中子星吸积柱中荷电粒子密度的分布及电场的形成

本文从刘维定理出发,通过相空间求平均,建立了中子星吸积柱中粒子流连续方程和动量迁移方程。并在静力学平衡下求出其解;建立了吸积柱中的荷电粒子分布;引出了某些有趣的新结果。      

Production and evolution of millisecond X-ray and radio pulsars

Observations using the Rossi X-ray Timing Explorer (RXTE) have discovered dozens of accreting neutron stars with millisecond spin periods in low-mass binary star systems. Eighteen are millisecond X-ray pulsars powered by accretion or nuclear burning or both. These stars have magnetic fields strong enough for them to become millisecond rotation-powered (radio) pulsars when accretion ceases. Few, if any, accretion- or rotation-powered pulsars have spin rates higher than 750 Hz. There is strong evidence that the spin-up of some accreting neutron stars is limited by magnetic spin-equilibrium whereas the spin-up of others is halted when accretion ends. Further study will show whether the spin rates of some accretion- or rotation-powered pulsars are or were limited by emission of gravitational radiation.     

Spectra of accreting X-ray pulsars

Using recent improved results on the frequency and polarization dependent opacities in a strong magnetic field we examine the physics of the lower accretion column on a magnetic neutron star to determine the spectrum radiated at infinity. We argue that photon convection by the fast infalling material (free fall velocities c/2) should substantially modify the spectrum radiated through the accretion column due to the frequency dependence of the opacity.     

Production of QPOs in accreting neutron star systems

Kilohertz QPOs have been detected from more than 20 neutron stars in low-mass X-ray binaries. Several different ideas have been proposed for their generation, involving resonances, magnetic interactions, and sharp transitions in the accretion flow. We show that although details are uncertain at this time, it is clear that the stellar magnetic field has a dynamic influence on the accretion flow. We also discuss the inferences about dense matter and strong gravity that can be drawn from all models, and the qualitative advances expected with a future X-ray timing mission.     

Exosat observations of the galactic bulge X-ray source GX17+2

We have measured the X-ray flux of the bright galactic bulge source GX17+2 in the energy range 1–20 keV using the EXOSAT ME experiment. During 8 hours of continuous observation an X-ray flare was observed (lasting ～1 hr) followed by an intensity increase. The data show intensity dips with a quasiperiod of ～1.4 hours and quasi-periodic oscillations on time scale of 200–500 sec, which are possibly connected with oscillations of an accretion disc. The spectrum can be fitted by two blackbody spectra with kT₁～1keV, and kT₂～2keV, respectively, and an iron line at 6.3 ± 0.3 keV having 130 eV equivalent width. While the low energy component is rather stable, the 2keV-component shows considerable intensity variations. We suggest that the latter component represents emission from the inner part of the accretion disc while the soft spectrum is blackbody emission from the surface of the neutron star.     

Challenges in X-ray binary timing: current and future

Millisecond X-ray time variability studies of accreting low-magnetic-field neutron stars and stellar-mass black holes in X-ray binaries probe the motion of matter in regions of strong gravity. In these regions, general relativity (GR) is no longer a small correction to the classical laws of motion, but instead dominates the dynamics: we are studying motion in strongly curved spacetime. Such millisecond X-ray variability studies can therefore provide unique tests of GR in the strong-field regime. The same studies also constrain neutron-star parameters such as stellar mass and radius, and thereby the equation of state (EOS) of ultradense matter. I briefly review the status, and discuss the prospects for mapping out space-time near accreting stellar-mass compact objects, and measuring the EOS of dense matter, through millisecond timing, particularly with an eye towards future missions. The overwhelming consideration for timing sensitivity is collecting area: contrary to most applications, the signal-to-noise ratio for the aperiodic timing phenomena produced by accretion flows increases proportionally with count rate rather than as the square root of it. A 10 times larger instrument turns 1σ effects into 10σ effects (or does as well in 1% of the time). With the Rossi X-ray Timing Explorer (RXTE), using 0.6 m² collecting area, we have found several timing diagnostics from the accretion flow in the strong field region around neutron stars and black holes, as well as signals from neutron star surface hot spots. Combined work between RXTE and the new sensitive X-ray spectrographs onboard Chandra and XMM can already begin to clinch the geometry and physical mechanisms underlying these signals. Future instruments, larger in area by an order of magnitude and in some cases with enhanced spectral capabilities, are expected to turn these diagnostics of GR into true tests of GR. They are also expected to put strong constraints on neutron-star structure, and thereby on the EOS of supranuclear density matter.     

