固体潮对地球重力场时变特征影响的潮波公式

精密和详细测定地球重力场及其随时间变化,是目前卫星重力测量的主要课题.基于目前高精度的固体潮展开,研究固体潮对地球重力场时变特征的影响.不同于IERS2000推荐的固体潮对重力影响的理论模型,独立于天体历书而基于精密引潮力位展开直接给出在毫微伽精度下的潮波公式,并考虑到四阶潮汐的效应.同时对重力数据归算中的永久性潮汐的处理进行总结和说明.本文的工作可为高精度的地球重力场的研究提供理论依据和参考.     

Noise properties of gravitational lens mass reconstruction

Gravitational lensing is potentially able to observe mass-selected haloes, and to measure the projected cluster mass function. An optimal mass selection requires a quantitative understanding of the noise behaviour in mass maps. This paper is an analysis of the noise properties in mass maps reconstructed from a maximum-likelihood method.
The first part of this work is the derivation of the noise power spectrum and the mass error bars as a straightforward extension of the Kaiser & Squires algorithm for the case of a correlated noise. Very good agreement is found between these calculations and the noise properties measured in the mass reconstructions limited to non-critical clusters of galaxies. It demonstrates that Kaiser & Squires and maximum-likelihood methods have similar noise properties and that the weak lensing approximation is valid for describing these properties .
In a second stage I show that the statistics of peaks in the noise follows accurately the peak statistics of a two-dimensional Gaussian random field (using the BBKS techniques) if the smoothing aperture contains enough galaxies. This analysis provides a full procedure for deriving the significance of any convergence peak as a function of its amplitude and profile.
I demonstrate that a detailed quantitative analysis of the structures in mass maps can be carried out, and that, to a very good approximation, a mass map is the sum of the lensing signal and known two-dimensional Gaussian random noise. A straightforward application is the measurement of the projected mass function in wide-field lensing surveys, down to small mass overdensities that are individually undetectable.     

The KX method for producing K-band flux-limited samples of quasars

The longstanding question of the extent to which the quasar population is affected by dust extinction, within host galaxies or galaxies along the line of sight, remains open. More generally, the spectral energy distributions of quasars vary significantly, and flux-limited samples defined at different wavelengths include different quasars. Surveys employing flux measurements at widely separated wavelengths are necessary to characterize fully the spectral properties of the quasar population. The availability of panoramic near-infrared detectors on large telescopes provides the opportunity to undertake surveys capable of establishing the importance of extinction by dust on the observed population of quasars. We introduce an efficient method for selecting K -band, flux-limited samples of quasars, termed 'KX' by analogy with the UVX method. This method exploits the difference between the power-law nature of quasar spectra and the convex spectra of stars: quasars are relatively brighter than stars at both short wavelengths (the UVX method) and long wavelengths (the KX method). We consider the feasibility of undertaking a large-area KX survey for damped Ly α galaxies and gravitational lenses using the planned UKIRT wide-field near-infrared camera.     

A multiwavelength view at the heart of the superwind in NGC 253

We present a new method of studying quadruple lenses in elliptical power-law potentials parametrized by ψ ( x , y )∝( x ²+ y ² q ²) ^{β /2} β (0 β <2). For this potential, the moments of the four image positions weighted by signed magnifications (magnification times parity) have very simple properties. In particular, we find that the zeroth moment – the sum of four signed magnifications satisfies ≃2/(2− β ); the relation is exact for β =0 (point-lens) and β =1 (isothermal potential), independent of the axial ratio. Similar relations can be derived when a shear is present along the major or minor axes. These relations, however, do not hold well for the closely related elliptical density distributions. For a singular isothermal elliptical density distribution without shear, the sum of signed magnifications for quadruple lenses is ≈2.8, again nearly independent of the ellipticity. For the same distribution with shear, the total signed magnification is around 2–3 for most cases, but can be significantly different for some combinations of the axial ratio and shear where six or eight images can appear.     

