638 research outputs found
Spectral boundary conditions and solitonic solutions in a classical Sellmeier dielectric
Electromagnetic field interactions in a dielectric medium represent a
longstanding field of investigation, both at the classical level and at the
quantum one. We propose a 1+1 dimensional toy-model which consists of an
half-line filling dielectric medium, with the aim to set up a simplified
situation where technicalities related to gauge invariance and, as a
consequence, physics of constrained systems are avoided, and still interesting
features appear. In particular, we simulate the electromagnetic field and the
polarization field by means of two coupled scalar fields ,
respectively, in a Hopfield-like model. We find that, in order to obtain a
physically meaningful behaviour for the model, one has to introduce spectral
boundary conditions depending on the particle spectrum one is dealing with.
This is the first interesting achievement of our analysis. The second relevant
achievement is that, by introducing a nonlinear contribution in the
polarization field , with the aim of mimicking a third order nonlinearity
in a nonlinear dielectric, we obtain solitonic solutions in the Hopfield model
framework, whose classical behaviour is analyzed too.Comment: 12 pages, 1 figur
Compact formulae, dynamics and radiation of charged particles under synchro-curvature losses
We consider the fundamental problem of charged particles moving along and
around a curved magnetic field line, revising the synchro-curvature radiation
formulae introduced by Cheng and Zhang (1996). We provide more compact
expressions to evaluate the spectrum emitted by a single particle, identifying
the key parameter that controls the transition between the curvature-dominated
and the synchrotron-dominated regime. This parameter depends on the local
radius of curvature of the magnetic field line, the gyration radius, and the
pitch angle. We numerically solve the equations of motion for the emitting
particle by considering self-consistently the radiative losses, and provide the
radiated spectrum produced by a particle when an electric acceleration is
balanced by its radiative losses, as it is assumed to happen in the outer gaps
of pulsar's magnetospheres. We compute the average spectrum radiated throughout
the particle trajectory finding that the slope of the spectrum before the peak
depends on the location and size of the emission region. We show how this
effect could then lead to a variety of synchro-curvature spectra. Our results
reinforce the idea that the purely synchrotron or curvature losses are, in
general, inadequate to describe the radiative reaction on the particle motion,
and the spectrum of emitted photons. Finally, we discuss the applicability of
these calculations to different astrophysical scenarios.Comment: 9 pages, 5 figures, 2 tables. Accepted for publication in MNRAS main
journal. References update
Synchro-curvature emitting regions in high-energy pulsar models
The detected high-energy pulsars' population is growing in number, and thus,
having agile and physically relevant codes to analyze it consistently is
important. Here, we update our existing synchro-curvature radiation model by
including a better treatment of the particle injection, particularly where the
large pitch angle particles dominate the spectra, and by implementing a fast
and accurate minimization technique. The latter allows a large improvement in
computational cost, needed to test model enhancements and to apply the model to
a larger pulsar population. We successfully fit the sample of pulsars with
X-ray and -ray data. Our results indicate that, for every emitting
particle, the spatial extent of their trajectory where the pitch angle is large
and most of the detected X-ray radiation is produced is a small fraction of the
light cylinder. We also confirm with this new approach that synchrotron
radiation is not negligible for most of the gamma-ray pulsars detected. In
addition, with the results obtained, we argue that J0357+3205 and J2055+2539
are MeV-pulsar candidates and are suggested for exhaustive observations in this
energy band.Comment: 12 pages, 7 figures. Accepted for publication in Monthly Notices of
the Royal Astronomical Societ
Pushing context-awareness down to the core: moreflexibility for the PerLa language
Information technology is increasingly pervading our envi-
ronment, making real Mark Weiser’s vision of a “disappear-
ing technology”. The work described in this paper focuses
on using context to enable pervasive system personaliza-
tion, allowing context-aware sensor-data tailoring. Since
sensor networks, besides data collection, are also able to pro-
duce active behaviours, the tailoring capabilities are also ex-
tended to these, thus applying context-awareness to generic
system operations. Moreover, because the number of pos-
sible context can grow rapidly with the complexity of the
application, the design phase is also supported by the possi-
bility to speed-up and modularize the definition of the data
and operations associated with each specific context, pro-
ducing a support tool that eases the job of the designers of
modern context-aware pervasive systems
