ≳ 10¹⁴  G) non-accreted and accreted outermost envelopes composed of different elements, from iron to hydrogen or helium. We discuss a combined effect of thermal conduction and neutrino emission in the outer neutron star crust and calculate the cooling of magnetars with a dipole magnetic field for various locations of the heat layer, heat rates and magnetic field strengths. Combined effects of strong magnetic fields and light-element composition simplify the interpretation of magnetars in our model: these effects allow one to interpret observations assuming less extreme (therefore, more realistic) heating. Massive magnetars, with fast neutrino cooling in their cores, can have higher thermal surface luminosity.     

The spin evolution of nascent neutron stars

The loss of angular momentum owing to unstable r-modes in hot young neutron stars has been proposed as a mechanism for achieving the spin rates inferred for young pulsars. One factor that could have a significant effect on the action of the r-mode instability is fallback of supernova remnant material. The associated accretion torque could potentially counteract any gravitational-wave-induced spin-down, and accretion heating could affect the viscous damping rates and hence the instability. We discuss the effects of various external agents on the r-mode instability scenario within a simple model of supernova fallback on to a hot young magnetized neutron star. We find that the outcome depends strongly on the strength of the magnetic field of the star. Our model is capable of generating spin rates for young neutron stars that accord well with initial spin rates inferred from pulsar observations. The combined action of r-mode instability and fallback appears to cause the spin rates of neutron stars born with very different spin rates to converge, on a time-scale of approximately 1 year. The results suggest that stars with magnetic fields ≤10¹³ G could emit a detectable gravitational wave signal for perhaps several years after the supernova event. Stars with higher fields (magnetars) are unlikely to emit a detectable gravitational wave signal via the r-mode instability. The model also suggests that the r-mode instability could be extremely effective in preventing young neutron stars from going dynamically unstable to the bar-mode.     

Evolution of neutron star magnetic fields

This paper reviews the current status of the theoretical models of the evolution of the magnetic fields of neutron stars other than magnetars. It appears that the magnetic fields of neutron stars decay significantly only if they are in binary systems. Three major physical models for this, namely spindown-induced flux expulsion, ohmic evolution of crustal field and diamagnetic screening of the field by accreted plasma, are reviewed.