4,264 research outputs found
Probing the Interstellar Dust towards the Galactic Centre: Dust Scattering Halo around AX J1745.6-2901
AX J1745.6-2901 is an X-ray binary located at only 1.45 arcmin from Sgr A*,
showcasing a strong X-ray dust scattering halo. We combine Chandra and
XMM-Newton observations to study the halo around this X-ray binary. Our study
shows two major thick dust layers along the line of sight (LOS) towards AX
J1745.6-2901. The LOS position and of these two layers depend on the
dust grain models with different grain size distribution and abundances. But
for all the 19 dust grain models considered, dust Layer-1 is consistently found
to be within a fractional distance of 0.11 (mean value: 0.05) to AX
J1745.6-2901 and contains only (19-34)% (mean value: 26%) of the total LOS
dust. The remaining dust is contained in Layer-2, which is distributed from the
Earth up to a mean fractional distance of 0.64. A significant separation
between the two layers is found for all the dust grain models, with a mean
fractional distance of 0.31. Besides, an extended wing component is discovered
in the halo, which implies a higher fraction of dust grains with typical sizes
590 \AA\ than considered in current dust grain models. Assuming AX
J1745.6-2901 is 8 kpc away, dust Layer-2 would be located in the Galactic disk
several kpc away from the Galactic Centre (GC). The dust scattering halo biases
the observed spectrum of AX J1745.6-2901 severely in both spectral shape and
flux, and also introduces a strong dependence on the size of the instrumental
point spread function and the source extraction region. We build Xspec models
to account for this spectral bias, which allow us to recover the intrinsic
spectrum of AX J1745.6-2901 free from dust scattering opacity. If dust Layer-2
also intervenes along the LOS to Sgr A* and other nearby GC sources, a
significant spectral correction for the dust scattering opacity would be
necessary for all these GC sources.Comment: 20 pages, published by MNRAS; revised values in Table-1 and Table-B
Towards Translating Graph Transformation Approaches by Model Transformations
Recently, many researchers are working on semantics preserving model transformation. In the field of graph transformation one can think of translating graph grammars written in one approach to a behaviourally equivalent graph grammar in another approach. In this paper we translate graph grammars developed with the GROOVE tool to AGG graph grammars by first investigating the set of core graph transformation concepts supported by both tools. Then, we define what it means for two graph grammars to be behaviourally equivalent, and for the regarded approaches we actually show how to handle different definitions of both - application conditions and graph structures. The translation itself is explained by means of intuitive examples
Npp1 promotes atherosclerosis in ApoE knockout mice.
Ecto-nucleotide pyrophosphatase/phosphodiesterase 1 (NPP1) generates inorganic pyrophosphate (PP(i)), a physiologic inhibitor of hydroxyapatite deposition. In a previous study, we found NPP1 expression to be inversely correlated with the degree of atherosclerotic plaque calcification. Moreover, function-impairing mutations of ENPP1, the gene encoding for NPP1, are associated with severe, artery tunica media calcification and myointimal hyperplasia with infantile onset in human beings. NPP1 and PP(i) have the potential to modulate atherogenesis by regulating arterial smooth muscle cell (SMC) differentiation and function, including increase of pro-atherogenic osteopontin (OPN) expression. Hence, this study tested the hypothesis that NPP1 deficiency modulates both atherogenesis and atherosclerotic intimal plaque calcification. Npp1/ApoE double deficient mice were generated by crossing mice bearing the ttw allele of Enpp1 (that encodes a truncation mutation) with ApoE null mice and fed with high-fat/high-cholesterol atherogenic diet. Atherosclerotic lesion area and calcification were examined at 13, 18, 23 and 28 weeks of age. The aortic SMCs isolated from both ttw/ttw ApoE(-/-) and ttw/+ ApoE(-/-) mice demonstrated decreased Opn expression. The 28-week-old ttw/ttw ApoE(-/-) and ttw/+ ApoE(-/-) had significantly smaller atherosclerotic lesions compared with wild-type congenic ApoE(-/-) mice. Only ttw/ttw but not ttw/+ mice developed artery media calcification. Furthermore in ttw/+ mice, there was a tendency towards increased plaque calcification compared to ApoE(-/-) mice without Npp1 deficiency. We conclude that Npp1 promotes atherosclerosis, potentially mediated by Opn expression in ApoE knockout mice
Effects of Interstellar Dust Scattering on the X-ray Eclipses of the LMXB AX J1745.6-2901 in the Galactic Center
AX J1745.6-2901 is an eclipsing low mass X-ray binary (LMXB) in the Galactic
Centre (GC). It shows significant X-ray excess emission during the eclipse
phase, and its eclipse light curve shows an asymmetric shape. We use archival
XMM-Newton and Chandra observations to study the origin of these peculiar X-ray
eclipsing phenomena. We find that the shape of the observed X-ray eclipse light
curves depends on both photon energy and the shape of the source extraction
region, and also shows differences between the two instruments. By performing
detailed simulations for the time-dependent X-ray dust scattering halo, as well
as directly modelling the observed eclipse and non-eclipse halo profiles of AX
J1745.6-2901, we obtained solid evidence that its peculiar eclipse phenomena
are indeed caused by the X-ray dust scattering in multiple foreground dust
layers along the line-of-sight (LOS). The apparent dependence on the
