2,869 research outputs found
Concept of modular flexure-based mechanisms for ultra-high precision robot design
This paper introduces a new concept of modular flexure-based mechanisms to design industrial ultra-high precision robots, which aims at significantly reducing both the complexity of their design and their development time. This modular concept can be considered as a robotic Lego, where a finite number of building bricks is used to quickly build a high-precision robot. The core of the concept is the transformation of a 3-D design problem into several 2-D ones, which are simpler and well-mastered. This paper will first briefly present the theoretical bases of this methodology and the requirements of both types of building bricks: the active and the passive bricks. The section dedicated to the design of the active bricks will detail the current research directions, mainly the maximisation of the strokes and the development of an actuation sub-brick. As for the passive bricks, some examples will be presented, and a discussion regarding the establishment of a mechanical solution catalogue will conclude the section. Last, this modular concept will be illustrated with a practical example, consisting in the design of a 5-degree of freedom ultra-high precision robot
Echoes of multiple outbursts of Sagittarius A* revealed by Chandra
The relatively rapid spatial and temporal variability of the X-ray radiation
from some molecular clouds near the Galactic center shows that this emission
component is due to the reflection of X-rays generated by a source that was
luminous in the past, most likely the central supermassive black hole,
Sagittarius A*. Studying the evolution of the molecular cloud reflection
features is therefore a key element to reconstruct Sgr A*'s past activity. The
aim of the present work is to study this emission on small angular scales in
order to characterize the source outburst on short time scales. We use Chandra
high-resolution data collected from 1999 to 2011 to study the most rapid
variations detected so far, those of clouds between 5' and 20' from Sgr A*
towards positive longitudes. Our systematic spectral-imaging analysis of the
reflection emission, notably of the Fe Kalpha line at 6.4 keV and its
associated 4-8 keV continuum, allows us to characterize the variations down to
15" angular scale and 1-year time scale. We reveal for the first time abrupt
variations of few years only and in particular a short peaked emission, with a
factor of 10 increase followed by a comparable decrease, that propagates along
the dense filaments of the Bridge cloud. This 2-year peaked feature contrasts
with the slower 10-year linear variations we reveal in all the other molecular
structures of the region. Based on column density constraints, we argue that
these two different behaviors are unlikely to be due to the same illuminating
event. The variations are likely due to a highly variable active phase of Sgr
A* sometime within the past few hundred years, characterized by at least two
luminous outbursts of a few-year time scale and during which the Sgr A*
luminosity went up to at least 10^39 erg/s.Comment: 17 pages, 16 figures, Accepted for publication in Astronomy &
Astrophysic
Quality interoperability within digital libraries: the DL.org perspective
Quality is the most dynamic aspect of DLs, and becomes even more complex with respect to interoperability. This paper formalizes the research motivations and hypotheses on quality interoperability conducted by the Quality Working Group within the EU-funded project DL.org (<a href="http://www.dlorg.eu">http://www.dlorg.eu/</a>). After providing a multi-level interoperability framework – adopted by DL.org - the authors illustrate key-research points and
approaches on the way to the interoperability of DLs quality, grounding them in the DELOS Reference Model. By applying the DELOS Reference Model Quality Concept Map to their interoperability motivating scenario, the authors subsequently present the two main research outcomes of their investigation - the Quality Core Model and the Quality Interoperability Survey
Variation of the X-ray non-thermal emission in the Arches cloud
The origin of the iron fluorescent line at 6.4 keV from an extended region
surrounding the Arches cluster is debated and the non-variability of this
emission up to 2009 has favored the low-energy cosmic-ray origin over a
possible irradiation by hard X-rays. By probing the variability of the Arches
cloud non-thermal emission in the most recent years, including a deep
observation in 2012, we intend to discriminate between the two competing
scenarios. We perform a spectral fit of XMM-Newton observations collected from
2000 to 2013 in order to build the Arches cloud lightcurve corresponding to
both the neutral Fe Kalpha line and the X-ray continuum emissions. We reveal a
30% flux drop in 2012, detected with more than 4 sigma significance for both
components. This implies that a large fraction of the studied non-thermal
emission is due to the reflection of an X-ray transient source.Comment: 5 pages, 3 figures, accepted for publication in MNRAS Letter
The primordial "f_NL" non-Gaussianity, and perturbations beyond the present horizon
We show a primordial non-linear "f_NL" term may produce unphysically large
CMB anisotropy for a red-tilted primordial power spectrum (n<1), because of
coupling to primordial fluctuation on the largest scale. We consider a
primordial power spectrum models of a running spectral index, and a transition
at very low wavenumbers. We find that only negative running spectral index
models are allowed, provided that there is no transition at a low wavenumbers
(i.e. k
-184, at 1 sigma level, on the transition scale of sharp cut-off models, using
recent CMB and SDSS data.Comment: v2: Journal information adde
Dust Emission from Active Galactic Nuclei
Unified schemes of active galactic nuclei (AGN) require an obscuring dusty
torus around the central source, giving rise to Seyfert 1 line spectrum for
pole-on viewing and Seyfert 2 characteristics in edge-on sources. Although the
observed IR is in broad agreement with this scheme, the behavior of the 10
micron silicate feature and the width of the far-IR emission peak remained
serious problems in all previous modeling efforts. We show that these problems
