Diffraction imaging by uniform asymptotic theory and double exponential fitting          下载免费PDF全文

Jingtao Zhao  Caixia Yu 《Geophysical Prospecting》2015,63(2):338-353

Seismic diffracted waves carry valuable information for identifying geological discontinuities. Unfortunately, the diffraction energy is generally too weak, and standard seismic processing is biased to imaging reflection. In this paper, we present a dynamic diffraction imaging method with the aim of enhancing diffraction and increasing the signal‐to‐noise ratio. The correlation between diffraction amplitudes and their traveltimes generally exists in two forms, with one form based on the Kirchhoff integral formulation, and the other on the uniform asymptotic theory. However, the former will encounter singularities at geometrical shadow boundaries, and the latter requires the computation of a Fresnel integral. Therefore, neither of these methods is appropriate for practical applications. Noting the special form of the Fresnel integral, we propose a least‐squares fitting method based on double exponential functions to study the amplitude function of diffracted waves. The simple form of the fitting function has no singularities and can accelerate the calculation of diffraction amplitude weakening coefficients. By considering both the fitting weakening function and the polarity reversal property of the diffracted waves, we modify the conventional Kirchhoff imaging conditions and formulate a diffraction imaging formula. The mechanism of the proposed diffraction imaging procedure is based on the edge diffractor, instead of the idealized point diffractor. The polarity reversal property can eliminate the background of strong reflection and enhance the diffraction by same‐phase summation. Moreover,the fitting weakening function of diffraction amplitudes behaves like an inherent window to optimize the diffraction imaging aperture by its decaying trend. Synthetic and field data examples reveal that the proposed diffraction imaging method can meet the requirement of high‐resolution imaging, with the edge diffraction fully reinforced and the strong reflection mostly eliminated.  相似文献   

The scientific rationale for the C1XS X-ray spectrometer on India's Chandrayaan-1 mission to the moon   总被引：1，自引：0，他引：1  

I.A. Crawford  K.H. Joy  B.J. Kellett  M. Anand  N. Bhandari  L. d’Uston  O. Gasnault  C.J. Howe  D. Koschny  B.J. Maddison  S. Narendranath  T. Okada  S.S. Russell  B. Swinyard  M. Wilding 《Planetary and Space Science》2009,57(7):725-734

The UK-built Chandrayaan-1 X-ray Spectrometer (C1XS) will fly as an ESA instrument on India's Chandrayaan-1 mission to the Moon, launched in October 2008. C1XS builds on experience gained with the earlier D-CIXS instrument on SMART-1, but will be a scientifically much more capable instrument. Here we describe the scientific objectives of this instrument, which include mapping the abundances of the major rock-forming elements (principally Mg, Al, Si, Ti, Ca and Fe) in the lunar crust. These data will aid in determining whether regional compositional differences (e.g., the Mg/Fe ratio) are consistent with models of lunar crustal evolution. C1XS data will also permit geochemical studies of smaller scale features, such as the ejecta blankets and central peaks of large impact craters, and individual lava flows and pyroclastic deposits. These objectives all bear on important, and currently unresolved, questions in lunar science, including the structure and evolution of any primordial magma ocean, as revealed by vertical and lateral geochemical variations in the crust, and the composition of the lunar mantle, which will further constrain theories of the Moon's origin, thermal history and internal structure.  相似文献   

XEUS: the physics of the hot evolving universe     

《Experimental Astronomy》2009,23(1):139-168

This paper describes the next generation X-ray observatory XEUS which has been submitted to the European Space Agency in the framework of the Cosmic Vision 2015–2025 competition and has been selected for an assessment study. The paper summarizes the scientific goals and instrumental concepts of the proposed X-ray telescope with 5 m² effective area and angular resolution better than 5 arc sec.  相似文献   

X-ray clusters of galaxies in conformal gravity     

Antonaldo Diaferio  Luisa Ostorero 《Monthly notices of the Royal Astronomical Society》2009,393(1):215-223