instruments is caused by different instrumental point-spread-functions. Our
results can be used to assess the influence of dust scattering in other
eclipsing X-ray sources, and raise the importance of considering the timing
effects of dust scattering halo when studying the variability of other X-ray
sources in the GC, such as Sgr A*. Moreover, our study of halo eclipse
reinforces the existence of a dust layer local to AX J1745.6-2901 as reported
by Jin et al. (2017), as well as identifying another dust layer within a few
hundred parsecs to Earth, containing up to several tens of percent LOS dust,
which is likely to be associated with the molecular clouds in the Solar
neighbourhood. The remaining LOS dust is likely to be associated with the
molecular clouds located in the Galactic disk in-between.Comment: 25 pages, 18 figures, 5 tables, accepted by MNRA
Structured jets and VHE emission of blazars and radiogalaxies
Recent observations in the TeV band challenge the simplest models developed
to describe the overall emission of blazars and radiogalaxies. In particular,
the observation of variable TeV emission from M87 and the fast variability
shown by PKS 2155-304 challenge the standard framework. We discuss how the
existence of a radial structure in the sub-pc scale jet, with faster a
component ("spine" or "needles") embedded in a slower layer can explain the
basic phenomenology of these sources.Comment: 4 pages, 3 figures. To be published in the proceedings of the 4th
Heidelberg International Symposium on High Energy Gamma-Ray Astronomy, July
7-11, 200
Strain Analysis by a Total Generalized Variation Regularized Optical Flow Model
In this paper we deal with the important problem of estimating the local
strain tensor from a sequence of micro-structural images realized during
deformation tests of engineering materials. Since the strain tensor is defined
via the Jacobian of the displacement field, we propose to compute the
displacement field by a variational model which takes care of properties of the
Jacobian of the displacement field. In particular we are interested in areas of
high strain. The data term of our variational model relies on the brightness
invariance property of the image sequence. As prior we choose the second order
total generalized variation of the displacement field. This prior splits the
Jacobian of the displacement field into a smooth and a non-smooth part. The
latter reflects the material cracks. An additional constraint is incorporated
to handle physical properties of the non-smooth part for tensile tests. We
prove that the resulting convex model has a minimizer and show how a
primal-dual method can be applied to find a minimizer. The corresponding
algorithm has the advantage that the strain tensor is directly computed within
the iteration process. Our algorithm is further equipped with a coarse-to-fine
strategy to cope with larger displacements. Numerical examples with simulated
and experimental data demonstrate the very good performance of our algorithm.
In comparison to state-of-the-art engineering software for strain analysis our
method can resolve local phenomena much better
A general framework for quantifying uncertainty at scale
In many fields of science, comprehensive and realistic computational models
are available nowadays. Often, the respective numerical calculations call for
the use of powerful supercomputers, and therefore only a limited number of
cases can be investigated explicitly. This prevents straightforward approaches
to important tasks like uncertainty quantification and sensitivity analysis.
This challenge can be overcome via our recently developed sensitivity-driven
dimension adaptive sparse grid interpolation strategy. The method exploits, via
adaptivity, the structure of the underlying model (such as lower intrinsic
dimensionality and anisotropic coupling of the uncertain inputs) to enable
efficient and accurate uncertainty quantification and sensitivity analysis at
scale. We demonstrate the efficiency of our approach in the context of fusion
research, in a realistic, computationally expensive scenario of turbulent
transport in a magnetic confinement tokamak device with eight uncertain
parameters, reducing the effort by at least two orders of magnitude. In
addition, we show that our method intrinsically provides an accurate surrogate
model that is nine orders of magnitude cheaper than the high-fidelity model.Comment: 19 pages, 6 figures, 1 tabl
Rapid variability at very high energies in Mrk 501
A major flaring state of the BL Lac object Mrk 501 was observed by the High
Energy Stereoscopic System (H.E.S.S.) in June, 2014. Flux levels higher than
one Crab unit were recorded and rapid variability at very high energies
(2-20 TeV) was revealed. The high statistics afforded by the flares
allowed us to probe the presence of minutes timescale variability and study its
statistical characteristics exclusively at TeV energies owing to the high
energy threshold of approximately 2 TeV. Doubling times of a few minutes are
estimated for fluxes greater than 2 TeV. Statistical tests on the light curves
show interesting temporal structure in the variations including deviations from
a normal flux distribution similar to those found in the PKS 2155-304 flare of
July 2006, at nearly an order of magnitude higher threshold energy. Rapid
variations at such high energies put strong constraints on the physical
mechanisms in the blazar jet.Comment: In Proceedings of the 34th International Cosmic Ray Conference
(ICRC2015), The Hague, The Netherland
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