脉冲星观测研究的某些进展

近年来,对日益增加的脉冲星观测资料的分析和对毫秒脉冲星、射电和X射线脉冲双星的观测,对脉冲星的形成、演化和辐射机制等提出了越来越多的新问题。本文对这些新问题的观测依据进行了介绍。主要涉及下述几方面的问题:是否存在着在结构、形成和演化上均不相同的两类脉冲星?脉冲星是否有两种或两种以上的辐射机制在同时起作用?脉冲星射电辐射中的两种相互垂直的偏振模式,以及近年来在毫秒脉冲星、射电和X射线脉冲双星的观测中所发现的新现象等。     

脉冲星参数统计和演化

统计分析了目前发现的几种脉冲星的自转周期和表面磁场以及空间的分布情况,揭示出毫秒脉冲星比普通射电脉冲星、LMXB(低质量X射线双星)比HMXB(高质量X射线双星)的空间分布要更加弥散;孤立毫秒脉冲星自转周期分布的峰值为4.7 ms,而普通脉冲星的相应值为0.6 s,双星中毫秒脉冲星这一值为3.5 ms; FERMI脉冲...     

基于泊松模型的X射线脉冲星信号的最大似然TOA估计

讨论了X射线脉冲星光子到达的周期平稳Poisson模型和到达时刻(Time of arrival,TOA)估计问题.在此基础上,得到了脉冲到达时刻的最大似然估计和克拉美-罗限(Cramer-Rao boundary,CRB),并推导了低信噪比情况下的似然函数近似表达和克拉美-罗限.利用解析的脉冲轮廓,对PSR B1821-24的TOA估计进行了Monte Carlo仿真,讨论了不同观测时间和信噪比下的估计误差,给出了相应的信噪比门限.研究表明:该分析方法能够有效估计X射线脉冲星的TOA定时精度,有利于评估其在各种应用中的性能.     

4U0115 63:一个高能γ射线双脉冲星

自从发现短周期X射线双脉冲星武仙座X-1发射1,000GeV γ脉冲辐射以后,学者们认为,可能其他的有类似短周期的X射线双脉冲星也有间歇的甚高能γ辐射,最近Chadwick等人对发射间歇性硬X射线双星系统4U0115 63中的3.6秒脉冲星进行了γ射线观测。观测是在1984年9月21日至29日用Dugway γ射线望远镜进行的,观测技术采用的是大气切伦柯夫光探测。选择这个天体作为可能的γ射线发射体,是由于它的光度、自转加速率及脉冲周期等是在北半球可观测天体中最接近武仙座X-1的相应值。     

银河系球状星团射电脉冲星的统计研究

球状星团是银河系中最古老的天体系统之一,其恒星密度极端高的核心有利于创造双星之间进行物质交换的环境,从而形成毫秒脉冲星双星、掩食脉冲双星、主序-毫秒脉冲双星、高轨道偏心率双星等双星系统,通过对这些系统进行研究有助于进一步认识球状星团的动力学、双星系统的演化和星际介质等相关问题。自30年前在球状星团中发现第一颗射电脉冲星至今,随着较高灵敏度射电望远镜的不断建成和使用,以及数据数字化处理能力的提高,天文学家在球状星团射电脉冲星的观测和理论研究方面取得很大进展。收集并分析了最新的球状星团脉冲星的数据,研究了球状星团射电脉冲星的自转周期和轨道周期的基本性质,讨论了球状星团脉冲星的搜寻,最后统计分析了双星系统,包括不同伴星类型的脉冲星的分布以及掩食双星系统的性质。     

从光速圆柱磁能的衰减研究毫秒脉冲双星的演化

本文用脉冲星光速圆柱附近磁能的衰减理论研究了毫秒脉冲星的演化。文中从理论上给出了脉冲星到达辐射截止线时的年龄和自转周期,及现在的辐射年龄和从现在辐射年龄到达辐射截止线所需的时间。并用此理论对7颗脉冲双星的演化作了数值计算。     

黑洞的搜寻和证认(II):脉冲星-黑洞(PSR-BH)双星系统的研究和搜寻

综述了脉冲星-黑洞（PSR－BH）系统的诞生率理论并指出近期发现该系统的可能性。通过综述和分析脉冲星-中子星（PSR－NS）系统的观测特性,特别是脉冲星精确守时性对精确确定双星轨道参量和验证引力理论的重要意义,推测了PSR－BH系统的可能性质,指出发现PSR-BH系统对黑洞的搜寻和最终证认的重要意义。最后简介目前搜寻PSR-BH系统的现状和我们搜寻短轨道周期脉冲双星的计划。     