2D and 3D MHD simulations of disk accretion by rotating magnetized stars: Search for variability

We performed 2D and full 3D magnetohydrodynamic simulations of disk accretion to a rotating star with an aligned or misaligned dipole magnetic field. We investigated the rotational equilibrium state and derived from simulations the ratio between two main frequencies: the spin frequency of the star and the orbital frequency at the inner radius of the disk. In 3D simulations we observed different features related to the non-axisymmetry of the magnetospheric flow. These features may be responsible for high-frequency quasi-periodic oscillations (QPOs). Variability at much lower frequencies may be connected with restructuring of the magnetic flux threading the inner regions of the disk. Such variability is specifically strong at the propeller stage of evolution.     

X射线脉冲星4U 1538—52 的时间和能谱特性

本文处理分析了EXOSAT卫星对4U1538-524次中能观测资料,并结合其他观测结果对4U1538-52的时间和能谱特性作了分析和解释:中子星自转仍在减慢;能谱为幂律谱,Tenma卫星以后的观测未见Fe发射线;X射线dip现象可解释为一前景星对中子星的遮掩而形成;X射线爆发在不同的轨道位相出现,持续近1小时;在1keV附近似有一发射线。      

Cyg X-2 — An accreting neutron star

HAKUCHO observation of Cyg X-2 over 40 days did not show a correlation between the hardness ratio and the intensity predicted for dwarf X-ray sources. The energy spectrum in the range 0.3 – 20 keV was found to deviate from the thermal bremsstrahlung spectrum below 2 keV. The X-ray spectrum can be accounted for in terms of the comptonization of blackbody radiation emitted from teh neutron star surface and the accretion disk.     

XMM-Newton observations of low luminosity Be/X-ray candidates

A small number of early Be stars exhibit X-ray luminosities intermediate between those typical of early type stars and those radiated by Be/X-ray binaries in the quiescent state. We report on XMM-Newton observations of two such Be stars, HD 161103 and SAO 49725 which were originally discovered in a systematic cross-correlation between the ROSAT all-sky survey and SIMBAD. The new observations confirm the X-ray luminosity detected by ROSAT (L_X  10³² erg s⁻¹) and the hardness of their X-ray spectra (thin thermal with kT  8–10 keV or power law with photon index of 1.7) which are both unusual for normal early type stars. We discuss the possible origin of this excess X-ray emission in the light of the models proposed for γ-Cas, magnetic disc-star interaction or accretion onto a compact companion object, neutron star or white dwarf, and compare the properties of these two sources with those of the new massive systems discovered in the XMM- Newton/SSC survey of the Galactic plane.     

Observational evidence for the binary model of X-ray bursters

Most, but not all, theoretical models of X-ray bursters require a binary system consisting of a mass donating star and a neutron star. The observational evidence in support of this model, however, is both indirect and meager. We have detected absorption dips in the X-ray spectrum of the Burster MXB 1916-05 with the IPC and the MPC on the Einstein Observatory which occur with a binary period of 2985 seconds. These dips are shown to be the result of a gas stream emanating from a companion star and hence this data represents the first direct evidence of the binary nature of X-ray bursters. Detailed models of the interaction of the gas stream with the accretion disk are presented. A 22nd mag. optical candidate for the system has been found.     

Polarization effects in radiation from compact X-ray sources

The theory of polarization effects in radiation from compact X-ray sources is presented. The following four problems are considered: 1) the polarization of X-rays from the magnetized neutron stars; 2) the polarization of X-rays from the accretion disk around a black hole; 3) the optical polarization from X-ray binaries, and 4) the results of X-ray polarimetric observations.     

Multiband echo tomography of Sco X-1

We present preliminary results of a simultaneous X-ray/optical campaign of the prototypical LMXB Sco X-1 at 1–10 Hz time resolution. Lightcurves of the high excitation Bowen/HeII emission lines and a red continuum at λ_c  6000 Å were obtained through narrow interference filters with ULTRACAM, and these were cross-correlated with simultaneous RXTE X-ray lightcurves. We find evidence for correlated variability, in particular when Sco X-1 enters the Flaring branch. The Bowen/HeII lightcurves lag the X-ray lightcurves with a light travel time which is consistent with reprocessing in the companion star while the continuum lightcurves have shorter delays consistent with reprocessing in the accretion disc.     

Evolution of binaries with chemically peculiar primaries

The chemically peculiar (CP) stars are classified into subgroups based on the type of peculiarities. A significant fraction of these are known to be binaries. The faster evolution of the massive component leads to a white dwarf or a neutron star. Further evolution of the binary is analysed taking into consideration, the orbital parameters, effect of magnetic field, spectroscopic peculiarities and finally the statistics of CP binaries and Low Mass X-ray Binaries (LMXB).

The possible consequences of the evolution to lead to the formation of Magnetic Cataclysmic Variables (MCV) and LMXB are discussed.     

Fundamental parameters of low mass X-ray binaries II: X-ray persistent systems

The determination of fundamental parameters in X-ray luminous (persistent) X-ray binaries has been classically hampered by the large optical luminosity of the accretion disc. New methods, based on irradiation of the donor star and burst oscillations, provide the opportunity to derive dynamical information and mass constraints in many persistent systems for the first time. These techniques are here reviewed and the latest results presented.     

