37,228 research outputs found
Improving the Scalability of Multi-Agent Systems
There is an increasing demand for designers and developers to construct ever larger multi-agent systems. Such systems will be composed of hundreds or even thousands of autonomous agents. Moreover, in open and dynamic environments, the number of agents in the system at any one time will fluctuate significantly. To cope with these twin issues of scalability and variable numbers, we hypothesize that multi-agent systems need to be both /self-building/ (able to determine the most appropriate organizational structure for the system by themselves at run-time) and /adaptive/ (able to change this structure as their environment changes). To evaluate this hypothesis we have implemented such a multi-agent system and have applied it to the domain of automated trading. Preliminary results supporting the first part of this hypothesis are presented: adaption and self-organization do indeed make the system better able to cope with large numbers of agents
Dust Transport in Protostellar Disks Through Turbulence and Settling
We apply ionization balance and MHD calculations to investigate whether
magnetic activity moderated by recombination on dust can account for the mass
accretion rates and the mid-infrared spectra and variability of protostellar
disks. The MHD calculations use the stratified shearing-box approach and
include grain settling and the feedback from the changing dust abundance on the
resistivity of the gas. The two-decade spread in accretion rates among T Tauri
stars is too large to result solely from variety in the grain size and stellar
X-ray luminosity, but can be produced by varying these together with the disk
magnetic flux. The diversity in the silicate bands can come from the coupling
of grain settling to the distribution of the magneto-rotational turbulence,
through three effects: (1) Recombination on grains yields a magnetically
inactive dead zone extending above two scale heights, while turbulence in the
magnetically active disk atmosphere overshoots the dead zone boundary by only
about one scale height. (2) Grains deep in the dead zone oscillate vertically
in waves driven by the turbulent layer above, but on average settle at the
laminar rates, so the interior of the dead zone is a particle sink and the disk
atmosphere becomes dust-depleted. (3) With sufficient depletion, the dead zone
is thinner and mixing dredges grains off the midplane. The MHD results also
show that the magnetic activity intermittently lifts clouds of dust into the
atmosphere. The photosphere height changes by up to one-third over a few
orbits, while the extinction along lines of sight grazing the disk surface
varies by factors of two over times down to 0.1 orbit. We suggest that the
changing shadows cast by the dust clouds on the outer disk are a cause of the
daily to monthly mid-infrared variability in some young stars. (Abridged.)Comment: ApJ in pres
Transient Relativistically-Shifted Lines as a Probe of Black Hole Systems
X-ray spectra of Seyfert galaxies have revealed a new type of X-ray spectral
feature, one which appears to offer important new insight into the black hole
system. XMM/Chandra revealed several narrow emission lines redward of Fe Kalpha
in NGC 3516. Since that discovery the phenomenon has been observed in other
Seyfert galaxies, e.g. NGC 7314 and ESO 198-G24. We present new evidence for a
redshifted Fe line in XMM spectra of Mrk 766. These data reveal the first
evidence for a significant shift in the energy of such a line, occurring over a
few tens of kiloseconds. This shift may be interpreted as deceleration of an
ejected blob of gas traveling close to the escape velocity.Comment: 13 pages, 5 figures (4 color) accepted by Ap
The Global Implications of the Hard X-ray Excess in Type 1 AGN
Recent evidence for a strong 'hard excess' of flux at energies > 20 keV in
some Suzaku observations of type 1 Active Galactic Nuclei (AGN) has motivated
an exploratory study of the phenomenon in the local type 1 AGN population. We
have selected all type 1 AGN in the Swift Burst Alert Telescope (BAT) 58-month
catalog and cross-correlated them with the holdings of the Suzaku public
archive. We find the hard excess phenomenon to be a ubiquitous property of type
1 AGN. Taken together, the spectral hardness and equivalent width of Fe K alpha
emission are consistent with reprocessing by an ensemble of Compton-thick
clouds that partially cover the continuum source. In the context of such a
model, ~ 80 % of the sample has a hardness ratio consistent with > 50% covering
of the continuum by low-ionization, Compton-thick gas. More detailed study of
the three hardest X-ray spectra in our sample reveal a sharp Fe K absorption
edge at ~ 7 keV in each of them, indicating that blurred reflection is not
responsible for the very hard spectral forms. Simple considerations place the