Crab脉冲星X射线计时观测数据处理与分析

对我国公开发布的空间试验卫星X射线探测器观测Crab脉冲星的数据进行了处理与分析,描述了利用X射线脉冲星观测数据与卫星轨道数据建立积分脉冲轮廓与标准脉冲轮廓的方法,给出了在频域测量脉冲星脉冲到达时刻(toa)的原理方法与算法.利用每组观测数据,计算得到Crab脉冲星脉冲toa,通过与Crab脉冲星星历预报脉冲toa的比较,分析得到卫星载荷X射线探测器观测Crab脉冲星,在消除掉系统误差后,50 min积分时间的观测精度约14μs.     

黑洞的搜寻和证认（Ⅱ）：脉冲星—黑洞（PSR—BH）双星系统的研究和搜寻

综述了脉冲星－黑洞系统的诞生率理论并指出近期发现该系统的可能性。通过综述和分析脉冲星－中子星系统的观测特性，特别是脉冲星精确守时性对精确确定双星轨道参量和验证引力理论的重要意义。     

利用费米卫星大面积望远镜对脉冲星的观测研究

费米γ射线空间望远镜(Fermi)自2008年8月观测任务开始以来,以其优越的观测性能开启了γ射线天文学研究的新纪元。其搭载的大面积望远镜(LAT)在脉冲星探测和研究方面取得了巨大的成功。除了探测到已知脉冲星的γ射线脉冲辐射信号外,LAT还能够利用γ射线数据的盲寻技术独立发现脉冲星,此外还提供了大量γ射线源进行射电脉冲搜寻。Fermi-LAT的观测带来了γ射线脉冲星数量的显著增长,确立了脉冲星是银河系内主要的γ射线源,并且探测到了毫秒脉冲星的γ射线辐射。一般来说,在长时标下脉冲星的γ射线辐射流量稳定且脉冲形状多呈现双峰结构,能谱可由具有指数截断的幂律谱描述。综述了利用Fermi-LAT数据寻找脉冲辐射信号的方法和研究γ射线脉冲星所得到的主要结果,并简要介绍了近年来在脉冲星研究方面取得的新成果。     

Surface-bounded atmosphere of Europa

A 1-D collisional Monte Carlo model of Europa's atmosphere is described in which the sublimation and sputtering sources of H₂O molecules and their molecular fragments are accounted for as well as the radiolytically produced O₂. Dissociation and ionization of H₂O and O₂ by magnetospheric electron, solar UV-photon and photo-electron impact, and collisional ejection from the atmosphere by the low-energy plasma are taken into account. Reactions with the surface are discussed, but only adsorption and atomic oxygen recombination are included in this model. The size of the surface-bounded oxygen atmosphere of Europa is primarily determined by a balance between atmospheric sources from irradiation of the satellite's icy surface by the high-energy magnetospheric charged particles and atmospheric losses from collisional ejection by the low-energy plasma, photo- and electron-impact dissociation, and ionization and pick-up from the surface-bounded atmosphere. A range of sources rates for O₂ to H₂O are used with a larger oxygen-to-water ratio than suggested by laboratory measurements in order to account for differences in adsorption onto grains in the regolith. These calculations show that the atmospheric composition is determined by both the water and oxygen photochemistry in the near-surface region, escape of suprathermal oxygen and water into the jovian system, and the exchange of radiolytic water products with the porous regolith. For the electron impact ionization rates used, pick-up ionization is the dominant oxygen loss process, whereas photo-dissociation and atmospheric sputtering are the dominant sources of neutral oxygen for Europa's neutral torus. Including desorption and loss of water enhances the supply of oxygen species to the neutral torus, but hydrogen produced by radiolysis is the dominant source of neutrals for Europa's torus in these models.     

A spectrophotometric study of the Wolf-Rayet star HD 50896

New spectrophotometric observations of the Wolf-Rayet system HD 50896 are presented and interpreted in terms of its binary nature. The lines of N V, N IV and C IV show moderate variations, which can be explained using a binary model with a compact companion. He n λ4686 appears to arise from a larger region compared to other lines. The distortion caused by the wind can partly explain its flux variations. The emission fluxes of He I lines are generally constant indicating their non-participation in the orbit. The results are compared with other Wolf-Rayet binaries (V444 Cyg and CQ Cep) and the evolutionary status is discussed.     

