Lunar seismic search for strange quark matter

It was pointed out in 1984 by Witten that strange quark matter (SQM) – matter made of up, down, and strange quarks (rather than just up and down, as are protons and neutrons) – might well be stable and the lowest energy state of matter. The reason is that it would be electrically neutral and have less Pauli-Principle repulsion. Binding would increase with numbers of quarks, and might not begin below thousands. It would have nuclear density. Neutron stars would be strange quark stars; and it might conceivably constitute dark matter.One way to detect ton-range SQM nuggets (SQNs) would be from seismic signals they would make passing through the Earth. We give a rough estimate on the relative advantage of attempting to detect SQNs on the Moon over Earth (about 50 times more detections).     

一种基于三正交主轴平面的快速星图模拟方法

为在地面上进行星图模拟以测试星图识别算法和星敏感器性能, 提出了一种新的快速星图模拟方法. 该方法利用截面圆和固定区域的性质, 将单位天球按照三个正交主轴平面进行等分, 根据赤纬值求解恒星矢量与主轴平面夹角来确定选星条件, 进而根据坐标变换原理进行星图模拟. 为验证所提方法与现有方法相比的优势, 进行了大量的仿真实验. 实验结果表明, 与现有星图模拟方法相比, 本文方法在保证星图模拟精度的条件下, 选星速度提高了3倍以上.      

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.     

Low-mass bare strange stars

Strange stars with low masses are suggested to exist in reality, the origin of which could be via accretion-induced collapse of white dwarfs. Such a strange star is likely bare, and would thus spin very fast, even to a period of <0.1 ms. Strange stars with low masses may differ from those with solar masses in various astrophysical appearances. Observations to test this “low-mass” idea are proposed.     

A star identification algorithm based on radial and dynamic cyclic features of star pattern

A full-sky star identification algorithm based on radial and dynamic cyclic patterns is presented with the aim of solving the “lost-in-space” problem. The dynamic cyclic pattern match is applied with a maximum cumulate comparison method to identify sensor-catalog pairings in initial match, which substantially eliminates the effects of the star position noise, magnitude noise, and false stars. After initial match pairings of stars are obtained, a chain part extension technique is employed to quickly search for the longest match chain as the final result. Experimental results indicate that the proposed algorithm is highly robust to star position noise, magnitude noise and false stars. In a series of simulations, the identification rate of the algorithm is 97.50% with 2.0 pixels star position noise, 96.90% with 0.4 Mv star magnitude noise and 95.30% with four false stars respectively. Moreover, the algorithm achieves an identification rate of 58.08% when only six stars are in the field of view.     

星敏感器导航星表建立

导航星表是星敏感器系统用来实现恒星星图识别和姿态确定的唯一依据,它的容量、内容、存储读取方式对于完成星敏感器功能和性能指标都极为重要．通过分析恒星在视场中的分布规律,建立起星表的完备性和冗余性与星敏感器视场内导航星的数量之间的关系,确定了导航星数量,提出了筛选导航星的原则和方法,为了实现星表的快速搜索,对导航星表的组织形式进行了详细讨论．构造一个由2510颗0～6等导航星组成的导航星表,满足了星敏感器星图识别和姿态确定的需求．     

高精度高动态星模拟器研究

给出了高精度高动态星模拟器系统组成和总体结构, 提出一种基于亚像素显示技术提高动态星模拟器星点位置显示精度从而减小星点几何中心与能量中心位置偏差的新方法. 针对动态星模拟器高动态性要求, 提出一种快速精确检索全天球导航星的新方法, 将已知SAO (Smithsonian Astrophysical Observatory) 星表进行分区, 计算当前姿态四元数并分析航天器视轴所在区域, 对于距离视轴较远区域停止搜索和匹配, 很大程度上缩小了导航星搜索范围, 进而缩短了全天球恒星遍历所需时间. 软件测试结果表明, 此快速检索法可使全天球导航星的选取速度提高近10倍.      

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.     

Measurement of neutron star parameters: A review of methods for low-mass X-ray binaries

Measurement of at least three independent parameters, for example, mass, radius and spin frequency, of a neutron star is probably the only way to understand the nature of its supranuclear core matter. Such a measurement is extremely difficult because of various systematic uncertainties. The lack of knowledge of several system parameter values gives rise to such systematics. Low mass X-ray binaries, which contain neutron stars, provide a number of methods to constrain the stellar parameters. Joint application of these methods has a great potential to significantly reduce the systematic uncertainties, and hence to measure three independent neutron star parameters accurately. Here, we review the methods based on: (1) thermonuclear X-ray bursts; (2) accretion-powered millisecond-period pulsations; (3) kilohertz quasi-periodic oscillations; (4) broad relativistic iron lines; (5) quiescent emissions; and (6) binary orbital motions.     