Distribution of caustic-crossing intervals for Galactic binary-lens microlensing events

Detection of caustic crossings of binary-lens gravitational microlensing events is important because by detecting them one can obtain useful information about both the lens and the source star. In this paper, we compute the distribution of the intervals between two successive caustic crossings, f ( t _cc), for Galactic bulge binary-lens events to investigate the observational strategy for the optimal detection and resolution of caustic crossings. From this computation, we find that the distribution is highly skewed towards short t _cc and peaks at t _cc∼1.5 d. For the maximal detection of caustic crossings, therefore, prompt initiation of follow-up observations for intensive monitoring of events will be important. We estimate that, under the strategy of the current follow-up observations with a second caustic-crossing preparation time of ∼2 d, the fraction of events with resolvable caustic crossing is ∼80 per cent. We find that if the follow-up observations can be initiated within 1 d after the first caustic crossing by adopting more aggressive observational strategies, the detection rate can be improved to ∼90 per cent.     

The effects of resolved sources and blending on the detection of planets via gravitational microlensing

It has been shown that gravitational microlensing events towards the Galactic Bulge are sensitive to the presence of a planet orbiting the lensing star. The probability of planet detection is calculated here as a function of the binary geometry for mass ratios of     taking the effects of resolving the source and the inclusion of unlensed light (blending) into account. Source radii up to     θ _E are considered, at which point the detection probability becomes negligible. Small     mass ratio planets become undetectable at source radii of     θ _E . Blending has a slight adverse effect on planet detection. It is worst when the unblended detection probability is small and causes planets to become undetectable at smaller source radii than would be the case in the absence of blending. An alternative to current gravitational microlensing follow-up observations is investigated, where only the peaks of high amplification events are followed. Such a strategy promises to be at least twice as efficient at detecting planets as current observations, but requires a large number of high amplification events.     

The non-Gaussian tail of cosmic-shear statistics

Owing to gravitational instability, an initially Gaussian density field develops non-Gaussian features as the Universe evolves. The most prominent non-Gaussian features are massive haloes, visible as clusters of galaxies. The distortion of high-redshift galaxy images because of the tidal gravitational field of the large-scale matter distribution, called cosmic shear, can be used to investigate the statistical properties of the large‐scale structure (LSS) . In particular, non-Gaussian properties of the LSS will lead to a non-Gaussian distribution of cosmic-shear statistic. The aperture mass ( M _ap) statistics, recently introduced as a measure for cosmic shear, is particularly well suited for measuring these non-Gaussian properties. In this paper we calculate the highly non-Gaussian tail of the aperture mass probability distribution, assuming Press–Schechter theory for the halo abundance and the 'universal' density profile of haloes as obtained from numerical simulations. We find that for values of M _ap much larger than its dispersion, this probability distribution is closely approximated by an exponential, rather than a Gaussian. We determine the amplitude and shape of this exponential for various cosmological models and aperture sizes, and show that wide-field imaging surveys can be used to distinguish between some of the currently most popular cosmogonies. Our study here is complementary to earlier cosmic-shear investigations, which focused more on two- and three-point statistical properties.     

The characteristics of magma reservoir failure beneath a volcanic edifice

Eruptions fed from subsurface reservoirs commonly construct volcanic edifices at the surface, and the growth of an edifice will in turn modify the subsurface stress state that dictates the conditions under which subsequent rupture of the inflating reservoir can occur. We re-examine this problem using axisymmetric finite element models of ellipsoidal reservoirs beneath conical edifices, explicitly incorporating factors (e.g., full gravitational loading conditions, an elastic edifice instead of a surface load, reservoir pressures sufficient to induce tensile rupture) that compromise previous solutions to illustrate why variations in rupture behavior can occur. Relative to half-space model results, the presence of an edifice generally rotates rupture toward the crest of a spherical reservoir, with increasing flank slope (for an edifice of constant volume) and larger edifices (or greater reservoir scaled depths) normally serving to enhance this trend. When non-spherical reservoirs are considered, the presence of an edifice amplifies previously identified half-space failure characteristics, shifting rupture to the crest more rapidly for prolate reservoirs while forcing rupture closer to the midpoint of oblate reservoirs. Rupture is always observed to occur in the σ_t orientation, and depending on where initial failure occurs rupture favors the initial emplacement of either lateral sills, circumferential intrusions or vertically ascending dikes. Ultimately, integration of our numerical model results with other information, for instance the sequence of intrusion/eruption events observed at a given volcano, can provide useful new insight into how a volcano's subsurface magma plumbing system evolved. We demonstrate this process through application of our model to Summer Coon, a well-studied stratocone on Earth, and Ilithyia Mons, a large conical shield volcano on Venus.