Meteoritic and other constraints on the internal structure and impact history of small asteroids

Studies of the internal structure of asteroids, which are crucial for understanding their impact history and for hazard mitigation, appear to be in conflict for the S-type asteroids, Eros, Gaspra, and Ida. Spacecraft images and geophysical data show that they are fractured, coherent bodies, whereas models of catastrophic asteroidal impacts, family and satellite formation, and studies of asteroid spin rates, and other diverse properties of asteroids and planetary craters suggest that such asteroids are gravitationally bound aggregates of rubble. These conflicting views may be reconciled if 10-50 km S-type asteroids formed as rubble piles, but were later consolidated into coherent bodies. Many meteorites are breccias that testify to a long history of impact fragmentation and consolidation by alteration, metamorphism, igneous and impact processes. Ordinary chondrites, which are the best analogs for S asteroids, are commonly breccias. Some may have formed in cratering events, but many appear to have formed during disruption and reaccretion of their parent asteroids. Some breccias were lithified during metamorphism, and a few were lithified by injected impact melt, but most are regolith and fragmental breccias that were lithified by mild or moderate shock, like their lunar analogs. Shock experiments show that porous chondritic powders can be consolidated during mild shock by small amounts of silicate melt that glues grains together, and by friction and pressure welding of silicate and metallic Fe,Ni grains. We suggest that the same processes that converted impact debris into meteorite breccias also consolidated asteroidal rubble. Internal voids would be partly filled with regolith by impact-induced seismic shaking. Consolidation of this material beneath large craters would lithify asteroidal rubble to form a more coherent body. Fractures on Ida that were created by antipodal impacts and are concentrated in and near large craters, and small positive gravity anomalies associated with the Psyche and Himeros craters on Eros, are consistent with this concept. Spin data suggest that smaller asteroids 0.6-6 km in size are unconsolidated rubble piles. C-type asteroids, which are more porous than S-types, and their analogs, the volatile-rich carbonaceous chondrites, were probably not lithified by shock.     

New spectroscopic classifications of 35 chemically peculiar candidate stars

The chemically peculiar (CP) stars of the upper main sequence are perfect tracers for several astrophysical processes. Their study especially in open clusters further helps to establish their evolutionary status. The latter is most important to understand the origin and evolution of the CP phenomenon, i.e. the connection between diffusion and a stellar magnetic field. There are two important topics, we cover with this paper. First of all, we investigate the reliability of the CCD Δa photometry for fainter objects in open clusters. The latter method is able to detect CP stars very efficiently, but still a spectroscopic verification is needed to verify the photometric candidates. On the other hand, already published spectral classifications on the basis of photographic plates and prism technology have tobe tested with modern instruments. Classification resolution spectroscopy is presented for thirty five bona‐fide CP candidates. Twenty six of them are located within the boundaries of fourteen open clusters, for which we also investigated their membership probabilities. Apart from five objects, they seem tobe members of the respective clusters. The objects were classified in the framework of a refined Morgan‐Keenan system with the extension of well established CP star spectra. We confirm the CP nature of all but one target. The results of Δa photometry and the spectral classifications are in excellent agreement. For the cluster members we find a continuous sequence of CP stars from 10 to 850 Myr, the whole range of investigated cluster ages (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Phase separation in giant planets: inhomogeneous evolution of Saturn

We present the first models of Jupiter and Saturn to couple their evolution to both a radiative-atmosphere grid and to high-pressure phase diagrams of hydrogen with helium and other admixtures. We find that prior calculated phase diagrams in which Saturn's interior reaches a region of predicted helium immiscibility do not allow enough energy release to prolong Saturn's cooling to its known age and effective temperature. We explore modifications to published phase diagrams that would lead to greater energy release, and propose a modified H-He phase diagram that is physically reasonable, leads to the correct extension of Saturn's cooling, and predicts an atmospheric helium mass fraction Y_atmos=0.185, in agreement with recent estimates. We also explore the possibility of internal separation of elements heavier than helium, and find that, alternatively, such separation could prolong Saturn's cooling to its known age and effective temperature under a realistic phase diagram and heavy element abundance (in which case Saturn's Y_atmos would be solar but heavier elements would be depleted). In none of these scenarios does Jupiter's interior evolve to any region of helium or heavy-element immiscibility: Jupiter evolves homogeneously to the present day. We discuss the implications of our calculations for Saturn's primordial core mass.     