We run adiabatic N -body/hydrodynamical simulations of isolated self-gravitating gas clouds to test whether conformal gravity, an alternative theory to general relativity, is able to explain the properties of X-ray galaxy clusters without resorting to dark matter. We show that the gas clouds rapidly reach equilibrium with a density profile which is well fitted by a β-model whose normalization and slope are in approximate agreement with observations. However, conformal gravity fails to yield the observed thermal properties of the gas cloud: (i) the mean temperature is at least an order of magnitude larger than the observed and (ii) the temperature profiles increase with the square of the distance from the cluster centre, in clear disagreement with real X-ray clusters. These results depend on a gravitational potential whose parameters reproduce the velocity rotation curves of spiral galaxies. However, this parametrization stands on an arbitrarily chosen conformal factor. It remains to be seen whether a different conformal factor, specified by a spontaneous breaking of the conformal symmetry, can reconcile this theory with observations.  相似文献   

AGN dust tori: the X-ray-infrared connection     

Michael Rowan-Robinson  Ivan Valtchanov  Kirpal Nandra 《Monthly notices of the Royal Astronomical Society》2009,397(3):1326-1337

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The structure of molecular clouds – I. All-sky near-infrared extinction maps     

Jonathan Rowles  Dirk Froebrich 《Monthly notices of the Royal Astronomical Society》2009,395(3):1640-1648

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Optical identification of the hard X-ray source IGR J18257-0707     

R. A. Burenin  I. F. Bikmaev  M. G. evnivtsev  J. A. Tomsick  S. Yu. Sazonov  M. N. Pavlinsky  R. A. Sunyaev 《Astronomy Letters》2009,35(2):71-74

We present the results of our optical identification of the hard X-ray source IGR J18257-0707 through its spectrophotometric observations with the optical RTT-150 telescope. The accurate position of the X-ray source determined using Chandra observations has allowed this source to be confidently associatedwith a faint optical object (m _R ≈ 20.4) whose optical spectrumexhibits a broad H _α emission line at redshift z = 0.037. Thus, the source IGR J18257-0707 is a type-1 Seyfert galaxy at redshift z = 0.037.  相似文献   

X-ray and strong lensing mass estimate of MS2137.3–2353     

A. Donnarumma  S. Ettori  M. Meneghetti  L. Moscardini 《Monthly notices of the Royal Astronomical Society》2009,398(1):438-450

We present new mass estimates of the cluster of galaxies MS2137.3–2353, inferred from X-ray and strong lensing analyses. This cluster exhibits an outstanding strong lensing configuration and indicates a well-relaxed dynamical state, being most suitable for a mass reconstruction which combines both techniques. Despite this, several previous studies have claimed a significant discrepancy between the X-ray and the strong lensing mass estimates. The primary aim of this paper is to address and explain this mismatch. For this purpose, we have analysed Chandra observations to recover the profiles of the intracluster medium properties and, assuming a functional form for the matter density, the total mass distribution. The notable strong-lensing features of MS2137.3 allow us to reconstruct its projected mass in the central regions with good accuracy, by taking advantage of the lensing inversion code lenstool . We compare the results obtained for both methods. Our mass estimates for MS2137.3 are in agreement within errors, leading to a mean, extrapolated value of   M ₂₀₀≃ 4.4 ± 0.3 × 10¹⁴ M_⊙  , under the assumption of the Navarro–Frenk–White (NFW) mass profile. However, the strong lensing mass estimate is affected by the details of the brightest cluster galaxy mass modelling, since the radial arc is a very sensitive probe of the total mass derivative in the central region. In particular, we do not find evidence for a high concentration for the NFW density profile, as reported in some earlier works.  相似文献   

The Influence of Viscosity on the Truncation of Accretion Disks Around Black Holes     

Qiang Niu 《Chinese Astronomy and Astrophysics》2009

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应用X射线源的航天器姿态测量   总被引：1，自引：0，他引：1  

杨廷高  陈鼎 《时间频率学报》2009,32(1):70-80

在介绍X射线源和X射线探测器、准直器的基础上,重点描述了应用X射线源测量航天器姿态参数的原理和方法。对X射线星扫描仪的结构、扫描观测X射线源方法和矩形准直器响应函数等做了介绍,较详细地论述了应用单准直器X射线星扫描仪和差分准直器X射线星扫描仪确定航天器姿态参数的测量模型,并进行了二者间的比较和分析。简单讨论了X射线星扫描仪测量建模和应用测量模型拟合航天器姿态参数的算法问题。最后,对采用编码孔径系统确定航天器姿态参数的方法做了概括性介绍。  相似文献