1,016 research outputs found
Minimum sensitivity design of attitude control systems Final report
Minimum sensitivity design of attitude control systems for spacecraf
Uncertainty and Sensitivity of the Feature Selective Validation (FSV) Method
The FSV method is a recognized validation tool that initially assesses the similarity between data sets for electromagnetic measurements and models. Its use may be extended to many problems and applications, and in particular, with relation to electrical systems, but it should be characterized in terms of its uncertainty, as for measurement tools. To this aim, the Guide to the Expression of Uncertainty in Measurement (GUM) is applied for the propagation of uncertainty from the experimental data to the Feature Selective Validation (FSV) quantities, using Monte Carlo analysis as confirmation, which ultimately remains the most reliable approach to determine the propagation of uncertainty, given the significant FSV non-linearity. Such non-linearity in fact compromises the accuracy of the Taylor approximation supporting the use of first-order derivatives (and derivative terms in general). MCM results are instead more stable and show sensitivity vs. input data uncertainty in the order of 10 to 100, highly depending on the local data samples value. To this aim, normalized sensitivity coefficients are also reported, in an attempt to attenuate the scale effects, redistributing the observed sensitivity values that, however, remain in the said range, up to about 100
Track insulation verification and measurement
Methods for the measurement of track insulation are reviewed pointing out minimum necessary conditions and practical aspects, as well as consistency and repeatability of results. Track configuration, need of electrical sectioning and use of external electrodes or conductive structure are key parameters to determine suitable methods
An Over-Massive Black Hole in a Typical Star-Forming Galaxy, 2 Billion Years After the Big Bang
Supermassive black holes (SMBHs) and their host galaxies are generally
thought to coevolve, so that the SMBH achieves up to about 0.2 to 0.5% of the
host galaxy mass in the present day. The radiation emitted from the growing
SMBH is expected to affect star formation throughout the host galaxy. The
relevance of this scenario at early cosmic epochs is not yet established. We
present spectroscopic observations of a galaxy at redshift z = 3.328, which
hosts an actively accreting, extremely massive BH, in its final stages of
growth. The SMBH mass is roughly one-tenth the mass of the entire host galaxy,
suggesting that it has grown much more efficiently than the host, contrary to
models of synchronized coevolution. The host galaxy is forming stars at an
intense rate, despite the presence of a SMBH-driven gas outflow.Comment: Author's version, including the main paper and the Supplementary
Materials (16+21 pages, 3+3 figures
Aging of materials at inlet and outlet fuel manifolds in a SOFC stack
Analyses performed on a stack operated for more than 4000h at 750\ub0C at high fuel utilization (80%) revealed significant differences in aging behaviour of stack components when comparing fuel inlet and fuel outlet areas. Two samples were cut from the fuel inlet and outlet manifolds of the operated SOFC stack. The glass-ceramic sealing material and the interconnect alloy were exposed to a dual atmosphere (external air and internal fuel stream). The fuel composition was dry H2/N2 (60:40 vol. %) at the inlet and H2O/H2/N2 (48:12:40 vol. %) at the outlet. The stack was operated in co-flow configuration, with a large excess of air. It was found that the interfaces between the seal and the alloy were significantly affected by the difference in fuel composition. The sealant exposed to the outgoing air was significantly polluted by chromium generated in the stack and transported by the air stream. These investigations therefore reveal that the long-term evolution of the sealing and of the metallic support depends on the local conditions in the stack, as well as on additional interactions with other degradation phenomena
The cosmic growth of the active black hole population at 1<z<2 in zCOSMOS, VVDS and SDSS
We present a census of the active black hole population at 1<z<2, by
constructing the bivariate distribution function of black hole mass and
Eddington ratio, employing a maximum likelihood fitting technique. The study of
the active black hole mass function (BHMF) and the Eddington ratio distribution
function (ERDF) allows us to clearly disentangle the active galactic nuclei
(AGN) downsizing phenomenon, present in the AGN luminosity function, into its
physical processes of black hole mass downsizing and accretion rate evolution.
