7,470 research outputs found
X-ray spectral modelling of the AGN obscuring region in the CDFS: Bayesian model selection and catalogue
AGN are known to have complex X-ray spectra that depend on both the
properties of the accreting SMBH (e.g. mass, accretion rate) and the
distribution of obscuring material in its vicinity ("torus"). Often however,
simple and even unphysical models are adopted to represent the X-ray spectra of
AGN. In the case of blank field surveys in particular, this should have an
impact on e.g. the determination of the AGN luminosity function, the inferred
accretion history of the Universe and also on our understanding of the relation
between AGN and their host galaxies. We develop a Bayesian framework for model
comparison and parameter estimation of X-ray spectra. We take into account
uncertainties associated with X-ray data and photometric redshifts. We also
demonstrate how Bayesian model comparison can be used to select among ten
different physically motivated X-ray spectral models the one that provides a
better representation of the observations. Despite the use of low-count
spectra, our methodology is able to draw strong inferences on the geometry of
the torus. For a sample of 350 AGN in the 4 Ms Chandra Deep Field South field,
our analysis identifies four components needed to represent the diversity of
the observed X-ray spectra: (abridged). Simpler models are ruled out with
decisive evidence in favour of a geometrically extended structure with
significant Compton scattering. Regarding the geometry of the obscurer, there
is strong evidence against both a completely closed or entirely open toroidal
geometry, in favour of an intermediate case. The additional Compton reflection
required by data over that predicted by toroidal geometry models, may be a sign
of a density gradient in the torus or reflection off the accretion disk.
Finally, we release a catalogue with estimated parameters such as the accretion
luminosity in the 2-10 keV band and the column density, , of the
obscurer.Comment: 28 pages, 18 figures, catalogue available from
https://www.mpe.mpg.de/~jbuchner/agn_torus/analysis/cdfs4Ms_cat/, software
available from https://github.com/JohannesBuchner/BX
Adaptive movement strategy may promote biodiversity in the rock-paper-scissors model
We study the role of the adaptive movement strategy in promoting biodiversity
in cyclic models described by the rock-paper-scissors game rules. We assume
that individuals of one out of the species may adjust their movement to escape
hostile regions and stay longer in their comfort zones. Running a series of
stochastic simulations, we calculate the alterations in the spatial patterns
and population densities in scenarios where not all organisms are physically or
cognitively conditioned to perform the behavioural strategy. Although the
adaptive movement strategy is not profitable in terms of territorial dominance
for the species, it may promote biodiversity. Our findings show that if all
individuals are apt to move adaptively, coexistence probability increases for
intermediary mobility. The outcomes also show that even if not all individuals
can react to the signals received from the neighbourhood, biodiversity is still
benefited, but for a shorter mobility range. We find that the improvement in
the coexistence conditions is more accentuated if organisms adjust their
movement intensely and can receive sensory information from longer distances.
We also discover that biodiversity is slightly promoted for high mobility if
the proportion of individuals participating in the strategy is low. Our results
may be helpful for biologists and data scientists to understand adaptive
process learning in system biology.Comment: 7 pages, 7 figure
Adaptive movement strategy in rock-paper-scissors models
Organisms may respond to local stimuli that benefit or threaten their
fitness. The adaptive movement behaviour may allow individuals to adjust their
speed to maximise the chances of being in comfort zones, where death risk is
minimal. We investigate spatial cyclic models where the rock-paper-scissors
game rules describe the nonhierarchical dominance. We assume that organisms of
one out of the species can control the mobility rate in response to the
information obtained from scanning the environment. Running a series of
stochastic simulations, we quantify the effects of the movement strategy on the
spatial patterns and population dynamics. Our findings show that the ability to
change mobility to adapt to environmental clues is not reflected in an
advantage in cyclic spatial games. The adaptive movement provokes a delay in
the spatial domains occupied by the species in the spiral waves, making the
group more vulnerable to the advance of the dominant species and less efficient
in taking territory from the dominated species. Our outcomes also show that the
effects of adaptive movement behaviour accentuate whether most individuals have
a long-range neighbourhood perception. Our results may be helpful for
biologists and data scientists to comprehend the dynamics of ecosystems where
adaptive processes are fundamental.Comment: 8 pages, 7 figure
Squeezing generation and revivals in a cavity-ion system in contact with a reservoir
We consider a system consisting of a single two-level ion in a harmonic trap,
which is localized inside a non-ideal optical cavity at zero temperature and
subjected to the action of two external lasers. We are able to obtain an
analytical solution for the total density operator of the system and show that
squeezing in the motion of the ion and in the cavity field is generated. We
also show that complete revivals of the states of the motion of the ion and of
the cavity field occur periodically.Comment: 9 pages, 3 figure
Metal hydrides for hydrogen storage at low charging pressures
LaNi5 alloys are been extensively study as hydrogen storage material due their easy activation and good kinetics. LaNi5 alloys, containing substitutional elements such as Al or Ce, allow adjustments of the equilibrium pressure and absorption kinetics of the metallic hydrides making them storage candidate materials susceptible to be used in fuel cells systems. In this work, the thermodynamic properties of LaNi5-yAly and LaNi5, La1-xCexNi5 alloys were evaluated using a purpose built Sievert-type apparatus
An electrochemical process for the production of synthetic fuels at low temperatures [Resumo]
Following work exploring the low temperature electrolysis in alkaline media, using graphite consumable anodes, from which syngas was obtained1, laboratory studies have been conducted in acid media pursuing higher efficiency in the production of hydrogen and synthetic fuels. Experiments were conducted in an own designed undivided planar cell with 25 cm2 geometrical area electrodes using a 0.5 M H2SO4 solution with and without Fe(II) additions. Fe2+ oxidizes to Fe3+ at the anode surface. The redox couple Fe3+/ Fe2+
acts as an oxidation mediator not only oxidizing the bulk and detached graphite but also the surface functional groups. The practical experimental potential for graphite oxidation is within the range for the electroxidation of the Fe redox couple giving as a result a 4-fold increase in the amount of produced CO2 at near room temperature, when using 0.025 M FeSO4
Hydrogen production by catalytic hydrolysis of sodium borohydride in batch reactors: new challenges
The present manuscript faces the study of H2 generation and storage from catalytic hydrolysis of sodium borohydride (NaBH4) under pressure. We revisit several works on this topic developed (or under development) by our team in the last four years on some of the most critical issues in this research area, namely catalyst durability/reutilization, gravimetric hydrogen storage density and recyclability. New results are also presented. Hydrogen generation rates and yields and hydrogen storage capacities can be augmented to reach 6 wt%, by adding small amounts of an organic polymer (CMC) to the classic NaBH4 hydrolysis, performed with stoichiometric amount of water in a batch reactor with a conical bottom shape and in the presence of Ni-Ru based catalyst, reused from 300 times. Sodium tetrahydroxoborate, NaB(OH)4), was produced in the presence of CMC additive, and did not show crystalline water in its crystal structure. This latter finding has potential to reduce recycling costs of NaBO2 back to NaBH4 and also increase the overall storage density of systems based on NaBH4 as hydrogen carrier
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