1,512 research outputs found
Position and velocity space diffusion of test particles in stochastic electromagnetic fields
The two--dimensional diffusive dynamics of test particles in a random
electromagnetic field is studied. The synthetic electromagnetic fluctuations
are generated through randomly placed magnetised ``clouds'' oscillating with a
frequency . We investigate the mean square displacements of particles
in both position and velocity spaces. As increases the particles
undergo standard (Brownian--like) motion, anomalous diffusion and ballistic
motion in position space. Although in general the diffusion properties in
velocity space are not trivially related to those in position space, we find
that energization is present only when particles display anomalous diffusion in
position space. The anomalous character of the diffusion is only in the
non--standard values of the scaling exponents while the process is Gaussian.Comment: 10 pages, 4 figure
Knowledge connectedness within and across home country borders: Spatial heterogeneity and the technological scope of firm innovations
Monitoring and modelling of soil–plant interactions: the joint use of ERT, sap flow and eddy covariance data to characterize the volume of an orange tree root zone
Abstract. Mass and energy exchanges between soil, plants and atmosphere control a number of key environmental processes involving hydrology, biota and climate. The understanding of these exchanges also play a critical role for practical purposes e.g. in precision agriculture. In this paper we present a methodology based on coupling innovative data collection and models in order to obtain quantitative estimates of the key parameters of such complex flow system. In particular we propose the use of hydro-geophysical monitoring via "time-lapse" electrical resistivity tomography (ERT) in conjunction with measurements of plant transpiration via sap flow and evapotranspiration (ET) from eddy covariance (EC). This abundance of data is fed to spatially distributed soil models in order to characterize the distribution of active roots. We conducted experiments in an orange orchard in eastern Sicily (Italy), characterized by the typical Mediterranean semi-arid climate. The subsoil dynamics, particularly influenced by irrigation and root uptake, were characterized mainly by the ERT set-up, consisting of 48 buried electrodes on 4 instrumented micro-boreholes (about 1.2 m deep) placed at the corners of a square (with about 1.3 m long sides) surrounding the orange tree, plus 24 mini-electrodes on the surface spaced 0.1 m on a square grid. During the monitoring, we collected repeated ERT and time domain reflectometry (TDR) soil moisture measurements, soil water sampling, sap flow measurements from the orange tree and EC data. We conducted a laboratory calibration of the soil electrical properties as a function of moisture content and porewater electrical conductivity. Irrigation, precipitation, sap flow and ET data are available allowing for knowledge of the system's long-term forcing conditions on the system. This information was used to calibrate a 1-D Richards' equation model representing the dynamics of the volume monitored via 3-D ERT. Information on the soil hydraulic properties was collected from laboratory and field experiments. The successful results of the calibrated modelling exercise allow for the quantification of the soil volume interested by root water uptake (RWU). This volume is much smaller (with a surface area less than 2 m2, and about 40 cm thick) than expected and assumed in the design of classical drip irrigation schemes that prove to be losing at least half of the irrigated water which is not taken up by the plants
P02.123. The anti-diabetic and cholesterol-lowering effects of common and cassia cinnamon (Cinnamomum verum and C. aromaticum): a randomized controlled trial
This paper accompanies a poster presentation on the anti-diabetic and cholesterol-lowering effects of common and cassia cinnamon (Cinnamomum verum and C. aromaticum)
NuSTAR and multifrequency study of the two high-redshift blazars S5 0836+710 and PKS 2149-306
The most powerful blazars are the flat spectrum radio quasars whose emission
is dominated by a Compton component peaking between a few hundred keV and a few
hundred MeV. We selected two bright blazars, PKS 2149-306 at redshift z=2.345
and S5 0836+710 at z=2.172, in order to observe them in the hard X-ray band
with the NuSTAR satellite. In this band the Compton component is rapidly rising
almost up to the peak of the emission. Simultaneous soft-X-rays and UV-optical
observations were performed with the Swift satellite, while near-infrared (NIR)
data were obtained with the REM telescope. To study their variability, we
repeated these observations for both sources on a timescale of a few months.
While no fast variability was detected during a single observation, both
sources were found to be variable in the X-ray band, up to 50%, between the two
observations, with larger variability at higher energies. No variability was
detected in the optical/NIR band. These data together with Fermi-LAT, WISE and
other literature data are then used to study the overall spectral energy
distributions (SEDs) of these blazars. Although the jet non-thermal emission
dominates the SED, it leaves the UV band unhidden, allowing us to detect the
thermal emission of the disc and to estimate the mass of the black hole. The
non-thermal emission is well reproduced by a one-zone leptonic model. The
non-thermal radiative processes are synchrotron, self-Compton and external
Compton using seed photons from both the broad-line region (BLR) and the torus.