We are utilizing type-1 AGN samples from three optical surveys (VVDS, zCOSMOS
and SDSS), that cover a wide range of 3 dex in luminosity over our redshift
interval of interest. We investigate the cosmic evolution of the AGN population
as a function of AGN luminosity, black hole mass and accretion rate. Compared
to z = 0, we find a distinct change in the shape of the BHMF and the ERDF,
consistent with downsizing in black hole mass. The active fraction or duty
cycle of type-1 AGN at z~1.5 is almost flat as a function of black hole mass,
while it shows a strong decrease with increasing mass at z=0. We are witnessing
a phase of intense black hole growth, which is largely driven by the onset of
AGN activity in massive black holes towards z=2. We finally compare our results
to numerical simulations and semi-empirical models and while we find reasonable
agreement over certain parameter ranges, we highlight the need to refine these
models in order to match our observations.Comment: 31 pages, 28 figures, accepted for publication in MNRA
AGN X-ray variability in the XMM-COSMOS survey
We took advantage of the observations carried out by XMM in the COSMOS field
during 3.5 years, to study the long term variability of a large sample of AGN
(638 sources), in a wide range of redshift (0.1<z<3.5) and X-ray luminosity
(L(2-10)). Both a simple statistical method to asses the
significance of variability, and the Normalized Excess Variance
() parameter, where used to obtain a quantitative measurement
of the variability. Variability is found to be prevalent in most AGN, whenever
we have good statistic to measure it, and no significant differences between
type-1 and type-2 AGN were found. A flat (slope -0.23+/-0.03) anti-correlation
between and X-ray luminosity is found, when significantly
variable sources are considered all together. When divided in three redshift
bins, the anti-correlation becomes stronger and evolving with z, with higher
redshift AGN being more variable. We prove however that this effect is due to
the pre-selection of variable sources: considering all the sources with
available measurement, the evolution in redshift disappears.
For the first time we were also able to study the long term X-ray variability
as a function of and Eddington ratio, for a large sample of AGN
spanning a wide range of redshift. An anti-correlation between
and is found, with the same slope of the
anti-correlation between and X-ray luminosity, suggesting
that the latter can be a byproduct of the former one. No clear correlation is
found between and the Eddington ratio in our sample.
Finally, no correlation is found between the X-ray and the
optical variability.Comment: 14 Pages, 13 figures. Accepted to the Astrophysical Journal on
December 6, 201
Blowin' in the wind: both `negative' and `positive' feedback in an obscured high-z Quasar
Quasar feedback in the form of powerful outflows is invoked as a key
mechanism to quench star formation in galaxies, preventing massive galaxies to
over-grow and producing the red colors of ellipticals. On the other hand, some
models are also requiring `positive' AGN feedback, inducing star formation in
the host galaxy through enhanced gas pressure in the interstellar medium.
However, finding observational evidence of the effects of both types of
feedback is still one of the main challenges of extragalactic astronomy, as few
observations of energetic and extended radiatively-driven winds are available.
Here we present SINFONI near infrared integral field spectroscopy of XID2028,
an obscured, radio-quiet z=1.59 QSO detected in the XMM-COSMOS survey, in which
we clearly resolve a fast (1500 km/s) and extended (up to 13 kpc from the black
hole) outflow in the [OIII] lines emitting gas, whose large velocity and
outflow rate are not sustainable by star formation only. The narrow component
of Ha emission and the rest frame U band flux from HST-ACS imaging enable to
map the current star formation in the host galaxy: both tracers independently
show that the outflow position lies in the center of an empty cavity surrounded
by star forming regions on its edge. The outflow is therefore removing the gas
from the host galaxy (`negative feedback'), but also triggering star formation
by outflow induced pressure at the edges (`positive feedback'). XID2028
represents the first example of a host galaxy showing both types of feedback
simultaneously at work.Comment: 9 pages, 5 figures, accepted for publication in Ap
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