856 research outputs found
Inverse Compton Scattering as the Source of Diffuse EUV Emission in the Coma Cluster of Galaxies
We have examined the hypothesis that the majority of the diffuse EUV flux in
the Coma cluster is due to inverse Compton scattering of low energy cosmic ray
electrons (0.16 < epsilon < 0.31 GeV) against the 3K black-body background. We
present data on the two-dimensional spatial distribution of the EUV flux and
show that these data provide strong support for a non-thermal origin for the
EUV flux. However, we show that this emission cannot be produced by an
extrapolation to lower energies of the observed synchrotron radio emitting
electrons and an additional component of low energy cosmic ray electrons is
required.Comment: 11 pages, 5 figure
X-ray Signatures of an Ionized Reprocessor in the Seyfert galaxy Ton S 180
We discuss the hard X-ray properties of the Seyfert galaxy Ton S 180, based
upon the analysis of ASCA data. We find the X-ray flux varied by a factor ~2 on
a time scale of a few thousand seconds. The source showed significantly higher
amplitude of variability in the 0.5-2 keV band than in the 2-10 keV band. The
continuum is adequately parameterized as a Gamma ~ 2.5 power-law across the
0.6--10 keV band . We confirm the recent discovery of an emission line of high
equivalent width, due to Fe K-shell emission from highly-ionized material.
These ASCA data show the Fe line profile to be broad and asymmetric and
tentatively suggest it is stronger during the X-ray flares, consistent with an
origin from the inner parts of an accretion disk. The X-ray spectrum is complex
below 2 keV, possibly due to emission from a blend of soft X-ray lines, which
would support the existence of an ionized reprocessor, most likely due to a
relatively high accretion rate in this source.Comment: 24 pages, 8 figures. LaTeX with encapsulated postscript. To appear in
the Astrophysical Journa
Towards Autopoietic Computing
A key challenge in modern computing is to develop systems that address
complex, dynamic problems in a scalable and efficient way, because the
increasing complexity of software makes designing and maintaining efficient and
flexible systems increasingly difficult. Biological systems are thought to
possess robust, scalable processing paradigms that can automatically manage
complex, dynamic problem spaces, possessing several properties that may be
useful in computer systems. The biological properties of self-organisation,
self-replication, self-management, and scalability are addressed in an
interesting way by autopoiesis, a descriptive theory of the cell founded on the
concept of a system's circular organisation to define its boundary with its
environment. In this paper, therefore, we review the main concepts of
autopoiesis and then discuss how they could be related to fundamental concepts
and theories of computation. The paper is conceptual in nature and the emphasis
is on the review of other people's work in this area as part of a longer-term
strategy to develop a formal theory of autopoietic computing.Comment: 10 Pages, 3 figure
Far Ultraviolet Absolute Flux of alpha Virginis
We present the far ultraviolet spectrum of alpha Virginis taken with EURD
spectrograph on-board MINISAT-01. The spectral range covered is from ~900 to
1080 A with 5 A spectral resolution. We have fitted Kurucz models to IUE
spectra of alpha Vir and compared the extension of the model to our wavelengths
with EURD data. This comparison shows that EURD fluxes are consistent with the
prediction of the model within 20-30%, depending on the reddening assumed. EURD
fluxes are consistent with Voyager observations but are ~60% higher than most
previous rocket observations of alpha Vir.Comment: 13 pages, 4 figures. Submitted to The Astrophysical Journa
Towards Pose-Invariant 2D Face Classification for Surveillance
A key problem for "face in the crowd" recognition from existing surveillance cameras in public spaces (such as mass transit centres) is the issue of pose mismatches between probe and gallery faces. In addition to accuracy, scalability is also important, necessarily limiting the complexity of face classification algorithms. In this paper we evaluate recent approaches to the recognition of faces at relatively large pose angles from a gallery of frontal images and propose novel adaptations as well as modifications. Specifically, we compare and contrast the accuracy, robustness and speed of an Active Appearance Model (AAM) based method (where realistic frontal faces are synthesized from non-frontal probe faces) against bag-of-features methods (which are local feature approaches based on block Discrete Cosine Transforms and Gaussian Mixture Models). We show a novel approach where the AAM based technique is sped up by directly obtaining pose-robust features, allowing the omission of the computationally expensive and artefact producing image synthesis step. Additionally, we adapt a histogram-based bag-of-features technique to face classification and contrast its properties to a previously proposed direct bag-of-features method. We also show that the two bag-of-features approaches can be considerably sped up, without a loss in classification accuracy, via an approximation of the exponential function. Experiments on the FERET and PIE databases suggest that the bag-of-features techniques generally attain better performance, with significantly lower computational loads. The histogram-based bag-of-features technique is capable of achieving an average recognition accuracy of 89% for pose angles of around 25 degrees
Implications of a Nonthermal Origin of the Excess EUV Emission from the Coma Cluster of Galaxies
The inverse Compton (IC) interpretation of the excess EUV emission, that was
recently reported from several clusters of galaxies, suggests that the amount
of relativistic electrons in the intracluster medium is highly significant,
W_e>10^{61} erg. Considering Coma as the prototype galaxy cluster of nonthermal
radiation, we discuss implications of the inverse Compton origin of the excess
EUV fluxes in the case of low intracluster magnetic fields of order 0.1 muG, as
required for the IC interpretation of the observed excess hard X-ray flux, and
in the case of high fields of order 1 muG as suggested by Faraday rotation
measurements. Although for such high intracluster fields the excess hard X-rays
will require an explanation other than by the IC effect, we show that the
excess EUV flux can be explained by the IC emission of a `relic' population of
