Millisecond Pulsar PSR 1937+21: A Highly Stable Clock

The stable rotation and sharp radio pulses of PSR 1937+21 make this pulsar a clock whose long-term frequency stability approaches and may exceed that of the best atomic clocks. Improvements in measurement techniques now permit pulse arrival times to be determined in 1 hour at the Arecibo radio telescope with uncertainties of about 300 nanoseconds relative to atomic time. Measurements taken approximately every 2 weeks since November 1982 yield estimates of fractional frequency stability that continue to improve with increasing averaging time. The pulsar's frequency stability is at least as good as 6 x 10(-14) for averaging times longer than 4 months, and over the longest intervals the measurements appear to be limited by the stability of the reference atomic docks. The data yield a firm upper limit of 7 x 10(-36) gram per cubic centimeter for the energy density of a cosmic background of gravitational radiation at frequencies of about 0.23 cycle per year. This limit corresponds to approximately 4 x 10(-7) of the density required to close the universe.     

A pulsar, the heliosphere, and pioneer 10: probable mimicking of a planet of PSR B1257+12 by solar rotation

Doppler data generated with the Pioneer 10 spacecraft's radio carrier wave between 1987 and 1995 show a 25.3-day periodicity which is related to the solar rotation. The timing data of the pulsar PSR B1257+12 also show a periodicity of 25.34 days, which has been explained as a signature of the pulsar's barycentric motion in response to the existence of a small moon-like object. However, because PSR B1257+12 is located close to the ecliptic and because the timing variations are in the range of microseconds, it is likely that the pulsar signal is affected by the same mechanism acting on the Pioneer 10 Doppler data. Hence, the hypothesized inner planet around PSR B1257+12 is probably an artifact of the heliosphere.     

Binary Pulsar PSR 1913 + 16: Model for Its Origin

The existing observational data for the binary pulsar PSR 1913 + 16 are sufficient to give a rather well-defined model for the system. On the basis of evolutionary considerations, the pulsar must be a neutron star near the upper mass limit of 1.2 solar masses (M.). The orbital inclination is probably high, i>/= 700, and the mass of the unseen companion probably lies close to the upper limit of the range 0.25 M. to 1.0 M.. The secondary cannot be a main sequence star and is probably a degenerate helium dwarf. At the 5.6-kiloparsec distance indicated by the dispersion measure, the magnetic dipole model gives an age of approximately 4 x 104 years, a rate of change of the pulsar period of P approximately 2 nanoseconds per day, and a surface magnetic field strength approximately (1/3) that of the Crab pulsar. The pulsar is fainter than an apparent magnitude V approximately + 26.5 and is at least approximately 80 times fainter than the Crab pulsar in the x-ray band. The companion star should be fainter than V approximately + 30, and a radio supernova remnant may be detectable near the position of the pulsar at a flux level of 相似文献   

基于外间隙模型的脉冲星高能辐射研究——以类Crab脉冲星PSR J2229+6114为例

脉冲星物理因脉冲星具有极高的密度、极强的磁场强度等极端物理环境而成为天体物理研究的热点.在两极辐射外间隙模型的基础上,对模型进行了如下考虑:①就对产生过程对脉冲星外间隙厚度因子的限制进行了深入分析;②考虑了脉冲星磁倾角对加速带电粒子运动的影响,计算了带电粒子投射角;③采用了新的计算伽玛射线辐射立体角的方法.在此基础上,将改进的模型运用到Fermi LAT新观测到的类Crab脉冲星(Crab-like Pul-sar):PSR J2229+6114,计算结果表明改进的外间隙模型能够较好的解释这颗脉冲星观测到的能谱和光变曲线.     

uc.12+A在B细胞淋巴瘤中的功能研究

从C57BL/6野生型小鼠骨髓中分选出正常B细胞,应用实时荧光定量PCR (qRT-PCR)检测uc.12+A在小鼠正常B细胞和B淋巴瘤细胞的表达水平.通过分子克隆将uc.12+A基因序列构建到pMSCV-PIG反转录病毒表达载体上,包装成反转录病毒感染小鼠B淋巴瘤细胞38B9和人Burkitt淋巴瘤细胞Romas,经...     

Confirmation of the Crystal Structure of Tetrahedrite, Cu12Sb4S13

The crystal structure of tetrahedrite, proposed in 1934 by Pauling and Neuman, has been confirmed. Least-squares refinement reduced the initial disagreement factor from 27.9 to 4.9 percent. Antimony displays three short orthogonal bonds to sulfur atoms. Two types of copper are tetrahedrally and triangularly coordinated by sulfur, respectively.     