A robust star identification algorithm with star shortlisting

A star tracker provides the most accurate attitude solution in terms of arc seconds compared to the other existing attitude sensors. When no prior attitude information is available, it operates in “Lost-In-Space (LIS)” mode. Star pattern recognition, also known as star identification algorithm, forms the most crucial part of a star tracker in the LIS mode. Recognition reliability and speed are the two most important parameters of a star pattern recognition technique. In this paper, a novel star identification algorithm with star ID shortlisting is proposed. Firstly, the star IDs are shortlisted based on worst-case patch mismatch, and later stars are identified in the image by an initial match confirmed with a running sequential angular match technique. The proposed idea is tested on 16,200 simulated star images having magnitude uncertainty, noise stars, positional deviation, and varying size of the field of view. The proposed idea is also benchmarked with the state-of-the-art star pattern recognition techniques. Finally, the real-time performance of the proposed technique is tested on the 3104 real star images captured by a star tracker SST-20S currently mounted on a satellite. The proposed technique can achieve an identification accuracy of 98% and takes only 8.2?ms for identification on real images. Simulation and real-time results depict that the proposed technique is highly robust and achieves a high speed of identification suitable for actual space applications.     

星敏感器中快速星跟踪技术

跟踪模式是星敏感器的主要工作模式之一,跟踪过程的快速性直接影响星敏感器的整体性能.提出了一种快速星跟踪算法.在跟踪算法的3个耗时环节,分别采用了分区星表、阈值映射、先排序后匹配识别这3种方法,以提高跟踪过程的快速性.其中分区星表法将整个天区分成了若干个子天区,使得在星体映射时,只是搜索星敏感器视轴指向附近的部分子天区,而不是搜索整个天区,减少了搜索星体的数量;阈值映射法,在满足精确度的情况下,设置被跟踪星体的数量阈值,只有被跟踪的星体数目少于此阈值时,才进行星体映射,减少了映射次数;先排序后匹配识别法,先根据星体在星图中的坐标值进行排序,然后再进行匹配识别,减少了那些距离较大的无谓的星体间的匹配识别.仿真测试结果表明,这3种方法的采用,提高了跟踪算法的快速性,提高了星敏感器的整体效能.      

Phase resolved study of the CRSF in MX 0656-072

MXB 0656-072 is an accreting X-ray pulsar with a Be star companion, showing notable emission in H. In October 2003 this system exhibited a large and extended X-ray outburst. RXTE observations during this outburst indicated a pulse period of 160.4 s and a cyclotron resonance scattering feature in the spectrum at 32 keV. This paper presents pulse profile analysis and phase-resolved X-ray spectroscopy of RXTE observations during this outburst.     

ROSAT observations of the Chamaeleon star forming cloud

Two soft X-ray images of the Chamaeleon I star forming cloud obtained with the ROSAT Position Sensitive Proportional Counter are presented. Seventy reliable, and perhaps 19 additional, X-ray sources are found. Up to Ninety percent of these sources are certainly or probably identified with T Tauri stars formed in the cloud. Twenty to 35 are probably previously unrecognized ‘weak’ T Tauri (WTT) stars. T Tauri X-ray luminosities range from log , or 10² – 10⁴ times solar levels, with mean in the 0.2–2.5 keV band. The X-ray luminosities of well-studied Chamaeleon cloud members are correlated with a complex of four stellar properties: effective temperature, mass, radius and bolometric luminosity. The spatial distribution, H-R diagram locations of the stars indicate WTT and CTT are coeval. The total premain sequence population of the cloud is likely to be > 100 stars, with WTT stars outnumbering ‘classical’ T Tauri (CTT) stars by 2:1.     

Relative fluxes of shock and prompt peaks in SEP event time profiles

Peak fluxes are an important property of gradual solar energetic particle (SEP) event time profiles from both astro/heliophysical and applications perspectives. However, the peak flux in an event may occur at the event onset, or at the time of the interplanetary shock arrival (the ESP or energetic storm particles). This makes an important difference in the interpretation of the peak flux, and in any attempts to characterize or model it. This paper describes a study of SEP data sets from ACE, IMP-8 and GOES toward determining the relative properties of these peak fluxes for protons with energies near 1, 10, and 50 MeV. The results suggest that for gradual events with both peaks, the ESP peak often dominates at 1 MeV energies and is dominant about half the time at 10 MeV. Moreover, the prompt peak fluxes can be used to estimate the shock peak (ESP event) up to days ahead, especially in the lower energy range.     

