小目标激光测距仪最佳发散角选取的改进方法     

胡冰  熊耀恒 《天文研究与技术》2008,5(1):21-24

讨论了激光测距仪测距本领的限制因素,找出了诸多限制因素的内在联系,得出了影响测距能力的关键因素是目标的照明概率.在前人已有的对激光测距仪最佳发散角的选取方法基础上,对其进行了改进.新得到的方法在选取最佳发散角时具有快速、简洁和精确度高等特点,并利用多项式拟合对其进行了简化,使其可用于粗略估算.最后,利用数值模拟对新方法进行了验证分析.  相似文献   

Determination of the Topology Skeleton of Magnetic Fields in a Solar Active Region     

Hui Zhao  Jing-Xiu Wang  Jun Zhang  Chi-Jie Xiao  Hai-Min Wang 《中国天文和天体物理学报》2008,8(2):133-145

Magnetic topology has been a key to the understanding of magnetic energy re-lease mechanism. Based on observed vector magnetograms, we have determined the three-dimensional (3D) topology skeleton of the magnetic fields in the active region NOAA 10720.The skeleton consists of six 3D magnetic nulls and a network of corresponding spines, fans,and null-null lines. For the first time, we have identified a spiral magnetic null in Sun's corona.The magnetic lines of force twisted around the spine of the null, forming a 'magnetic wreath'with excess of free magnetic energy and resembling observed brightening structures at extra-ultraviolet (EUV) wavebands. We found clear evidence of topology eruptions which are re-ferred to as catastrophic changes of topology skeleton associated with a coronal mass ejection(CME) and an explosive X-ray flare. These results shed new lights on the structural complex-ity and its role in explosive magnetic activity. The concept of flux rope has been widely used in modelling explosive magnetic activity, although their observational identity is rather ob-scure or, at least, lacking of necessary details up to date. We suggest that the magnetic wreath associated with the 3D spiral null is likely an important class of the physical entity of flux ropes.  相似文献   

DWT Power Spectrum of the Two-Degree Field Galaxy Redshift Survey     

Yan-Chuan Cai  Jun Pan  Yong-Heng Zhao  Long-Long Feng  Li-Zhi Fang 《中国天文和天体物理学报》2008,8(2):159-178

The power spectrum of the two-degree Field Galaxy Redshift Survey (2dFGRS) sample is estimated with the discrete wavelet transform (DWT) method. The DWT power spectra within 0.035 < k < 2.2 h Mpc-1 are measured for three volume-limited samples de-fined in consecutive absolute magnitude bins -19 ～-18, -20～ -19 and -21～-20. We show that the DWT power spectrum can effectively distinguish ACDM models of σs = 0.84 and σ8 = 0.74. We adopt maximum likelihood method to perform three-parameter fitting of the bias parameter b, pairwise velocity dispersion σpv and redshift distortion parame-ter β = Ω0.6m/b to the measured DWT power spectrum. The fitting results state that in a σ8= 0.84 universe the best-fit values of σm given by the three samples are mutually consis-tent within the range 0.28 ～ 0.36, and the best fitted values of σpv are 398 35-27475 37-29 and 550 ± 20km s-1 for the three samples, respectively. In the model of σ8 = 0.74, our three samples give very different values of Ωm. We repeated the fitting using the empirical formula of redshift distortion. The result of the model of low σ8 is still poor, especially, one of the best-fit values of σpv is as large as 103 km s-1. We also repeated our fitting by incorporating a scale-dependent galaxy bias. This gave a slightly lower value of Ωm. Differences between the models of σ8 = 0.84 and σ8 = 0.74 still exist in the fitting results. The power spectrum of 2dFGRS seems to disfavor models with low amplitude of density fluctuations if the bias parameter is assumed to be scale independent. For the fitting value of Ωm to be consistent with that given by WMAP3, strong scale dependence of the bias parameters is needed.  相似文献   

