Early optical emission from the gamma-ray burst of 4 October 2002

Observations of the long-lived emission--or 'afterglow'--of long-duration gamma-ray bursts place them at cosmological distances, but the origin of these energetic explosions remains a mystery. Observations of optical emission contemporaneous with the burst of gamma-rays should provide insight into the details of the explosion, as well as into the structure of the surrounding environment. One bright optical flash was detected during a burst, but other efforts have produced negative results. Here we report the discovery of the optical counterpart of GRB021004 only 193 seconds after the event. The initial decline is unexpectedly slow and requires varying energy content in the gamma-ray burst blastwave over the course of the first hour. Further analysis of the X-ray and optical afterglow suggests additional energy variations over the first few days.     

A common origin for cosmic explosions inferred from calorimetry of GRB030329

Past studies have suggested that long-duration gamma-ray bursts have a 'standard' energy of E(gamma) approximately 10(51) erg in the ultra-relativistic ejecta, after correcting for asymmetries in the explosion ('jets'). But a group of sub-energetic bursts, including the peculiar GRB980425 associated with the supernova SN1998bw (E(gamma) approximately 10(48) erg), has recently been identified. Here we report radio observations of GRB030329 that allow us to undertake calorimetry of the explosion. Our data require a two-component explosion: a narrow (5 degrees opening angle) ultra-relativistic component responsible for the gamma-rays and early afterglow, and a wide, mildly relativistic component that produces the radio and optical afterglow more than 1.5 days after the explosion. The total energy release, which is dominated by the wide component, is similar to that of other gamma-ray bursts, but the contribution of the gamma-rays is energetically minor. Given the firm link of GRB030329 with SN2003dh, our result indicates a common origin for cosmic explosions in which, for reasons not yet understood, the energy in the highest-velocity ejecta is extremely variable.     

Possible thermal and chemical stabilization of body-centred-cubic iron in the Earth's core

The nature of the stable phase of iron in the Earth's solid inner core is still highly controversial. Laboratory experiments suggest the possibility of an uncharacterized phase transformation in iron at core conditions and seismological observations have indicated the possible presence of complex, inner-core layering. Theoretical studies currently suggest that the hexagonal close packed (h.c.p.) phase of iron is stable at core pressures and that the body centred cubic (b.c.c.) phase of iron becomes elastically unstable at high pressure. In other h.c.p. metals, however, a high-pressure b.c.c. form has been found to become stabilized at high temperature. We report here a quantum mechanical study of b.c.c.-iron able to model its behaviour at core temperatures as well as pressures, using ab initio molecular dynamics free-energy calculations. We find that b.c.c.-iron indeed becomes entropically stabilized at core temperatures, but in its pure state h.c.p.-iron still remains thermodynamically more favourable. The inner core, however, is not pure iron, and our calculations indicate that the b.c.c. phase will be stabilized with respect to the h.c.p. phase by sulphur or silicon impurities in the core. Consequently, a b.c.c.-structured alloy may be a strong candidate for explaining the observed seismic complexity of the inner core.     

The bright optical afterglow of the nearby gamma-ray burst of 29 March 2003

Past studies of cosmological gamma-ray bursts (GRBs) have been hampered by their extreme distances, resulting in faint afterglows. A nearby GRB could potentially shed much light on the origin of these events, but GRBs with a redshift z 相似文献   

The mammalian sodium channel BNC1 is required for normal touch sensation

Of the vertebrate senses, touch is the least understood at the molecular level The ion channels that form the core of the mechanosensory complex and confer touch sensitivity remain unknown. However, the similarity of the brain sodium channel 1 (BNC1) to nematode proteins involved in mechanotransduction indicated that it might be a part of such a mechanosensor. Here we show that disrupting the mouse BNC1 gene markedly reduces the sensitivity of a specific component of mechanosensation: low-threshold rapidly adapting mechanoreceptors. In rodent hairy skin these mechanoreceptors are excited by hair movement. Consistent with this function, we found BNC1 in the lanceolate nerve endings that lie adjacent to and surround the hair follicle. Although BNC1 has been proposed to have a role in pH sensing, the acid-evoked current in cultured sensory neurons and the response of acid-stimulated nociceptors were normal in BNC1 null mice. These data identify the BNC1 channel as essential for the normal detection of light touch and indicate that BNC1 may be a central component of a mechanosensory complex.     

