河蟹生态养殖池浮游植物功能类群的结构特征   总被引：2，自引：0，他引：2  

郝俊  马旭洲  王武  王高龙  王友成  郎月林  李星星 《湖泊科学》2016,28(5):1047-1056

为探究河蟹生态养殖池浮游植物群落的结构及动态,2013年4-10月对上海松江泖港地区河蟹养殖池的浮游植物进行了调查,分析其种类构成、密度、生物量和生物多样性.共检测到浮游植物105种,其中河蟹生态养殖池优势种11种,分别属于B、D、J、LO、X1、WO、X2、S1和MP功能群.河蟹养殖池浮游植物平均密度为282.58×10~4cells/L,平均生物量为1.59 mg/L,Shannon-Wiener多样性指数变化范围为2.11~3.09.水源浮游植物平均密度为175.13×104cells/L,平均生物量为0.99 mg/L,Shannon-Wiener多样性指数变化范围为1.70~2.79.河蟹养殖池浮游植物平均密度分别于5月14日-6月14日和7月29日-8月29日与水源差异显著;河蟹养殖池浮游植物平均生物量分别于5月30日、6月14日和7月29日-8月29日与水源差异显著;河蟹养殖池浮游植物Shannon-Wiener生物多样性指数分别于5月14日-6月14日和8月14日-9月15日与水源差异显著.河蟹养殖池浮游植物的平均密度、平均生物量和生物多样性指数与水源水体变化趋势基本一致,均高于水源,群落稳定.  相似文献   

基于X射线脉冲星的航天器动力学定轨   总被引：1，自引：1，他引：0  

毛悦  宋小勇  贾小林  吴显兵 《武汉大学学报(信息科学版)》2010,(4)

系统研究了利用X射线脉冲星观测量进行动力学定轨的全过程,给出了观测数据仿真的基本流程,并利用仿真数据对各项因素对定轨精度的影响进行了量化分析。  相似文献   

Pulse profile stability of the Crab pulsar     

Chetana Jain  Biswajit Paul 《中国天文和天体物理学报》2011,(10)

We present an X-ray timing analysis of the Crab pulsar, PSR B0531+21, using archival RXTE data. We have investigated the stability of the Crab pulse profile, in soft (2-20 keV) and hard (30-100 keV) X-ray energies, over the last decade of RXTE operation. The analysis includes measurement of the separation between the two pulse peaks and the intensity and widths of the two peaks. We did not find any significant time dependency in the pulse shape. The two peaks have been stable in phase, intensity and width f...  相似文献   

Performance of the MAGIC stereo system obtained with Crab Nebula data     

《Astroparticle Physics》2012,35(7):435-448

MAGIC is a system of two Imaging Atmospheric Cherenkov Telescopes located in the Canary island of La Palma. Since autumn 2009 both telescopes have been working together in stereoscopic mode, providing a significant improvement with respect to the previous single-telescope observations. We use observations of the Crab Nebula taken at low zenith angles to assess the performance of the MAGIC stereo system. The trigger threshold of the MAGIC telescopes is 50 − 60 GeV. Advanced stereo analysis techniques allow MAGIC to achieve a sensitivity as good as (0.76 ± 0.03)% of the Crab Nebula flux in 50 h of observations above 290 GeV. The angular resolution at those energies is better than ∼0.07°. We also perform a detailed study of possible systematic effects which may influence the analysis of the data taken with the MAGIC telescopes.  相似文献   

Marine debris impacts to a tidal fringing-marsh in North Carolina     

Uhrin AV  Schellinger J 《Marine pollution bulletin》2011,62(12):2605-2610

We evaluated injuries to Spartina alterniflora by debris items common to North Carolina coastal waters as a function of debris type (wire blue crab pots, vehicle tires, and anthropogenic wood) and deployment duration, and monitored S. alterniflora recovery following debris removal. Injuries sustained by S. alterniflora and subsequent recovery, varied considerably between debris types. Differences were likely due to dissimilarities in the structure and composition of debris. Tires caused an immediate (within 3 weeks) and long-term impact to S. alterniflora; tire footprints remained devoid of vegetation 14 months post-removal. Conversely, crab pot impacts were not as abrupt and recovery was short-term (<10 months). We suggest that removal programs specifically target habitats that are susceptible to negative impacts (e.g., salt marsh) and prone to debris accumulation. Management would benefit from the inclusion of habitat information in removal databases.  相似文献   

一种新的XPNAV系统解整周模糊度搜索算法     

张华  许录平  谢强 《武汉大学学报(信息科学版)》2011,(1):39-43

提出了一种新的XPNAV系统解相位模糊度搜索算法,依据检核脉冲星的特征周期,将正确的模糊向量限定在多个子空间内,使用线性搜索寻找子空间种子向量,采用基于满二叉树的穷举搜索算法遍历子空间中向量,并建立向量间的关系。理论分析和仿真实验表明,新的解相位模糊度算法能显著降低系统的运算量,计算过程中的矩阵均是由脉冲星特征属性决定的常数矩阵,便于工程实现,此外二叉树搜索方法还能用于并行计算。  相似文献   