Are Seyfert 2 Galaxies without Polarized Broad Emission Lines More Obscured？     

Xin-Wen Shu  Jun-Xian Wang  Peng Jiang 《中国天文和天体物理学报》2008,8(2):204-210

New XMM-Newton data of seven Seyfert 2 galaxies with optical spectropolarimet- ric observations are presented. An analysis of the 0.5 - 10 keV spectra shows that four Seyfert 2 galaxies with polarized broad lines (PBLs) are absorbed by NH < 1024 cm-2, while two of three Seyfert 2 galaxies without PBLs show evidence of Compton-thick obscuration, support- ing the conclusion that Seyfert 2 galaxies without PBLs are more obscured than those with PBLs. Adding the measured obscuration indicators (NH, T ratio, and Fe Kα line EW) of six luminous AGNs to our previous sample improves the significance level of the difference in absorption from 92.3% to 96.3% for NH, 99.1% to 99.4% for T ratio, and 95.3% to 97.4% for Fe Kα line EW. The present results support and enhance the suggestions that the absence of PBLs in Seyfert 2 galaxies can be explained by larger viewing angles of the line of sight to the putative dusty torus, which leads to the obscuration of the broad-line scattering screen, as expected in the unification model.  相似文献   

Clustering Property of Wolf-Rayet Galaxies in the SDSS     

Wei Zhang  Xu Kong  Fu-Zhen Cheng 《中国天文和天体物理学报》2008,8(2):211-218

We have analysed, for the first time, the clustering properties of Wolf-Rayet (W-R) galaxies, using a large sample of 846 W-R galaxies selected from the Data Release 4 (DR4) of the Sloan Digital Sky Survey (SDSS). We compute the cross-correlation function between W- R galaxies and a reference sample of galaxies drawn from the DR4. We compare the function to the results for control samples of non-W-R star-forming galaxies that are matched closely in redshift, luminosity, concentration, 4000-A break strength and specific star formation rate (SSFR). On scales larger than a few Mpc, W-R galaxies have almost the same clustering amplitude as the control samples, indicating that W-R galaxies and non-W-R control galax- ies populate dark matter haloes of similar masses. On scales between 0.1-1 h-1 Mpc, W-R galaxies are less clustered than the control samples, and the size of the difference depends on the SSFR. Based on both observational and theoretical considerations, we speculate that this negative bias can be interpreted by W-R galaxies residing preferentially at the centers of their dark matter haloes. We examine the distribution of W-R galaxies more closely using the SDSS galaxy group catalogue of Yang et al., and find that ～82% of our W-R galaxies are the central galaxies of groups, compared to ～74% for the corresponding control galaxies. We find that W-R galaxies are hosted, on average, by dark matter haloes of masses of 1012,3 M☉, compared to 1012,1 M? For centrally-located W-R galaxies and 1012,7 M☉ For satellite ones. We would like to point out that this finding, which provides a direct observational support to our conjecture, is really very crude due to the small number of W-R galaxies and the incom- pleteness of the group catalogue, and needs more work in future with larger samples.  相似文献   

Distribution of 56Ni Yields of Type Ia Supernovae and its Implication for Progenitors     

Bo Wang  Xiang-Cun Meng  Xiao-Feng Wang  Zhan-Wen Han 《中国天文和天体物理学报》2008,8(1):71-80