We find that our data are better reproduced if we assume that the location of
the dissipation region of the jet, R_diss, is in-between the torus, (at
R_torus), and the BLR (R_torus>R_diss>R_BLR). The observed variability is
explained by changing a minimum number of model parameters by a very small
amount.Comment: 11 pages, 5 figures, accepted for publication in Ap
The DLV System for Knowledge Representation and Reasoning
This paper presents the DLV system, which is widely considered the
state-of-the-art implementation of disjunctive logic programming, and addresses
several aspects. As for problem solving, we provide a formal definition of its
kernel language, function-free disjunctive logic programs (also known as
disjunctive datalog), extended by weak constraints, which are a powerful tool
to express optimization problems. We then illustrate the usage of DLV as a tool
for knowledge representation and reasoning, describing a new declarative
programming methodology which allows one to encode complex problems (up to
-complete problems) in a declarative fashion. On the foundational
side, we provide a detailed analysis of the computational complexity of the
language of DLV, and by deriving new complexity results we chart a complete
picture of the complexity of this language and important fragments thereof.
Furthermore, we illustrate the general architecture of the DLV system which
has been influenced by these results. As for applications, we overview
application front-ends which have been developed on top of DLV to solve
specific knowledge representation tasks, and we briefly describe the main
international projects investigating the potential of the system for industrial
exploitation. Finally, we report about thorough experimentation and
benchmarking, which has been carried out to assess the efficiency of the
system. The experimental results confirm the solidity of DLV and highlight its
potential for emerging application areas like knowledge management and
information integration.Comment: 56 pages, 9 figures, 6 table
A refined position catalog of the Swift XRT afterglows
We present a catalogue of refined positions of 68 gamma ray burst (GRB)
afterglows observed by the Swift X-ray Telescope (XRT) from the launch up to
2005 Oct 16. This is a result of the refinement of the XRT boresight
calibration. We tested this correction by means of a systematic study of a
large sample of X-ray sources observed by XRT with well established optical
counterparts. We found that we can reduce the systematic error radius of the
measurements by a factor of two, from 6.5" to 3.2" (90% of confidence). We
corrected all the positions of the afterglows observed by XRT in the first 11
months of the Swift mission. This is particularly important for the 37 X-ray
afterglows without optical counterpart. Optical follow-up of dark GRBs, in
fact, will be more efficient with the use of the more accurate XRT positions.Comment: 4 pages, 4 figures, 1 table ; accepted for publication in A&A
Letters. The revised version contains updated position
Swift observations of GRB 060614: an anomalous burst with a well behaved afterglow
GRB 060614 is a remarkable GRB observed by Swift with puzzling properties,
which challenge current progenitor models. The lack of any bright SN down to
very strict limits and the vanishing spectral lags are typical of short GRBs,
strikingly at odds with the long (102s) duration of this event. Here we present
spectral and temporal analysis of the Swift observations. We show that the
burst presents standard optical, UV and X-ray afterglows. An achromatic break
is observed simultaneously in optical and X-rays, at a time consistent with the
break in the R-band light curve measured by the VLT. The achromatic behaviour
and the consistent post-break decay slopes make GRB 060614 one of the best
examples of a jet break for a Swift burst. The optical, UV and X-rays afterglow
light curves have also an earlier break at ~30 ks. In the optical, there is
strong spectral evolution around this break, suggesting the passage of a break
frequency through the optical/UV band. The very blue spectrum at early times
and the trend in the light curves (rising at low frequencies, and decaying at
higher energies) suggest this may be the injection frequency. The early X-ray
light curve is well interpreted as the X-ray counterpart of the burst extended
emission. Spectral analysis of BAT/XRT data in the 80s overlap time show that
the Ep of the burst has decreased to as low as 8keV at the beginning of the XRT
observation. The Ep continues to decrease through the XRT energy band and exits
it at about 500s after the trigger. The average Ep of the burst is likely < 24
keV but larger than 8 keV. The initial peak observed by BAT is however
distinctly harder than the rest with Ep ~300 keV as measured by Konus Wind.
Considering the time-averaged spectral properties, GRB 060614 is consistent
with the Eiso-Ep_rest, Egamma-Ep_rest, and Liso-Ep correlations.Comment: 20 pages, 9 figures, 8 tables. Abstract shortened for posting on
astro-ph. Accepted for publication by A&
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