electrons driven into the incipient intracluster medium at the epoch of
starburst activity by galactic winds, and later on reenergized by adiabatic
compression and/or large-scale shocks transmitted through the cluster as the
consequence of more recent merger events. For high magnetic fields B > 1 muG
the interpretation of the radio fluxes of Coma requires a second population of
electrons injected recently. They can be explained as secondaries produced by a
population of relativistic protons. We calculate the fluxes of gamma-rays to be
expected in both the low and high magnetic field scenarios, and discuss
possibilities to distinguish between these two principal options by future
gamma-ray observations.Comment: LaTeX, 6 figures; accepted for publication in Ap
The Coronae of AR Lac
We observed the coronally active eclipsing binary, AR Lac, with the High
Energy Transmission Grating on Chandra for a total of 97 ks, spaced over five
orbits, at quadratures and conjunctions. Contemporaneous and simultaneous EUV
spectra and photometry were also obtained with the Extreme Ultraviolet
Explorer. Significant variability in both X-ray and EUV fluxes were observed,
dominated by at least one X-ray flare and one EUV flare. We saw no evidence of
primary or secondary eclipses. X-ray flux modulation was largest at high
temperature, indicative of flare heating of coronal plasma. Line widths
interpreted in terms of Doppler broadening suggest that both binary stellar
components are active. From line fluxes obtained from total integrated spectra,
we have modeled the emission measure and abundance distributions. A strong
maximum was found in the differential emission measure, characterized by peaks
at log T = 6.9 and 7.4, together with a weak but significant cooler maximum
near log T=6.2, and a moderately strong hot tail from log T= 7.6-8.2. Coronal
abundances have a broad distribution and show no simple correlation with first
ionization potential. While the resulting model spectrum generally agrees very
well with the observed spectrum, there are some significant discrepancies,
especially among the many Fe L-lines. Both the emission measure and abundance
distributions are qualitatively similar to prior determinations from other
X-ray and ultraviolet spectra, indicating some long-term stability in the
overall coronal structure.Comment: 31 pages, 8 figures, 3 tables; Accepted for publication in the
Astrophysical Journal (tentatively October 1, 2003
Host genetic and environmental factors shape the human gut resistome
BACKGROUND: Understanding and controlling the spread of antimicrobial resistance is one of the greatest challenges of modern medicine. To this end many efforts focus on characterising the human resistome or the set of antibiotic resistance determinants within the microbiome of an individual. Aside from antibiotic use, other host environmental and genetic factors that may shape the resistome remain relatively underexplored. METHODS: Using gut metagenome data from 250 TwinsUK female twins, we quantified known antibiotic resistance genes to estimate gut microbiome antibiotic resistance potential for 41 types of antibiotics and resistance mechanisms. Using heritability modelling, we assessed the influence of host genetic and environmental factors on the gut resistome. We then explored links between gut resistome, host health and specific environmental exposures using linear mixed effect models adjusted for age, BMI, alpha diversity and family structure. RESULTS: We considered gut microbiome antibiotic resistance to 21 classes of antibiotics, for which resistance genes were detected in over 90% of our population sample. Using twin modelling, we estimated that on average about 25% of resistome variability could be attributed to host genetic influences. Greatest heritability estimates were observed for resistance potential to acriflavine (70%), dalfopristin (51%), clindamycin (48%), aminocoumarin (48%) and the total score summing across all antibiotic resistance genes (38%). As expected, the majority of resistome variability was attributed to host environmental factors specific to an individual. We compared antibiotic resistance profiles to multiple environmental exposures, lifestyle and health factors. The strongest associations were observed with alcohol and vegetable consumption, followed by high cholesterol medication and antibiotic usage. Overall, inter-individual variation in host environment showed modest associations with antibiotic resistance profiles, and host health status had relatively minor signals. CONCLUSION: Our results identify host genetic and environmental influences on the human gut resistome. The findings improve our knowledge of human factors that influence the spread of antibiotic resistance genes and may contribute towards helping to attenuate it
CANGAROO-III search for TeV Gamma-rays from two clusters of galaxies
Because accretion and merger shocks in clusters of galaxies may accelerate
particles to high energies, clusters are candidate sites for the origin of
ultra-high-energy (UHE) cosmic-rays. A prediction was presented for gamma-ray
emission from a cluster of galaxies at a detectable level with the current
generation of imaging atmospheric Cherenkov telescopes. The gamma-ray emission
was produced via inverse Compton upscattering of cosmic microwave background
(CMB) photons by electron-positron pairs generated by collisions of UHE cosmic
rays in the cluster. We observed two clusters of galaxies, Abell 3667 and Abell
4038, searching for very-high-energy gamma-ray emission with the CANGAROO-III
atmospheric Cherenkov telescope system in 2006. The analysis showed no
significant excess around these clusters, yielding upper limits on the
gamma-ray emission. From a comparison of the upper limit for the north-west
radio relic region of Abell 3667 with a model prediction, we derive a lower
limit for the magnetic field of the region of ~0.1 micro G. This shows the
potential of gamma-ray observations in studies of the cluster environment. We
also discuss the flux upper limit from cluster center regions using a model of
gamma-ray emission from neutral pions produced in hadronic collisions of
cosmic-ray protons with the intra-cluster medium (ICM). The derived upper limit
of the cosmic-ray energy density within this framework is an order of magnitude
higher than that of our Galaxy.Comment: 7 pages, 6 figures, Accepted in Ap
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