The amount of 56Ni produced in Type Ia supernova (SN Ia) explosion is probably the most important physical parameter underlying the observed correlation of SN Ia lumi-nosities with their light curves. Based on an empirical relation between the 56Ni mass and the light curve parameter △m15, we obtained rough estimates of the 56Ni mass for a large sample of nearby SNe Ia with the aim of exploring the diversity in SN Ia. We found that the derived 56Ni masses for different SNe Ia could vary by a factor of ten (e.g., MNi = 0.1 - 1.3 M⊙),which cannot be explained in terms of the standard Chandraseldaar-mass model (with a 56Ni mass production of 0.4 - 0.8 M⊙). Different explosion and/or progenitor models are clearly required for various SNe Ia, in particular, for those extremely nickel-poor and nickel-rich producers. The nickel-rich (with MNi 0.8 M⊙) SNe Ia are very luminous and may have massive progenitors exceeding the Chandrasekhar-mass limit since extra progenitor fuel is required to produce more 56Ni to power the light curve. This is also consistent with the find-ing that the intrinsically bright SNe Ia prefer to occur in stellar environments of young and massive stars. For example, 75% SNe Ia in spirals have △m15 < 1.2 while this ratio is only 18% in E/S0 galaxies. The nickel-poor SNe Ia (with MNi < 0.2 M⊙) may invoke the sub-Chandrasekhar model, as most of them were found in early-type E/S0 galaxies dominated by the older and low-mass stellar populations. This indicates that SNe Ia in spiral and E/S0 galaxies have progenitors of different properties.  相似文献   

An Approach of Tropospheric Correction for VLBI Phase-Referencing using GPS Data     

Bo Zhang  Xing-Wu Zheng  Jin-Ling Li  Ye Xu 《中国天文和天体物理学报》2008,8(1):127-132

The dominant source of error in VLBI phase-referencing is the troposphere at observing frequencies above 5 GHz. We compare the tropospheric zenith delays derived from VLBI and GPS data at VLBA stations collocated with GPS antennas. The systematic biases and standard deviations both are at the level of sub-centimeter. Based on this agreement, we suggest a new method of tropospheric correction in phase-referencing using combined VLBI and GPS data.  相似文献   

基于小波变换的脉冲星弱信号的去噪方法研究   总被引：1，自引：0，他引：1  

汪丽  柯熙政  倪广仁 《天文研究与技术》2008,5(1):49-54

利用小波变换去除噪声的基本原理和方法及消色散方法对毫秒脉冲星极弱信号进行了提取,并对噪声进行了最佳阈值选取实验研究,实现了脉冲星弱信号消色散积累后的检测和抑噪.并研究了软硬阈值估计子等不同阈值消噪技术,进而对PSR 0437-4715信号采用不同方法去噪后的结果进行了分析比较,认为在最大分解层数J=5时对脉冲星信号处理有普适意义.研究结果认为基于小波变换的消噪方法是一种提高信噪比、展示噪声和突变信号的优越方法.  相似文献   

Possible Contribution of Mature γ-ray Pulsars to Cosmic-ray Positrons     

Quan-Gui Gao  Ze-Jun Jiang  Li Zhang 《中国天文和天体物理学报》2008,8(1)

We restudy the possible contribution of mature gamma-ray pulsars to cosmic ray positrons based on the new version of outer gap model. In this model, the inclination angle and average properties of the outer gap are taken into account, and more mature pulsars can have the outer gap and emit high energy photons. Half of the primary particles in the outer gaps will flow back toward the star surface and emit synchrotron photons, which can produce electron/positron pairs by the cascade of pair production. Some of these pairs will escape from the light cylinder and be accelerated to relativistic energies in the pulsar wind driven by low-frequency electromagnetic waves. Using a Monte Carlo method, we obtain a sample of mature gamma-ray pulsars and then calculate the production of the positrons from these pulsars. The observed excess of cosmic positrons can be well explained by this model.  相似文献   

SPOT 5卫星遥感影像在土地利用数据库系统更新中的应用     

刘天光  曹培国  孔琴 《山东国土资源》2007,23(6):2007/5/16-2007/6/25

随着国民经济的快速发展，土地现状变更十分频繁，如何保持数据库系统的现势性成为国土资源管理部门的一项重要工作。该文介绍了利用卫星遥感数据对土地利用数据库变更的系统设计、实现方法及结果，说明了利用遥感卫星数据对数据库进行更新的优势。  相似文献