10,781 research outputs found
The effects of organic farming on the soil physical environment
The aim of this research was to investigate the effects of organic farming practices on the development of soil physical properties, and in particular, soil structure in comparison with conventional agricultural management. The soil structure of organically and conventionally managed soils at one site was compared in a quantitative manner at different scales of observations using image analysis. Key soil physical and chemical properties were measured as well as the pore fractal geometry to characterise pore roughness. Organically managed soils had higher organic matter content and provided a more stable soil structure than conventionally managed soils. The higher porosity (%) at the macroscale in soil under conventional management was due to fewer larger pores while mesoand microscale porosity was found to be greater under organic management. Organically managed soils typically provided spatially well distributed pores of all sizes and of greater roughness compared to those under conventional management. These variations in the soil physical environment are likely to impact significantly on the performance of these soils for a number of key processes such as crop establishment and water availabilit
X-ray, optical and infrared investigation of the candidate Supergiant Fast X-ray Transient IGR J18462-0223
We report on a broad-band X-ray study (0.5-60 keV) of the poorly known
candidate Supergiant Fast X-ray Transient (SFXT) IGR J18462-0223, and on
optical and near-infrared (NIR) followup observations of field objects. The
out-of-outburst X-ray state has been investigated for the first time with
archival INTEGRAL/IBIS, ASCA, Chandra and Swift/XRT observations. This allowed
us to place stringent 3 sigma upper limits on the soft (0.5-10 keV) and hard
(18-60 keV) X-ray emission of 2.9x10^-13 erg cm^-2 s^-1 and 8x10^-12 erg cm^-2
s^-1, respectively; the source was also detected during an intermediate soft
X-ray state with flux equal to 1.6x10^-11 erg cm^-2 s^-1 (0.5-10 keV). In
addition, we report on the INTEGRAL/IBIS discovery of three fast hard X-ray
flares (18-60 keV) having a duration in the range 1-12 hours: the flaring
behavior was also investigated in soft X-rays (3-10 keV) with archival
INTEGRAL/JEM-X observations. The duty cycle (1.2%) and the dynamic ranges (>
1,380 and > 190 in the energy bands 0.5-10 keV and 18-60 keV, respectively)
were measured for the first time. Archival UKIDSS JHK NIR data, together with
our deep R-band imaging of the field, unveiled a single, very red object inside
the intersection of the Swift/XRT and XMM-Newton error circles: this source has
optical/NIR photometric properties compatible with a very heavily absorbed blue
supergiant located at about 11 kpc, thus being a strong candidate counterpart
for IGR J18462-0223. NIR spectroscopy is advised to confirm the association.
Finally, a hint of a possible orbital period was found at about 2.13 days. If
confirmed by further studies, this would make IGR J18462-0223 the SFXT with the
shortest orbital period among the currently known systems.Comment: accepted for publication in A&A, 9 pages, 7 figures, 2 table
Dynamic van der Waals Theory of two-phase fluids in heat flow
We present a dynamic van der Waals theory. It is useful to study phase
separation when the temperature varies in space. We show that if heat flow is
applied to liquid suspending a gas droplet at zero gravity, a convective flow
occurs such that the temperature gradient within the droplet nearly vanishes.
As the heat flux is increased, the droplet becomes attached to the heated wall
that is wetted by liquid in equilibrium. In one case corresponding to partial
wetting by gas, an apparent contact angle can be defined. In the ther case with
larger heat flux, the droplet completely wets the heated wall expelling liquid.Comment: 6pages, 8figure
Direct conversion of rheological compliance measurements into storage and loss moduli
We remove the need for Laplace/inverse-Laplace transformations of
experimental data, by presenting a direct and straightforward mathematical
procedure for obtaining frequency-dependent storage and loss moduli
( and respectively), from time-dependent experimental
measurements. The procedure is applicable to ordinary rheological creep
(stress-step) measurements, as well as all microrheological techniques, whether
they access a Brownian mean-square displacement, or a forced compliance. Data
can be substituted directly into our simple formula, thus eliminating
traditional fitting and smoothing procedures that disguise relevant
experimental noise.Comment: 4 page
Statistics of Entropy Production in Linearized Stochastic System
We consider a wide class of linear stochastic problems driven off the
equilibrium by a multiplicative asymmetric force. The force brakes detailed
balance, maintained otherwise, thus producing entropy. The large deviation
function of the entropy production in the system is calculated explicitly. The
general result is illustrated using an example of a polymer immersed in a
gradient flow and subject to thermal fluctuations.Comment: 4 pages, 1 figur
Percolation properties of 3-D multiscale pore networks: how connectivity controls soil filtration processes
Quantifying the connectivity of pore networks is a key issue not only for modelling fluid flow and solute transport in porous media but also for assessing the ability of soil ecosystems to filter bacteria, viruses and any type of living microorganisms as well inert particles which pose a contamination risk. Straining is the main mechanical component of filtration processes: it is due to size effects, when a given soil retains a conveyed entity larger than the pores through which it is attempting to pass. We postulate that the range of sizes of entities which can be trapped inside soils has to be associated with the large range of scales involved in natural soil structures and that information on the pore size distribution has to be complemented by information on a critical filtration size (CFS) delimiting the transition between percolating and non percolating regimes in multiscale pore networks. We show that the mass fractal dimensions which are classically used in soil science to quantify scaling laws in observed pore size distributions can also be used to build 3-D multiscale models of pore networks exhibiting such a critical transition. We extend to the 3-D case a new theoretical approach recently developed to address the connectivity of 2-D fractal networks (Bird and Perrier, 2009). Theoretical arguments based on renormalisation functions provide insight into multi-scale connectivity and a first estimation of CFS. Numerical experiments on 3-D prefractal media confirm the qualitative theory. These results open the way towards a new methodology to estimate soil filtration efficiency from the construction of soil structural models to be calibrated on available multiscale data
Statistical mechanical theory of an oscillating isolated system. The relaxation to equilibrium
In this contribution we show that a suitably defined nonequilibrium entropy
of an N-body isolated system is not a constant of the motion in general and its
variation is bounded, the bounds determined by the thermodynamic entropy, i.e.,
the equilibrium entropy. We define the nonequilibrium entropy as a convex
functional of the set of n-particle reduced distribution functions
(n=0,......., N) generalizing the Gibbs fine-grained entropy formula.