基于压力-状态-响应模型的吐鲁番地区生态安全动态评价

【目的】研究区域生态安全在不同时期的动态变化。【方法】在国内外已有研究基础上,基于压力-状态-响应(PSR)概念模型,选取26项指标,构建吐鲁番地区生态安全评价指标体系。利用层次分析法确定各项指标的权重,采用综合指数评价法从时间尺度上(2000～2009年)对历史文化资源丰富的吐鲁番地区生态安全进行定量评估。【结果】吐鲁番地区生态安全综合指数从2000年0.381增长到2009年的0.512,研究时间尺度内的生态安全综合指数变化趋势方程为Y_3=0.016 X+0.346 4(通过显著性检验,R~2=0.956 6),生态安全整体水平呈上升趋势,2000～2005年为临界安全状态,2006～2009年为较安全状态。从三个子系统看,对吐鲁番地区生态安全的贡献依次为:系统压力>系统响应>系统状态。【结论】依据评价结果及吐鲁番地区现状,为吐鲁番地区的生态建设及管理提供科学依据。     

试论物权确认

确认物权既是物权保护的前提和起点 ,又是物权保护的方法之一 ,对物权保护具有极其重要的意义。确认物权的请求权不是一项独立的物权请求权和民事实体权利 ,而是一种裁判请求权 ,但这并不妨碍其作为一种独立的物权保护方式。物权确认应当通过行政程序和诉讼程序 (包括民事诉讼程序和行政诉讼程序 )进行 ,但不应通过仲裁程序进行。物权确认所应遵循的基本规则是诉讼法上的证据规则和民法上的权利推定规则 ,前者不应在民法上规定 ,后者应在民法上予以明确规定     

Defect in vitamin B12 release from lysosomes: newly described inborn error of vitamin B12 metabolism

Cultured diploid fibroblasts from a patient with a previously undescribed inborn error of cobalamin metabolism accumulate unmetabolized, nonprotein-bound vitamin B12 in lysosomes. These cells are able to endocytose the transcobalamin II-B12 complex and to release B12 from transcobalamin II. The freed vitamin B12 is not released from lysosomes into the cytoplasm of the cell. This suggests that there is a specific lysosomal transport mechanism for vitamin B12 in the human.     

Pulsar test of a variation of the speed of light with frequency

The sharply defined optical and radio pulses from pulsars make possible a test of the variation of the speed of light with frequency, and of the possible existence of a photon mass. The data indicate that the mass of a real photon is less than 10(-44) gram. Detection of extragalactic pulsars could allow a substantial improvement of this limit.     

饲粮氰钴素(VB_(12))及叶酸(VB_(11))水平对蛋鸡生产性能及血液生化指标的影响

选择 33周龄健康的海兰褐壳蛋鸡 12 0只,随机分成 3个处理(即VB12 及VB11添加水平分别为:A组:0,0;B组:0 0 1, 5;C组:0 10,5 0 0mg/kg)。每个处理 4个重复,每个重复 10只鸡,4周的试验结果表明:叶酸及氰钴素的水平对蛋鸡生产性能及蛋壳质量无显著影响(P >0 0 5 ),但对蛋鸡的血红蛋白含量有影响,C组与A组相比,差异显著(P <0 0 5)。     

M12B型冲压机连杆机构设计优化

本文对M12B型冲压机的机构尺寸进行了优化.在满足冲压机工作要求的情况下,使其四杆机构的最小传动角大于45°并保证其机构运转良好及效率高.     

基于PSR模型的旅游生态系统健康评价——以南京玄武湖风景区为例

针对旅游生态系统特点,以生态系统健康理论为基础,以"压力—状态—响应"(PSR)为指标框架,结合信息熵理论、专家评判系统与模糊综合评价方法,建立了旅游生态系统健康评价模型,将生态系统健康程度划分为健康、较健康、亚健康、不健康和病态5个级别,作为评价依据。实证研究以玄武湖风景区为评价单元,提取其压力、状态、响应指标信息,同时整合社会经济统计、问卷调查等多源数据,通过指标权重分析,识别出游客量、游客增长率等影响玄武湖生态系统健康的主要因子,并对玄武湖风景区生态系统健康状况进行分析、评价。研究结果显示,2007～2009年玄武湖风景区生态系统的综合健康度分别为0.60、0.61、0.63。可见,该旅游生态系统已由"亚健康"过渡为"较健康",综合健康指数正逐步提高。     

维生素B1和B12对米氏凯伦藻增殖的影响

[目的]研究维生素B1、B12在米氏凯伦藻有毒赤潮爆发增殖过程中的作用。[方法]设置7个不同浓度的维生素B1与B12的K培养基培养米氏凯伦藻,通过定期浮游计数框计数与活体叶绿素荧光值的测定,研究不同浓度维生素B1、B12下,米氏凯伦藻细胞密度与叶绿素荧光值的变化情况。[结果]在维生素B1浓度低于0.312 5×10-5g/L时,对米氏凯伦藻细胞密度与单位细胞叶绿素荧光值有显著影响（P〈0.05）;维生素B12浓度变化对米氏凯伦藻细胞密度与单位细胞叶绿素荧光值影响不显著（P〉0.05）。[结论]维生素B1对米氏凯伦藻细胞增殖有显著影响,维生素B12对米氏凯伦藻种群增殖的影响不显著。     

基于PSR模型的艾比湖流域生态安全评价研究

基于压力-状态-响应(PSR)概念模型,结合层次分析法并采用综合评价模型对艾比湖流域生态安全进行综合评价。研究结果表明:(1)在13项评价指标中,影响流域生态安全的主导因素为森林覆盖率、草地面积和水土流失治理面积,其所占权重均在0.1以上;(2)自2000—2010年以来,艾比湖流域生态安全指数由0.230 9增加至0.755 6,流域生态环境呈现良性发展趋势,生态安全状况有所好转。当地政府通过提高森林覆盖率、增加草地面积以及治理水土流失等措施,不断改善了该流域的自然生态环境。