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In this paper synchrotron microtomography on Plasma Sprayed Tungsten (PS-W) is presented and discussed. PS-W is a challenging material for microtomography since it exhibits a random porous network at different length-scales (from nanometers to micrometers) and is hardly penetrable by X-rays. Furthermore, inner porosity causes strong internal scattering. The key challenges were, firstly, to optimize the beam parameters considering the inherent trade-off between photon energy (penetration depth) and spatial resolution and, secondly, to develop effective signal filtering algorithms. Despite the limited signal-to-noise ratio detected, large volumes of PS-W could be reconstructed with good image quality and micrometric resolution (voxel size = 1.4 {\mu}m). As an important result, we report excellent image quality and higher penetration depth by applying the same setup on a ferrous microstructure, namely a 10%W/Steel MMC used as interlayer between PS-W and a ferritic/martensitic steel substrate. The paper reports a detailed 3D morphological analysis of all inclusion types in PS-W and W/Steel, which led to disclosure of a complex connected porous network in both media. The analysis is presented in terms of multiphase volume fraction, ratio of percolation and 3D shape descriptors. 3D percolation patterns are analyzed in detail and sensitivity towards segmentation threshold for the noise-affected PS-W region is discussed. Remarkably, percolation of the porous phase was found throughout the entire coating thickness of PS-W. In W-Steel MMC percolation was found in the perpendicular plane and interpreted as onset of delamination caused by thermomechanical stress., Comment: 18 pages, 12 figures

Recursive loops in a logic program present a challenging problem to the PLP framework. On the one hand, they loop forever so that the PLP backward-chaining inferences would never stop. On the other hand, they generate cyclic influences, which are disallowed in Bayesian networks. Therefore, in existing PLP approaches logic programs with recursive loops are considered to be problematic and thus are excluded. In this paper, we propose an approach that makes use of recursive loops to build a stationary dynamic Bayesian network. Our work stems from an observation that recursive loops in a logic program imply a time sequence and thus can be used to model a stationary dynamic Bayesian network without using explicit time parameters. We introduce a Bayesian knowledge base with logic clauses of the form $A \leftarrow A_1,...,A_l, true, Context, Types$, which naturally represents the knowledge that the $A_i$s have direct influences on $A$ in the context $Context$ under the type constraints $Types$. We then use the well-founded model of a logic program to define the direct influence relation and apply SLG-resolution to compute the space of random variables together with their parental connections. We introduce a novel notion of influence clauses, based on which a declarative semantics for a Bayesian knowledge base is established and algorithms for building a two-slice dynamic Bayesian network from a logic program are developed., Comment: 29 pages

When thermal relativistic electrons with isotropic distribution of velocities move in a gas region, or impinge upon the surface of a cloud that consists of a dense gas or doped dusts, the Cerenkov effect produces peculiar atomic or ionic emission lines -- the Cerenkov line-like radiation. This newly recognized emission mechanism may find wide applications in high-energy astrophysics. In this paper, we tentatively adopt this new line emission mechanism to discuss the origin of iron K$_{\alpha}$ feature of AGNs. Motivation of this research is to attempt a solution to a problem encountered by the ``disk-fluorescence line'' model, i.e. the lack of temporal response of the observed iron K$_{\alpha}$ line flux to the changes of the X-ray continuum flux. If the Cerenkov line emission is indeed responsible significantly for the iron K$_{\alpha}$ feature, the conventional scenario around the central supermassive black holes of AGNs would need to be modified to accommodate more energetic, more violent and much denser environments than previously thought., Comment: 22 pages, 4 figures, 1 table. ApJ in press (December)

The recent discoveries of X-ray lines in the afterglows of gamma-ray bursts (GRBs) provide significant clues to the nature of GRB progenitors and central environments. However, the iron line interpretation by fluorescence or recombination mechanism requires a large amount of iron material. We argue that the very strong iron line could be attributed to an alternative mechanism: Cerenkov line emission since relativistic electrons and dense medium exist near GRB sites. Therefore, the broad iron lines are expected, and line intensity will be nearly independent of the iron abundance, the medium with the anomalously high Fe abundance is not required., Comment: 4 pages, revised version accepted for the publication in ApJL

Recently, Narayan & Raymond (1999) proposed that the thermal emission lines from the hot plasma in advection dominated accretion flows (ADAFs) are potentially observable with the next generation of X-ray observatories, with which the physical properties of some X-ray sources can be probed. In ADAFs, the temperature of the ion is so high that the thermal broadening of the line is important. We calculate the profiles of thermal line emission from ADAFs, in which both the thermal and Doppler broadening have been considered. It is found that the double-peaked profiles are present for high inclination angles between the axis of disk and the line of sight. The double-peaked profiles are smeared in low inclination cases, and completely disappear while the inclination angle is less than $15^{\circ}$, where the thermal and turbulent broadening dominated on the line profiles. We also note that the thermal line profile is affected by the location of the transition radius of ADAF. The self-similar height-integrated disk structure and the emissivity with power-law dependence of radius are adopted in our calculations. The results obtained in this work can be used as a diagnosis on the future X-ray observations of the thermal lines. Some important physical quantities of ADAFs could be inferred from future thermal line observations., Comment: 7 pages

You & Cheng (1980) argued that, for relativistic electrons moving through a dense gas, the Cerenkov effect will produce peculiar atomic and/or molecular emission lines--Cerenkov lines. They presented a series of formulae to describe the new line-mechanism. Elegant experimental confirmation has been obtained by Xu. et al. in the laboratory which definitely verified the existence of Cerenkov lines. Owing to the potential importance in high energy astrophysics, in this paper we give a more detailed physical discussion of the emission mechanisms and improve the previous formulae system into a form which is more convenient for astrophysical application. Specifically, the extended formulae in this paper can be applied to other species of atoms and/or ions rather than hydrogen as in the previous paper, and also to X-ray astronomy. They can also be used for the calculation of a nonuniform plane-parallel slab of the emissive gas., Comment: accepted for publication in A&A

A simplified frequency-integrated radiative transfer equation is solved to study Compton scatterings in the corona of the disk by using numerical iterating method. We find that the vertical thickness of the corona cannot be used as the typical length to measure the optical depth of the corona. A semi-analytical approach is proposed to calculate the energy dissipations in the corona of the disk. We demonstrate that our approach can reproduce the numerical solutions to an accuracy of <2 %., Comment: 5 pages, 4 Postscript figures, uses l-aa.sty. to appear in A&A