distribution of Compton-thick clouds at or within the optical broad line
region.Comment: Accepted for publication in Ap
The Variable X-ray Spectrum of Markarian 766 - II. Time-Resolved Spectroscopy
CONTEXT: The variable X-ray spectra of AGN systematically show steep
power-law high states and hard-spectrum low states. The hard low state has
previously been found to be a component with only weak variability. The origin
of this component and the relative importance of effects such as absorption and
relativistic blurring are currently not clear. AIMS: In a follow-up of previous
principal components analysis, we aim to determine the relative importance of
scattering and absorption effects on the time-varying X-ray spectrum of the
narrow-line Seyfert 1 galaxy Mrk~766. METHODS: Time-resolved spectroscopy,
slicing XMM and Suzaku data down to 25 ks elements, is used to investigate
whether absorption or scattering components dominate the spectral variations in
Mrk 766.Time-resolved spectroscopy confirms that spectral variability in Mrk
766 can be explained by either of two interpretations of principal components
analysis. Detailed investigation confirm rapid changes in the relative
strengths of scattered and direct emission or rapid changes in absorber
covering fraction provide good explanations of most of the spectral
variability. However, a strong correlation between the 6.97 keV absorption line
and the primary continuum together with rapid opacity changes show that
variations in a complex and multi-layered absorber, most likely a disk wind,
are the dominant source of spectral variability in Mrk 76
X-ray variability analysis of a large series of XMM-Newton + NuSTAR observations of NGC 3227
We present a series of X-ray variability results from a long XMM-Newton +
NuSTAR campaign on the bright, variable AGN NGC 3227. We present an analysis of
the lightcurves, showing that the source displays typically
softer-when-brighter behaviour, although also undergoes significant spectral
hardening during one observation which we interpret as due to an occultation
event by a cloud of absorbing gas. We spectrally decompose the data and show
that the bulk of the variability is continuum-driven and, through rms
variability analysis, strongly enhanced in the soft band. We show that the
source largely conforms to linear rms-flux behaviour and we compute X-ray power
spectra, detecting moderate evidence for a bend in the power spectrum,
consistent with existing scaling relations. Additionally, we compute X-ray
Fourier time lags using both the XMM-Newton and - through maximum-likelihood
methods - NuSTAR data, revealing a strong low-frequency hard lag and evidence
for a soft lag at higher frequencies, which we discuss in terms of
reverberation models.Comment: Accepted for publication in MNRAS; 19 pages, 13 figures, 4 tables;
minor typographical errors corrected and reference list update
Improving root cause analysis through the integration of PLM systems with cross supply chain maintenance data
The purpose of this paper is to demonstrate a system architecture for integrating Product Lifecycle Management (PLM) systems with cross supply chain maintenance information to support root-cause analysis. By integrating product-data from PLM systems with warranty claims, vehicle diagnostics and technical publications, engineers were able to improve the root-cause analysis and close the information gaps. Data collection was achieved via in-depth semi-structured interviews and workshops with experts from the automotive sector. Unified Modelling Language (UML) diagrams were used to design the system architecture proposed. A user scenario is also presented to demonstrate the functionality of the system
Evidence for Orbital Motion of Material Close to the Central Black Hole of Mrk 766
Time-resolved X-ray spectroscopy has been obtained for the narrow line
Seyfert galaxy Mrk766 from XMM-Newton observations. We present analysis in the
energy-time plane of EPIC pn data in the 4-8 keV band with energy resolution
R~50. A component of Fe Ka emission detected in the maps shows a variation of
photon energy with time that appears both to be statistically significant and
to be consistent with sinusoidal variation. We investigate the interpretation
that there exists a component of line emission from matter in a Keplerian orbit
around a supermassive black hole. The orbit has a period ~165 ks and a
line-of-sight velocity ~13,500 km/s. This yields a lower limit for the central
mass of M > 4.9x10^5 solar masses within a radius of 3.6 x 10^13 cm (2.4 A.U.).
The orbit parameters are consistent with higher black hole masses, but the lack
of any substantial gravitational redshift of the orbit implies an upper limit
to the black hole mass of 4.5x10^7 solar masses.Comment: 20 pages, 6 figures (some colour). Accepted for publication in A&A.
Only minor changes since V1 (including reordering of Figs 1a & b
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