Unipolar induction of a magnetized accretion disk around a black hole

The structure and magnitude of the electromagnetic field produced by a rotating accretion disk around a black hole were determined. The disk matter is assumed to be a magnetized plasma with a frozenin poloidal magnetic field. The vacuum approximation is used outside the disk.     

Laboratory measurements of the microwave opacity of hydrochloric acid vapor in a carbon dioxide atmosphere

Laboratory measurements of the microwave opacity of HCl in a CO₂ atmosphere have been conducted in the S (13.3 cm), X (3.6 cm), and K (1.4 cm) microwave bands at a pressure of 7.2 bar and at two different mixing ratios. The results are consistent with an opacity model employing the Van Vleck-Weisskopf lineshape applied to the published submillimeter line intensities of HCl (JPL Catalog [Pickett et al., 1998, J. Quant. Spectrosc. Rad. Trans. 60, 883-890]) and empirically fitted with a modeled parameter for CO₂ broadening. Based on the deep atmospheric abundance of HCl inferred from near-infrared measurements [Dalton et al., 2000, Bull. Am. Astron. Soc. 32, 1120], the resulting modeled HCl opacity is constrained to have a small effect on the overall microwave absorption spectrum of Venus, but can be used in developing a more accurate radiative transfer model.     

Sources and losses of energetic protons in Saturn's magnetosphere

We present Cassini data revealing that protons between a few keV and about 100 keV energy are not stably trapped in Saturn's inner magnetosphere. Instead these ions are present only for relatively short times following injections. Injected protons are lost principally because the neutral gas cloud converts these particles to energetic neutral atoms via charge exchange. At higher energies, in the MeV to GeV range, protons are stably trapped between the orbits of the principal moons because the proton cross-section for charge exchange is very small at such energies. These protons likely result from cosmic ray albedo neutron decay (CRAND) and are lost principally to interactions with satellite surfaces and ring particles during magnetospheric radial diffusion. A main result of this work is to show that the dominant energetic proton loss and source processes are a function of proton energy. Surface sputtering by keV ions is revisited based on the reduced ion intensities observed. Relatively speaking, MeV ion and electron weathering is most important closer to Saturn, e.g. at Janus and Mimas, whereas keV ion weathering is most important farther out, at Dione and Rhea.     

The contribution of grains to the activity of comets: II. The brightness of the coma

We use the model of grain behavior in the coma developed by Beer et al. [Beer, E.H., Podolak, M., Prialnik, P., 2006. Icarus 180, 473-486] to compute the contribution of ice grains to the brightness of the coma. The motion of an ice grain along the comet-Sun axis is computed, taking into account gas drag, the gravity of the nucleus, and radiation pressure of sunlight. The sublimation of the grains is also included. We assume that the maximum distance that a grain travels along this axis is indicative of the size of the coma, and we compute the resultant brightness as a function of heliocentric distance. The results are then compared to observations.     

The rotation of Janus and Epimetheus

Epimetheus, a small moon of Saturn, has a rotational libration (an oscillation about synchronous rotation) of 5.9°±1.2°, placing Epimetheus in the company of Earth’s Moon and Mars’ Phobos as the only natural satellites for which forced rotational libration has been detected. The forced libration is caused by the satellite’s slightly eccentric orbit and non-spherical shape.Detection of a moon’s forced libration allows us to probe its interior by comparing the measured amplitude to that predicted by a shape model assuming constant density. A discrepancy between the two would indicate internal density asymmetries. For Epimetheus, the uncertainties in the shape model are large enough to account for the measured libration amplitude. For Janus, on the other hand, although we cannot rule out synchronous rotation, a permanent offset of several degrees between Janus’ minimum moment of inertia (long axis) and the equilibrium sub-Saturn point may indicate that Janus does have modest internal density asymmetries.The rotation states of Janus and Epimetheus experience a perturbation every 4 years, as the two moons “swap” orbits. The sudden change in the orbital periods produces a free libration about synchronous rotation that is subsequently damped by internal friction. We calculate that this free libration is small in amplitude (<0.1°) and decays quickly (a few weeks, at most), and is thus below the current limits for detection using Cassini images.