Towards autonomic pervasive systems: the PerLa context language
The property of context-awareness, inherent to a Pervasive
System, requires a clear definition of context and of how the
context parameter values must be extracted from the real
world. Since often the same variables are common to the
operational system and to the context it operates into, the
usage of the same language to manage both the application
and the context can lead to substantial savings in application
development time and costs. In this paper we propose a
context-management extension to the PerLa language and
middleware that allows for declarative gathering of context
data from the environment, feeding this data to the internal
context model and, once a context is active, acting on the
relevant resources of the pervasive system, according to the
chosen contextual policy
An assessment of the pulsar outer gap model. II: Implications for the predicted -ray spectra
One of the most important predictions of any gap model for pulsar
magnetospheres is the predicted -ray spectra. In the outer gap model,
the properties of the synchro-curvature radiation are sensitive to many
parameters, whose realistic ranges have been studied in detail in an
accompanying paper. There we demonstrated that the uncertainty in the radius of
curvature, the magnetic field geometry, and the X-ray surface flux may affect
by orders of magnitude the predicted flux and spectral peak in the -ray
regime. Here, we present a systematic, numerical study of the impact of the
different parameters on the particle dynamics along the gap and calculate the
emitted synchro-curvature radiation along the trajectory. By integrating the
emitted radiation along the gap and convolving it with a parametrized particle
distribution, we discuss how the comparison with the wealth of {\em Fermi}-LAT
data can be used to constrain the applicability of the model. The resulting
spectra show very different energy peaks, fluxes and shapes, qualitatively
matching the great variety of the observed {\em Fermi}-LAT pulsars. In
particular, if we see a large fraction of photons emitted from the initial part
of the trajectory, we show that the spectra will be flatter at the low-energy
{\it Fermi}-LAT regime (100 MeV -- 1 GeV). This provides a solution for such
observed flat spectra, while still maintain synchro-curvature radiation as the
origin of these photons.Comment: 9 pages, 6 figures, 1 table. Accepted for pubblication in MNRAS main
journal. References update
An assessment of the pulsar outer gap model. I: Assumptions, uncertainties, and implications for the gap size and the accelerating field
The popular outer gap model of magnetospheric emission from pulsars has been
widely applied to explain the properties observed in -rays. However,
its quantitative predictions rely on a number of approximations and assumptions
that are usually overlooked. Here we examine them, reviewing the main
ingredients entering in the model, evaluating their range of uncertainties.
Usually, in the quantitative applications of the model, key parameters like the
radius of curvature and the energies of the interacting photons are taken to be
a fixed, single value. Instead, here we explore their realistic ranges, and the
impact of these on the consistency of the model itself. We conclude that the
popular evaluation of the trans-field size of the gap as a function of period
and period derivative, is unreliable and affected by a huge dispersion. Last,
the exploration of the possible values for the radius of curvature, the local
magnetic field and other quantities deserve more attention for quantitative
applications of the outer gap model, like the calculation of -ray
spectra, which is the subject of an accompanying paper.Comment: 19 pages, 5 figures, 3 tables. Accepted for pubblication in MNRAS
main journal. References update
Assessing the potential of molten carbonate fuel cell-based schemes for carbon capture in natural gas-fired combined cycle power plants
Abstract This work explores two configurations of natural gas-fired combined cycles (NGCC) with molten carbonate fuel cells (MCFC) for CO2 capture. Special attention is devoted to the selection of MCFC operating conditions (trade-off between CO2 capture and voltage losses), heat integration scheme, fuel use and CO2 purification. Two schemes are considered: (i) in the first "integrated" scheme, MCFC modules are installed between the gas turbine and the heat recovery steam generator (HRSG) to maximize the efficiency of the integrated power plant; (ii) in the second "non-integrated" layout, the MCFC is located downstream of the HRSG and a regenerative heat exchanger is designed to preheat cathode reactants up to the MCFC working temperature. This study includes a full techno-economic analysis of the two layouts based on a preliminary sizing of the key-components, and a sensitivity analysis on the CO2 utilization factor. Compared to a benchmark amine scrubbing process, the "integrated" configuration shows considerably better performance (Specific Primary Energy Consumption for CO2 Avoided - SPECCA = 0.31 MJ kgCO2-1; Cost of CO2 avoided - CCA = 50 tCO2−1)
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