11,740 research outputs found
Monte Carlo Simulation of Comptonization in Inhomogeneous Media
Comptonization is the process in which photon spectrum changes due to
multiple Compton scatterings in the electronic plasma. It plays an important
role in the spectral formation of astrophysical X-ray and gamma-ray sources.
There are several intrinsic limitations for the analytical method in dealing
with the Comptonization problem and Monte Carlo simulation is one of the few
alternatives. We describe an efficient Monte Carlo method that can solve the
Comptonization problem in a fully relativistic way. We expanded the method so
that it is capable of simulating Comptonization in the media where electron
density and temperature varies discontinuously from one region to the other and
in the isothermal media where density varies continuously along photon paths.
The algorithms are presented in detail to facilitate computer code
implementation. We also present a few examples of its application to the
astrophysical research.Comment: 12 pages, 4 figures, Postscript file, in press ("Computers in
Physics", Vol. 11, No. 6
Phase Difference and Coherence as Diagnistics of Accreting Compact Sources
We present calculations of the time lags and the coherence function of X-ray
photons for a novel model of radiation emission from accretion powered,
high-energy sources. Our model involves only Comptonization of soft photons
injected near the compact object in an extended but non-uniform atmosphere
around the compact object. Our results show that this model produces time lags
between the hard and soft bands of the X-ray spectrum which increase with
Fourier period, in agreement with recent observations; it also produces a
coherence function equal to one over a wide range of frequencies if the system
parameters do not have significant changes, also in agreement with the limited
existing observations. We explore various conditions that could affect
coherence functions. We indicate that measurements of these statistical
quantities could provide diagnostics of the radial structure of the density of
this class of sources.Comment: 15 pages, Latex, using AAS macro, to be published in Astrophysical
Journal Letters
Compton backscattered annihilation line emission: A new diagnostic of accreting compact sources
It is shown that Compton scattering of 511 keV electron-positron annihilation radiation produces a line like feature at approx. 170 keV from backscattered photons. Assuming a simple model of an accretion disk around a compact source, the spectrum is explored of the spectrum of Compton scattered annihilation line emission for a range of conditions. It is further shown that such Compton baskscattering of annihilation line emission from the inner edge of an accretion disk could account for the previously unidentified 170 keV line emission and high energy continuum observed from a variable, compact source, or sources, of annihilation radiation near the Galactic Center. Identification of the observed 170 keV line as an annihilation line reflection feature provides strong new evidence that the source of the emission is an accreting compact object. Further study of these features in existing spectra and in forthcoming GRO observation of these and other sources can provide unique new diagnostics of the innermost regions of accretion disks around compact objects
Understanding the Roles of sketching in product design
Sketching is an essential part of the tradition and practice of design. However, despite the extensive literature on the subject, the roles of sketching in the design are still not fully understood. Therefore, this PhD study was carried out to understand the roles of sketching in the product design field and to develop the means to support the design sketching process. Hypotheses emerged from the review of the literature and led to three descriptive studies: a pilot study to identify the roles of sketching in design; a survey of design students; a survey of design professionals. Data collected from these studies were used to test and verify the hypotheses.
It was found that the major roles of sketching in design were related to design ideation and design communication. The comparison between design students and professionals showed that they tend to perceive the major roles of sketching differently, but shared similar opinions on the major roles of talking sketches, sketching expertise and as a practical supporting tool. In addition, an Ideation Segment Model was proposed, which describes the process that sketching ideas evolve into final design outcomes and locates the potential supporting tools in the sketching process.
Consequently, a practical supporting tool was developed to enhance understanding of design sketching, to provide external stimuli, and to improve sketching expertise. The supporting tool, entitled the PD-Sketching Primer and PD-Sketching Toolkit, was tested with education and positive feedback was received.
This research has contributed towards understanding the roles of sketching in design, e.g. it is the first empirical study of ânon-working sketchesâ in the design field. It has generated new knowledge and tools to support the design sketching process. This supporting tool is one of only a few resources designed to support sketching activity and creative performance in the product design field.Open Acces
Structural auditing methodologies for controlled terminologies
Several auditing methodologies for large controlled terminologies are developed. These are applied to the Unified Medical Language System XXXX and the National Cancer Institute Thesaurus (NCIT). Structural auditing methodologies are based on the structural aspects such as IS-A hierarchy relationships groups of concepts assigned to semantic types and groups of relationships defined for concepts. Structurally uniform groups of concepts tend to be semantically uniform. Structural auditing methodologies focus on concepts with unlikely or rare configuration. These concepts have a high likelihood for errors.
One of the methodologies is based on comparing hierarchical relationships between the META and SN, two major knowledge sources of the UMLS. In general, a correspondence between them is expected since the SN hierarchical relationships should abstract the META hierarchical relationships. It may indicate an error when a mismatch occurs.
The UMLS SN has 135 categories called semantic types. However, in spite of its medium size, the SN has limited use for comprehension purposes because it cannot be easily represented in a pictorial form, it has many (about 7,000) relationships. Therefore, a higher-level abstraction for the SN called a metaschema, is constructed. Its nodes are meta-semantic types, each representing a connected group of semantic types of the SN. One of the auditing methodologies is based on a kind of metaschema called a cohesive metaschema. The focus is placed on concepts of intersections of meta-semantic types. As is shown, such concepts have high likelihood for errors.
Another auditing methodology is based on dividing the NCIT into areas according to the roles of its concepts. Moreover, each multi-rooted area is further divided into pareas that are singly rooted. Each p-area contains a group of structurally and semantically uniform concepts. These groups, as well as two derived abstraction networks called taxonomies, help in focusing on concepts with potential errors. With genomic research being at the forefront of bioscience, this auditing methodology is applied to the Gene hierarchy as well as the Biological Process hierarchy of the NCIT, since processes are very important for gene information. The results support the hypothesis that the occurrence of errors is related to the size of p-areas. Errors are more frequent for small p-areas
The kHz QPOs as a probe of the X-ray color-color diagram and accretion-disk structure for the atoll source 4U 1728-34
We have taken the kHz QPOs as a tool to probe the correlation between the
tracks of X-ray color-color diagram (CCD) and magnetosphere-disk positions for
the atoll source 4U 1728-34, based on the assumptions that the upper kHz QPO is
ascribed to the Keplerian orbital motion and the neutron star (NS)
magnetosphere is defined by the dipole magnetic field. We find that from the
island to the banana state, the inner accretion disk gradually approaches the
NS surface with the radius decreasing from r ~33.0km to ~15.9 km, corresponding
to the magnetic field from B(r) ~4.8*10^6 G to ~4.3*10^7 G. In addition, we
note the characteristics of some particular radii of magnetosphere-disk -r are:
firstly, the whole atoll shape of the CCD links the disk radius range of ~15.9
- 33.0 km, which is just located inside the corotation radius of 4U 1728-34
-r_co ( ~34.4 km), implying that the CCD shape is involved in the NS spin-up
state. Secondly, the island and banana states of CCD correspond to the two
particular boundaries: (I)-near the corotation radius at r ~27.2 - 33.0 km,
where the source lies in the island state; (II)-near the NS surface at r ~15.9
- 22.3 km, where the source lies in both the island and banana states. Thirdly,
the vertex of the atoll shape in CCD, where the radiation transition from the
hard to soft photons occurs, is found to be near the NS surface at r ~16.4 km.
The above results suggest that both the magnetic field and accretion
environment are related to the CCD structure of atoll track, where the
corotation radius and NS hard surface play the significant roles in the
radiation distribution of atoll source.Comment: 6 pages, 3 figures, 1 table, accepted by Astronomy & Astrophysic
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