Review on latest progress on supergiant fast X-ray transients and future direction

In the recent years, the discovery of a new class of Galactic transients with fast and bright flaring X-ray activity, the supergiant fast X-ray transients, has completely changed our view and comprehension of massive X-ray binaries. These objects display X-ray outbursts which are difficult to be explained in the framework of standard theories for the accretion of matter onto compact objects, and could represent a dominant population of X-ray binaries. I will review their main observational properties (neutron star magnetic field, orbital and spin period, long term behavior, duty cycle, quiescence and outburst emission), which pose serious problems to the main mechanisms recently proposed to explain their X-ray behavior. I will discuss both present results and future perspectives with the next generation of X-ray telescopes.     

Contraction of magnetized gas to the central gravitator and the production of magnetically accelerated jets

A magnetodynamic model proposed and worked on by Uchida and collaborators for the star formation jets (bipolar flows), by taking a genetic point of view into account, is reviewed. A large scale magnetic field, which is week in the primordial gas but intensified in the gravitational contraction of the gas, assists the continuous accretion of the gas to the central gravitator by extracting angular momentum from the rotating disk, and this process creates a large amplitude torsional Alfven wave that swirls up the gas into the direction of the axis. This torsional Alfven wave, as it propagates, pinches the large scale field into a slender strong field structure which we identify with the collimated jet. The rationale for extending this mechanism to the AGN jet cases is given, and some results of application to the AGN jet case are presented, with interpretation of some characteristic features like the wiggling of the jets, extended radio lobes and the hotspots at the end of the jets.     

Fluid instabilities around accreting X-ray sources

Some aspects of fluid instabilities occurring in the magnetospheres of accreting neutron stars are discussed. It is pointed out that (i) in the absence of strong differential rotation, the accreting plasma should be drawn out into spiralling, sheet-like structures, resulting in efficient mixing between the two media; (ii) the Rayleigh-Taylor instability also acts to limit the X-ray luminosity in super-critical sources; and (iii) magnetic shear has a strong stabilizing influence on Kelvin-Helmholtz modes, and its presence may allow substantial amounts of material to be supported around the magnetosphere.     

X-ray timing beyond the Rossi X-ray Timing Explorer

With its ability to look at bright galactic X-ray sources with sub-millisecond time resolution, the Rossi X-ray Timing Explorer (RXTE) discovered that the X-ray emission from accreting compact stars shows quasi-periodic oscillations on the dynamical timescales of the strong field region. RXTE showed also that waveform fitting of the oscillations resulting from hot spots at the surface of rapidly rotating neutron stars constrain their masses and radii. These two breakthroughs suddenly opened up a new window on fundamental physics, by providing new insights on strong gravity and dense matter. Building upon the RXTE legacy, in the Cosmic Vision exercise, testing General Relativity in the strong field limit and constraining the equation of state of dense matter were recognized recently as key goals to be pursued in the ESA science program for the years 2015–2025. This in turn identified the need for a large (10 m² class) aperture X-ray observatory. In recognition of this need, the XEUS mission concept which has evolved into a single launch L2 formation flying mission will have a fast timing instrument in the focal plane. In this paper, I will outline the unique science that will be addressed with fast X-ray timing on XEUS.     

Characteristics and accuracies of the GRACE inter-satellite pointing

For almost 10 years, the Gravity Recovery and Climate Experiment (GRACE) has provided information about the Earth gravity field with unprecedented accuracy. Efforts are ongoing to approach the GRACE baseline accuracy as there still remains an order of magnitude between the present error level of the gravity field solutions and the GRACE baseline. At the current level of accuracy, thorough investigation of sensor related effects is necessary as they are one of the potential contributors to the error budget. In the science mode operations, the twin satellites are kept precisely pointed with their KBR antennas towards each other. It is the task of the onboard attitude and orbit control system (AOCS) to keep the satellites in the required formation. We analyzed long time series of the inter-satellite pointing variations as they reflect the AOCS performance and characteristics. We present significant systematic effects in the inter-satellite pointing and discuss their possible sources. Prominent features are especially related to the magnetic torquer characteristics, star cameras’ performance and KBR antenna calibration parameters. The relation between the magnetic torquer attitude control and the Earth magnetic field, impact of the different performance of the two star camera heads on the attitude control and the features due to uncertainties in the calibration parameters relating the star camera frame to K-frame are discussed in detail. Proper understanding of these effects will help to reduce their impact on the science data and subsequently increase the accuracy of the gravity field solutions. Moreover, understanding the complexity of the onboard system is essential not only for increasing the accuracy of the GRACE data but also for the development of the future gravity field satellite missions.