A triangle voting algorithm based on double feature constraints for star sensors

A novel autonomous star identification algorithm is presented in this study. In the proposed algorithm, each sensor star constructs multi-triangle with its bright neighbor stars and obtains its candidates by triangle voting process, in which the triangle is considered as the basic voting element. In order to accelerate the speed of this algorithm and reduce the required memory for star database, feature extraction is carried out to reduce the dimension of triangles and each triangle is described by its base and height. During the identification period, the voting scheme based on double feature constraints is proposed to implement triangle voting. This scheme guarantees that only the catalog star satisfying two features can vote for the sensor star, which improves the robustness towards false stars. The simulation and real star image test demonstrate that compared with the other two algorithms, the proposed algorithm is more robust towards position noise, magnitude noise and false stars.     

太阳爆发抵近探测——“触碰计划”

本文旨在介绍一项具有重大科学意义和应用价值的深空探测任务构想.该任务将对驱动恒星大尺度爆发过程的中心结构(即磁重联电流片)进行抵近(原位)探测,主要目的是详细研究发生在离地球最近的恒星—太阳上的大尺度磁重联过程的精细物理特征,揭示太阳系中最为剧烈的能量释放过程(即太阳爆发或太阳风暴)的奥秘.该任务的科学目标:磁重联过程...     

一种基于恒星分布的星敏感器导航星库制作方法

为了进一步提升星敏感器的定姿精度,分析了恒星的空间几何分布对姿态精度的影响,提出了一种基于恒星分布的星敏感器导航星库制作方法。在分析对比了现有星库的基础上,选择了精度较高的依巴谷星库作为基础星库,并剔除了暗星和双星以精简星库。遍历全天球对每个视场的星库中的空间几何分布进行分析对比,删除定姿较差的3颗星。对星库进行遍历补偿,保证了星库中恒星分布的均匀性。实验结果表明,全天球轨道测试中,本文方法制作星库的全天识别率为100%,且星颗分布数10颗以上天区占到97.64%,具有良好的覆盖性,同时有效提升了姿态精度。     

Progress Report on Insight-HXMT: China's First X-ray Astronomy Satellite

Insight-HXMT is China's first X-ray astronomy satellite. It was launched on 15 June 2017 and is currently in service smoothly. Insight-HXMT has been used to scan the Galactic plane repeatedly, making pointing observations to neutron stars and black holes, and monitor the whole sky continuously in the MeV band. Insight-HXMT is also very flexible in making ToO observations, with the response time from about 3 hours to within a day. So far more than 50 refereed publications have been made with data from its observations; many more publications have used the data or results of Insight-HXMT one way or another. The scientific impacts of Insight-HXMT have been growing rapidly since launch. We expect Insight-HXMT to continue to operate for several more years.      

依巴谷卫星扫描观测不均匀性的研究

以观测时间T=0.5年,四种不同的旋转角ξ=30°、36°、43°和50°,计算了依巴谷卫星对50颗全天均匀分布的恒星模拟观测次数。发现随着黄纬的增加,观测次数逐渐增加。当|β|=40°～60°时,观测次数分布达极大。而|β|=0°～10°之间,观测次数约只有极大值的1/2。计算符合Hoyer等人的理论估计,即在|β|=90°—ξ附近观测次数最多,在黄道面附近的恒星观测次数最少。本文指出依巴谷卫星对|β|为50°～60°区域内的恒星观测精度高。原因是在这一区域内,依巴谷扫描观测的次数最多。      

High energy emission from pulsars: Outer gap scenario

I will give a brief review of the recent development in the emission models of isolated, rapidly rotating neutron stars, focusing on the γ-ray radiation mechanism in their outer magnetospheres. By examining the Poisson equation for the electrostatic potential, I show that an active particle accelerator must extend from the vicinity of the neutron star surface to the vicinity of light cylinder. Furthermore, combining the Poisson equation with the Boltzmann equations for electrons/positrons and γ-rays, and assuming that the gap trans-field thickness is large compared to the longitudinal width, I demonstrate that the energy distribution of ultra-relativistic particles cannot be described by a power-law but by a quasi-monoenergetic distribution at the terminal Lorentz factor. The particles are accelerated in the gap and escape from it with large Lorentz factors. Is is shown that such energetic particles migrating outside of the gap contribute significantly to the γ-ray luminosity and reproduce the observed soft γ-ray spectrum between 100 MeV and 3 GeV for the Vela pulsar.