大规模脉冲星候选体信号的无监督聚类分析研究          下载免费PDF全文

刘莹  马智  游子毅  王培  党世军  赵汝双  董爱军 《天文学报》2022,63(3):36-138

随着500 m口径球面射电望远镜(Five-hundred-meter Aperture Spherical radio Telescope, FAST)等大型射电望远镜的建设和使用,脉冲星巡天数据进入PB时代.为解决如此大量高速采样的标量数据挖掘问题,促进新天文现象的发现,提出一种基于无监督聚类的脉冲星候选体筛选方案.该方案采用基于密度层次、划分方法的混合聚类算法,结合MapReduce/Spark并行计算模型和基于滑动窗口的分组策略,进而提高大量候选体信号筛选的效率.通过在脉冲星数据集HTRU2 (High Time Resolution Universe)上的对比实验,结果表明该算法能取得较高的精确度和召回率,分别是0.946和0.905,并且当并行节点足够时,该算法的时间复杂度相比串行执行明显下降.可见,该方法为脉冲星观测大数据的分析挖掘提供一种可行思路.  相似文献   

Polarization of the Crab Nebula with disordered magnetic components     

Y. Nakamura  S. Shibata 《Monthly notices of the Royal Astronomical Society》2007,381(4):1489-1498

In this paper, we present an expanding disc model to derive the polarization properties of the Crab nebula. The distribution function of the plasma and the energy density of the magnetic field are prescribed as functions of the distance from the pulsar using the model derived by Kennel and Coroniti with  σ= 0.003  , where σ is the ratio of the Poynting flux to the kinetic energy flux in the bulk motion just before the termination shock. Unlike the case for previous models, we introduce a disordered magnetic field, which is parametrized by the fractional energy density of the disordered component. The flow dynamics are not solved, and the mean field is toroidal.
The averaged degree of polarization over the disc is obtained as a function of the inclination angle and fractional energy density of the disordered magnetic field. It is found for the Crab Nebula that the disordered component contains about 60 per cent of the magnetic field energy. This value is supported by the facts that the disc appears not as 'lip-shaped' but as 'rings' in the observed intensity map, and that the highest degree of polarization of ∼40 per cent is reproduced for rings, which is consistent with the observations.
We suggest that, because the disordered field contributes to the pressure rather than to the tension, the pinch force may have been overestimated in previous relativistic magnetohydrodynamic simulations. The disruption of the oppositely directed magnetic fields, which is proposed by Lyuvarsky, may actually take place. The relativistic flow speed, which is indicated by the front–back contrast, can be detected in the asymmetry of distributions of the position angle and depolarization.  相似文献   

3颗脉冲星自转性质的研究          下载免费PDF全文

胡悦  李琳  党世军  王兆军  袁建平  王榆斌 《天文学报》2020,61(6):63

正常脉冲星和亳秒脉冲星都表现出计时噪声.脉冲星的自转变化是造成计时噪声的重要原因之一.通过联合新疆天文台南山25m射电望远镜和澳大利亚Parkes64m射电望远镜的脉冲到达时间数据，使用脉冲星计时的方法对PSRJ1539--5626、J1832- -0827和J1847- -0402的自转进行分析研究，它们在到达时间残差上均表现出很强的计时噪声，低频噪声的功率谱分别符合谱指数为-6、-6、 -4.5的幂律,功率谱强度分别为1.77 x 10^-17 yr³、4.43x 10^-18 yr³和2.09x 10^-18yr3.这3颗脉冲星在自转频率1阶导数的变化上都表现出较为明显的振荡,振荡幅度分别为0.61(3)x 10^-15s^-2、0.54(5) x 10^-16s^-2和0.11(2) x 10^-15s^-2 (其中括号内数字代表末位数字的有效误差,下同)，自转频率1阶导数振荡变化的相对大小分别是0.75(5)%、0.035(9)%、 0.076(2)%.利用Parkes 64 m射电望远镜的观测数据,分别获得了这3颗脉冲星积分脉冲轮廓及其半高全宽,发现3颗脉冲星的脉冲轮廓的宽度均有一定 的变化，变化幅度分别为0.0028(6)、0.00059(3)和0.00011(4)个相位.没有探测到自转减慢率的变化与辐射变化之间存在明显的相关性.  相似文献   

Relationship between the nonstationary radio-luminosity variations of the Crab Nebula and the activity of its pulsar     

K. S. Stankevich  V. P. Ivanov 《Astronomy Letters》2005,31(2):98-102

Based on the half-century-long history of radio observations of the Crab Nebula, we investigate the evolution of its radio luminosity. We found a secular decrease in the radio luminosity; it has decreased by 9% since the discovery of the radio source in 1948. Apart from the secular decrease in the luminosity of the Crab Nebula, we identified two time intervals, 1981–1987 and 1992–1998, when radio bursts with energy release ～10⁴¹ erg took place. In these years, the spectral indices of the instantaneous spectra decreased significantly due to the increase in the flux densities at short (centimeter and millimeter) wavelengths. These events were preceded by sudden increases in the pulsar’s rotation rates, the largest of which, with an amplitude of ΔΩ/Ω = 3 × 10^?8, occurred in 1975 and 1989. We show that the magnetospheric instability mechanism that accompanies strong glitches can provide the energetics of the excess luminosity of the Nebula through the ejection of relativistic electrons with a total energy higher than 6 × 10⁴² erg from the pulsar’s magnetosphere.  相似文献