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   

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.     

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.     

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。     

饲粮氰钴素(VB12)及叶酸(VB11)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)。     

基于PSR模型的江西省生态文明建设评价

为科学评价江西省生态文明建设状况,促进其生态文明建设科学发展,基于“压力-状态-响应”模型构建了3个子系统共24个指标的生态文明建设评价指标体系,对江西省11个地市2012年生态文明建设状况进行了定量分析。结果显示：南昌市生态文明建设综合评价位居11个地市的第一位,鹰潭、新余、景德镇、赣州、抚州的这一指标高于全省平均水平。南昌、鹰潭和新余的生态系统压力较大,其中鹰潭在响应系统方面相对最为薄弱;上饶、抚州、萍乡和宜春的生态系统压力较小,其中抚州的生态系统健康状态和生态管理控制能力相对较强,而萍乡和宜春的生态文明建设状态系统比较薄弱,尚存在较大的改善空间。     

The Morphogenesis of Bands and Zonal Winds in the Atmospheres on the Giant Outer Planets

The atmospheres of Jupiter, Saturn, Uranus, and Neptune were modeled as shallow layers of turbulent fluid overlying a smooth, spherical interior. With only the observed values of radius, rotation rate, average wind velocity, and mean layer thickness as model parameters, bands and jets spontaneously emerged from random initial conditions. The number, width, and amplitude of the jets, as well as the dominance of anticyclonic vortices, are in good agreement with observations for all four planets.     

维生素B12添加剂测定方法之比较分析

维生素B12粉剂是饲料生产中常用的维生素B12添加剂.本品是以维生素B12为原料,加入玉米淀粉或碳酸氢钙等稀释剂制成的粉剂,常见的3种产品分别含有1%、5%、10%维生素B12.维生素B12又称氰钴胺,因其含有元素钴,其粉剂为浅红色至棕色粉末,市售1%粉剂为浅红色,因其价格较高,该原料常有掺假现象.     

基于PSR模型的岳阳市土地可持续利用评价

选取湖南省岳阳市为研究对象,利用PSR模型构建岳阳市土地可持续利用评价指标体系,在对指标值进行标准化的基础上,采用改进熵值法确定指标权重,最后计算岳阳市土地可持续利用程度。结果表明,岳阳市土地可持续利用水平在2005-2014年总体处于较高层次,但中间年份由于自然原因和社会经济原因,导致岳阳市土地可持续利用水平出现较小波动;岳阳市土地可持续利用水平与岳阳市自然条件、岳阳市城市的建设机遇及政府的建设态度相关。     

基于PSR模型的太原市土地资源生态安全评价

以太原市作为研究对象,利用压力—状态—响应(PSR)模型,以国家土地资源安全评价应用性评估指标体系为基础,建立了土地资源生态安全评价指标体系,并对太原市2006—2012年土地资源生态安全水平进行了评价。结果表明,2006—2012年太原市土地资源生态安全水平基本处于较不安全的状态;生态环境、人文社会压力是影响太原市土地资源安全压力重要的因素,而太原市的响应力度不够,土地资源的压力仍有增长的趋势,政府必须采取更多适当的手段和政策来保护土地资源。针对评价结果和影响因素分析结果提出了相应的安全对策。     

植物B12D基因克隆研究进展

综述了植物B12D基因的克隆、特性、时空表达、亚细胞定位、异源表达、过表达、生物和非生物胁迫应答、响应脱落酸与赤霉素等激素调节等方面的研究进展,展望了采用分子生物学新技术研究其调节植物胁迫应答反应机制和改良作物品质的发展趋势,为利用其进行植物耐胁迫和品质改良提供参考。