38 research outputs found
SWIFT observations of TeV BL Lac objects
Context: We present the results of a set of observations of nine TeV detected
BL Lac objects performed by the XRT and UVOT detectors on board the Swift
satellite between March and December 2005. Aims: We are mainly interested in
measuring the spectral parameters, and particularly the intrinsic curvature in
the X-ray band. Methods: We perform X-ray spectral analysis of observed BL Lac
TeV objects using either a log-parabolic or a simple power-law model . Results:
We found that many of the objects in our sample do show significant spectral
curvature, whereas those having the peak of the spectral energies distribution
at energies lower than ~0.1 keV show power law spectra. In these cases,
however, the statistics are generally low thus preventing a good estimate of
the curvature. Simultaneous UVOT observations are important to verify how X-ray
spectra can be extrapolated at lower frequencies and to search for multiple
emission components. Conclusions: The results of our analysis are useful for
the study of possible signatures of statistical acceleration processes
predicting intrinsically curved spectra and for modelling the SED of BL
Lacertae objects up to TeV energies where a corresponding curvature is likely
to be present.Comment: 10 pages, 7 figures. Accepted for publication in the Astronomy and
Astrophysic
Associative and Spatial Relationships in Thesaurus-based Retrieval
The OASIS (Ontologically Augmented Spatial Information System) project explores terminology systems for thematic and spatial access in digital library applications. A prototype implementation uses data from the Royal Commission on the Ancient and Historical Monuments of Scotland, together with the Getty AAT and TGN thesauri. This paper describes its integrated spatial and thematic schema and discusses novel approaches to the application of thesauri in spatial and thematic semantic distance measures. Semantic distance measures can underpin interactive and automatic query expansion techniques by ranking lists of candidate terms. We first illustrate how hierarchical spatial relationships can be used to provide more flexible retrieval for queries incorporating place names in applications employing online gazetteers and geographical thesauri. We then employ a set of experimental scenarios to investigate key issues affecting use of the associative (RT) thesaurus relationships in semantic distance measures. Previous work has noted the potential of RTs in thesaurus search aids but the problem of increased noise in result sets has been emphasised. Specialising RTs allows the possibility of dynamically linking RT type to query context. Results presented in this paper demonstrate the potential for filtering on the context of the RT link and on subtypes of RT relationships
Physics of Solar Prominences: II - Magnetic Structure and Dynamics
Observations and models of solar prominences are reviewed. We focus on
non-eruptive prominences, and describe recent progress in four areas of
prominence research: (1) magnetic structure deduced from observations and
models, (2) the dynamics of prominence plasmas (formation and flows), (3)
Magneto-hydrodynamic (MHD) waves in prominences and (4) the formation and
large-scale patterns of the filament channels in which prominences are located.
Finally, several outstanding issues in prominence research are discussed, along
with observations and models required to resolve them.Comment: 75 pages, 31 pictures, review pape
Discovery and characterization of a unique mycobacterial heme acquisition system
Mycobacterium tuberculosis must import iron from its host for survival, and its siderophore-dependent iron acquisition pathways are well established. Here we demonstrate a newly characterized pathway, whereby M. tuberculosis can use free heme and heme from hemoglobin as an iron source. Significantly, we identified the genomic region, Rv0202c–Rv0207c, responsible for the passage of heme iron across the mycobacterial membrane. Key players of this heme uptake system were characterized including a secreted protein and two transmembrane proteins, all three specific to mycobacteria. Furthermore, the crystal structure of the key heme carrier protein Rv0203 was found to have a unique fold. The discovery of a unique mycobacterial heme acquisition pathway opens new avenues of exploration into mycobacterial therapeutics