Evidence for a link between global lightning activity and upper tropospheric water vapour

Tropospheric water vapour is a key element of the Earth's climate, which has direct effects as a greenhouse gas, as well as indirect effects through interaction with clouds, aerosols and tropospheric chemistry. Small changes in upper-tropospheric water vapour have a much larger impact on the greenhouse effect than small changes in water vapour in the lower atmosphere, but whether this impact is a positive or negative feedback remains uncertain. The main challenge in addressing this question is the difficulty in monitoring upper-tropospheric water vapour globally over long timescales. Here I show that upper-tropospheric water-vapour variability and global lightning activity are closely linked, suggesting that upper-tropospheric water-vapour changes can be inferred from records of global lightning activity, readily obtained from observations at a single location on the Earth's surface. This correlation reflects the fact that continental deep-convective thunderstorms transport large amounts of water vapour into the upper troposphere and thereby dominate the variations of global upper-tropospheric water vapour while producing most of the lightning on Earth. As global lightning induces Schumann resonances, an electromagnetic phenomenon in the atmosphere that can be observed easily at low cost, monitoring of these resonances might provide a convenient method for tracking upper-tropospheric water-vapour variability and hence contribute to a better understanding of the processes affecting climate change.     

Constraints on the composition of the Earth's core from ab initio calculations

Knowledge of the composition of the Earth's core is important for understanding its melting point and therefore the temperature at the inner-core boundary and the temperature profile of the core and mantle. In addition, the partitioning of light elements between solid and liquid, as the outer core freezes at the inner-core boundary, is believed to drive compositional convection, which in turn generates the Earth's magnetic field. It is generally accepted that the liquid outer core and the solid inner core consist mainly of iron. The outer core, however, is also thought to contain a significant fraction of light elements, because its density--as deduced from seismological data and other measurements--is 6-10 per cent less than that estimated for pure liquid iron. Similar evidence indicates a smaller but still appreciable fraction of light elements in the inner core. The leading candidates for the light elements present in the core are sulphur, oxygen and silicon. Here we obtain a constraint on core composition derived from ab initio calculation of the chemical potentials of light elements dissolved in solid and liquid iron. We present results for the case of sulphur, which provide strong evidence against the proposal that the outer core is close to being a binary iron-sulphur mixture.     

基于Weibull函数和价格折扣的易腐品库存订货与定价策略

【目的】研究需求率为售价的指数函数，变质率服从三参数Weibull分布的订货与定价策略。【方法】系统以利润最大化为目标函数，考虑商品变质时带来的成本，引入价格折扣，构建了库存模型。【结果】采用直接法和泰勒展开式对模型进行近似求解，提供算例分析以得到问题的近似解，通过对折扣率、变质率灵敏度分析给出了相应的合理化管理建议。【结论】零售商在处理保鲜期后的商品时引入的价格折扣越高销量越大。商品的流动环境因子的增加会使得零售商成本增加，在销售过程中可采用更为先进的交通工具和储存手段。     

The elastic constants of MgSiO3 perovskite at pressures and temperatures of the Earth's mantle.

The temperature anomalies in the Earth's mantle associated with thermal convection can be inferred from seismic tomography, provided that the elastic properties of mantle minerals are known as a function of temperature at mantle pressures. At present, however, such information is difficult to obtain directly through laboratory experiments. We have therefore taken advantage of recent advances in computer technology, and have performed finite-temperature ab initio molecular dynamics simulations of the elastic properties of MgSiO3 perovskite, the major mineral of the lower mantle, at relevant thermodynamic conditions. When combined with the results from tomographic images of the mantle, our results indicate that the lower mantle is either significantly anelastic or compositionally heterogeneous on large scales. We found the temperature contrast between the coldest and hottest regions of the mantle, at a given depth, to be about 800 K at 1,000 km, 1,500 K at 2,000 km, and possibly over 2,000 K at the core-mantle boundary.