find a natural explanation if the dust is contained in about 5-10 clouds along
radial rays through the torus. The spectral energy distributions (SED) of both
type 1 and type 2 sources are properly reproduced from different viewpoints of
the same object if the visual optical depth of each cloud is larger than about
60 and the clouds' mean free path increases roughly in proportion to radial
distance.Comment: 11 pages, submitted to ApJ Letter
Modelling and analyzing adaptive self-assembling strategies with Maude
Building adaptive systems with predictable emergent behavior is a challenging task and it is becoming a critical need. The research community has accepted the challenge by introducing approaches of various nature: from software architectures, to programming paradigms, to analysis techniques. We recently proposed a conceptual framework for adaptation centered around the role of control data. In this paper we show that it can be naturally realized in a reflective logical language like Maude by using the Reflective Russian Dolls model. Moreover, we exploit this model to specify and analyse a prominent example of adaptive system: robot swarms equipped with obstacle-avoidance self-assembly strategies. The analysis exploits the statistical model checker PVesta
The XMM-Newton view of the central degrees of the Milky Way
The deepest XMM-Newton mosaic map of the central 1.5 deg of the Galaxy is
presented, including a total of about 1.5 Ms of EPIC-pn cleaned exposures in
the central 15" and about 200 ks outside. This compendium presents broad-band
X-ray continuum maps, soft X-ray intensity maps, a decomposition into spectral
components and a comparison of the X-ray maps with emission at other
wavelengths. Newly-discovered extended features, such as supernova remnants
(SNRs), superbubbles and X-ray filaments are reported. We provide an atlas of
extended features within +-1 degree of Sgr A*. We discover the presence of a
coherent X-ray emitting region peaking around G0.1-0.1 and surrounded by the
ring of cold, mid-IR-emitting material known from previous work as the "Radio
Arc Bubble" and with the addition of the X-ray data now appears to be a
candidate superbubble. Sgr A's bipolar lobes show sharp edges, suggesting that
they could be the remnant, collimated by the circumnuclear disc, of a SN
explosion that created the recently discovered magnetar, SGR J1745-2900. Soft
X-ray features, most probably from SNRs, are observed to fill holes in the dust
distribution, and to indicate a direct interaction between SN explosions and
Galactic center (GC) molecular clouds. We also discover warm plasma at high
Galactic latitude, showing a sharp edge to its distribution that correlates
with the location of known radio/mid-IR features such as the "GC Lobe". These
features might be associated with an inhomogeneous hot "atmosphere" over the
GC, perhaps fed by continuous or episodic outflows of mass and energy from the
GC region.Comment: MNRAS published online. See www.mpe.mpg.de/heg/gc/ for a higher
resolution version of the figure
The distribution of silicate strength in Spitzer spectra of AGNs and ULIRGs
A sample of 196 AGNs and ULIRGs observed by the Infrared Spectrograph (IRS)
on Spitzer is analyzed to study the distribution of the strength of the 9.7
micron silicate feature. Average spectra are derived for quasars, Seyfert 1 and
Seyfert 2 AGNs, and ULIRGs. We find that quasars are characterized by silicate
features in emission and Seyfert 1s equally by emission or weak absorption.
Seyfert 2s are dominated by weak silicate absorption, and ULIRGs are
characterized by strong silicate absorption (mean apparent optical depth about
1.5). Luminosity distributions show that luminosities at rest frame 5.5 micron
are similar for the most luminous quasars and ULIRGs and are almost 10^5 times
more luminous than the least luminous AGN in the sample. The distributions of
spectral characteristics and luminosities are compared to those of optically
faint infrared sources at z~2 being discovered by the IRS, which are also
characterized by strong silicate absorption. It is found that local ULIRGs are
a similar population, although they have lower luminosities and somewhat
stronger absorption compared to the high redshift sources.Comment: Accepted for publication on ApJ
Absorption lines from magnetically-driven winds in X-ray binaries
High resolution X-ray spectra of black hole X-ray binaries (BHBs) show
blueshifted absorption lines from disk winds which seem to be equatorial. Winds
occur in the Softer (disk-dominated) states of the outburst and are less
prominent or absent in the Harder (power-law dominated) states. We use
self-similar magneto-hydrodynamic (MHD) accretion-ejection models to explain
the disk winds in BHBs. In our models, the density at the base of the outflow
from the accretion disk is not a free parameter, but is determined by solving
the full set of dynamical MHD equations. Thus the physical properties of the
outflow are controlled by the global structure of the disk. We studied
different MHD solutions characterized by different values of (a) the disk
aspect ratio () and (b) the ejection efficiency (). We use two
kinds of MHD solutions depending on the absence (cold solution) or presence
(warm solution) of heating at the disk surface. Such heating could be from e.g.
dissipation of energy due to MHD turbulence in the disk or from illumination.
We use each of these MHD solutions to predict the physical parameters of an
outflow; put limits on the ionization parameter (), column density and
timescales, motivated by observational results; and thus select regions within
the outflow which are consistent with the observed winds. The cold MHD
solutions cannot account for winds due to their low ejection efficiency. But
warm solutions can explain the observed physical quantities in the wind because
they can have sufficiently high values of (, implying larger
mass loading at the base of the outflow). Further from our thermodynamic
equilibrium curve analysis for the outflowing gas, we found that in the Hard
state a range of is thermodynamically unstable, and had to be excluded.
This constrain made it impossible to have any wind at all, in the Hard state.Comment: 16 Pages, 10 figures in the main body and 4 figures in the appendix.
Accepted for publication in A&
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