Additionally, as a consequence of our microscopic analysis we find that this
nonequilibrium entropy behaves as a free entropic oscillator. In the approach
to the equilibrium regime we find relaxation equations of the Fokker-Planck
type, particularly for the one-particle distribution function
Slip boundary conditions for shear flow of polymer melts past atomically flat surfaces
Molecular dynamics simulations are carried out to investigate the dynamic
behavior of the slip length in thin polymer films confined between atomically
smooth thermal surfaces. For weak wall-fluid interactions, the shear rate
dependence of the slip length acquires a distinct local minimum followed by a
rapid growth at higher shear rates. With increasing fluid density, the position
of the local minimum is shifted to lower shear rates. We found that the ratio
of the shear viscosity to the slip length, which defines the friction
coefficient at the liquid/solid interface, undergoes a transition from a nearly
constant value to the power law decay as a function of the slip velocity. In a
wide range of shear rates and fluid densities, the friction coefficient is
determined by the product of the value of surface induced peak in the structure
factor and the contact density of the first fluid layer near the solid wall.Comment: 27 pages, 11 figure
Swift-XRT observation of 34 new INTEGRAL/IBIS AGNs: discovery of Compton thick and other peculiar sources
For a significant number of the sources detected at high energies (>10 keV)
by the INTEGRAL/IBIS and Swift/BAT instruments there is either a lack
information about them in the 2-10 keV range or they are totally unidentified.
Herein, we report on a sample of 34 IBIS AGN or AGN candidate objects for which
there is X-ray data in the Swift/XRT archive. Thanks to these X-ray follow up
observations, the identification of the gamma ray emitters has been possible
and the spectral shape in terms of photon index and absorption has been
evaluated for the first time for the majority of our sample sources. The
sample, enlarged to include 4 more AGN already discussed in the literature, has
been used to provide photon index and column density distribution. We obtain a
mean value of 1.88 with a dispersion of 0.12, i.e. typical of an AGN sample.
Sixteen objects (47%) have column densities in excess of 10^{22} cm^{-2} and,
as expected, a large fraction of the absorbed sources are within the Sey 2
sample. We have provided a new diagnostic tool (NH versus
F(2-10)keV/F(20-100)keV softness ratio) to isolate peculiar objects; we find at
least one absorbed Sey 1 galaxy, 3 Compton thick AGN candidates; and one secure
example of a "true" type 2 AGN. Within the sample of 10 still unidentified
objects, 3 are almost certainly AGN of type 2; 3 to 4 have spectral slopes
typical of AGN; and two are located high on the galactic plane and are strong
enough radio emitters so that can be considered good AGN candidates.Comment: 15 pages, 5 figures, ApJ accepte
IGR J22517+2218=MG3 J225155+2217: a new gamma-ray lighthouse in the distant Universe
We report on the identification of a new soft gamma ray source, namely IGR
J22517+2218, detected with IBIS/INTEGRAL. The source, which has an observed
20-100 keV flux of ~4 x10^-11 erg cm-2 s-1, is spatially coincident with MG3
J225155+2217, a quasar at z=3.668. The Swift/XRT 0.5-10 keV continuum is flat
(Gamma=1.5) with evidence for a spectral curvature below 1-2 keV either due to
intrinsic absorption (NH=3 +/- 2 x 10^22 cm-2) or to a change in slope (Delta
Gamma= 0.5). X-ray observations indicate flux variability over a 6 days period
which is further supported by a flux mismatch between Swift and INTEGRAL
spectra. IGR J22517+2218 is radio loud and has a flat radio spectrum; optically
it is a broad line emitting quasar with the atypical property of hosting a
narrow line absorption system. The Source Spectral Energy Distribution is
unusual compared to blazars of similar type: either it has the synchrotron peak
in the X/gamma-ray band (i.e. much higher than generally observed) or the
Compton peak in the MeV range (i.e. lower than typically measured). IGR
J22517+2218=MG3 J225155+2217 is the second most distant blazar detected above
20 keV and a gamma-ray lighthouse shining from the edge of our Universe.Comment: 4 pages, 4 figures, Accepted for publication in Astrophysical Journal
Letter
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