107 research outputs found
Opportunistic routing in wireless mesh networks
Opportunistic Routing (OR) has been proposed as a way to increase the performance of wireless networks by exploiting its broadcast nature. In OR, instead of pre-selecting a single specific node to be the next-hop as a forwarder for a packet, multiple nodes can potentially be selected as the next-hop forwarder. Thus the source can use multiple potential paths to deliver the packets to the destination. More specially, when the current node transmits a packet, all the candidates that receive the packet successfully will coordinate with each other to determine which one would actually forward the packet according to some criteria, while the other nodes will simply discard the packet. In this chapter, we survey the state of the art in OR, then focus on the candidates selection algorithms and carry out a comparative performance evaluation of the most relevant proposals appeared in the literature.Peer ReviewedPostprint (author’s final draft
Notre environnement devient intelligent
Les capteurs sans fil s’organisent automatiquement en réseaux
pour enregistrer les données de notre environnement physique
et transmettre les informations. C’est la théorie, mais la réalité
pratique n’est pas si simple. L’article décrit un réseau de capteurs
sans fil installé à l’EPFL
Magnetic fields in cosmic particle acceleration sources
We review here some magnetic phenomena in astrophysical particle accelerators
associated with collisionless shocks in supernova remnants, radio galaxies and
clusters of galaxies. A specific feature is that the accelerated particles can
play an important role in magnetic field evolution in the objects. We discuss a
number of CR-driven, magnetic field amplification processes that are likely to
operate when diffusive shock acceleration (DSA) becomes efficient and
nonlinear. The turbulent magnetic fields produced by these processes determine
the maximum energies of accelerated particles and result in specific features
in the observed photon radiation of the sources. Equally important, magnetic
field amplification by the CR currents and pressure anisotropies may affect the
shocked gas temperatures and compression, both in the shock precursor and in
the downstream flow, if the shock is an efficient CR accelerator. Strong
fluctuations of the magnetic field on scales above the radiation formation
length in the shock vicinity result in intermittent structures observable in
synchrotron emission images. Resonant and non-resonant CR streaming
instabilities in the shock precursor can generate mesoscale magnetic fields
with scale-sizes comparable to supernova remnants and even superbubbles. This
opens the possibility that magnetic fields in the earliest galaxies were
produced by the first generation Population III supernova remnants and by
clustered supernovae in star forming regions.Comment: 30 pages, Space Science Review
Magnetic field amplification and X-ray emission in galaxy minor mergers
We investigate the magnetic field evolution in a series of galaxy minor
mergers using the N-body/smoothed particle hydrodynamics (SPH) code
\textsc{Gadget}. The simulations include the effects of radiative cooling, star
formation and supernova feedback. Magnetohydrodynamics (MHD) is implemented
using the SPH method. We present 32 simulations of binary mergers of disc
galaxies with mass ratios of 2:1 up to 100:1, whereby we have additionally
varied the initial magnetic field strengths, disc orientations and resolutions.
We investigate the amplification of a given initial magnetic field within the
galaxies and an ambient intergalactic medium (IGM) during the interaction. We
find that the magnetic field strengths of merger remnants with mass ratios up
to 10:1 saturate at a common value of several G. For higher mass ratios,
the field strength saturates at lower values. The saturation values correspond
to the equipartition value of magnetic and turbulent energy density. The
initial magnetization, disc orientation and numerical resolution show only
minor effects on the saturation value of the magnetic field. We demonstrate
that a higher impact energy of the progenitor galaxies leads to a more
efficient magnetic field amplification. The magnetic and turbulent energy
densities are higher for larger companion galaxies, consistent with the higher
impact energy supplied to the system. We present a detailed study of the
evolution of the temperature and the bolometric X-ray luminosity within the
merging systems. Thereby we find that magnetic fields cause a more efficient
increase of the IGM temperature and the corresponding IGM X-ray luminosity
after the first encounter. However, the presence of magnetic fields does not
enhance the total X-ray luminosity. Generally, the final value of the X-ray
luminosity is even clearly lower for higher initial magnetic fields.Comment: 20 pages, 12 figures. Submitted to MNRA
The geometry of the magnetic field in the central molecular zone measured by PILOT
We present the first far infrared (FIR) dust emission polarization map covering the full extent of Milky Way’s central molecular zone (CMZ). The data, obtained with the PILOT balloon-borne experiment, covers the Galactic center region − 2° < ℓ < 2°, − 4° < b < 3° at a wavelength of 240 μm and an angular resolution of 2.2′. From our measured dust polarization angles, we infer a magnetic field orientation projected onto the plane of the sky (POS) that is remarkably ordered over the full extent of the CMZ, with an average tilt angle of ≃22° clockwise with respect to the Galactic plane. Our results confirm previous claims that the field traced by dust polarized emission is oriented nearly orthogonally to the field traced by GHz radio synchrotron emission in the Galactic center region. The observed field structure is globally compatible with the latest Planck polarization data at 353 and 217 GHz. Upon subtraction of the extended emission in our data, the mean field orientation that we obtain shows good agreement with the mean field orientation measured at higher angular resolution by the JCMT within the 20 and 50 km s−1 molecular clouds. We find no evidence that the magnetic field orientation is related to the 100 pc twisted ring structure within the CMZ. The low polarization fraction in the Galactic center region measured with Planck at 353 GHz combined with a highly ordered projected field orientation is unusual. This feature actually extends to the whole inner Galactic plane. We propose that it could be caused by the increased number of turbulent cells for the long lines of sight towards the inner Galactic plane or to dust properties specific to the inner regions of the Galaxy. Assuming equipartition between magnetic pressure and ram pressure, we obtain magnetic field strength estimates of the order of 1 mG for several CMZ molecular clouds
Detection of the Small Magellanic Cloud in gamma-rays with Fermi/LAT
The flux of gamma rays with energies >100MeV is dominated by diffuse emission
from CRs illuminating the ISM of our Galaxy through the processes of
Bremsstrahlung, pion production and decay, and inverse-Compton scattering. The
study of this diffuse emission provides insight into the origin and transport
of CRs. We searched for gamma-ray emission from the SMC in order to derive
constraints on the CR population and transport in an external system with
properties different from the Milky Way. We analysed the first 17 months of
continuous all-sky observations by the Large Area Telescope of the Fermi
mission to determine the spatial distribution, flux and spectrum of the
gamma-ray emission from the SMC. We also used past radio synchrotron
observations of the SMC to study the population of CR electrons specifically.
We obtained the first detection of the SMC in high-energy gamma rays, with an
integrated >100MeV flux of (3.7 +/-0.7) x10e-8 ph/cm2/s, with additional
systematic uncertainty of <16%. The emission is steady and from an extended
source ~3{\deg} in size. It is not clearly correlated with the distribution of
massive stars or neutral gas, nor with known pulsars or SNRs, but a certain
correlation with supergiant shells is observed. The observed flux implies an
upper limit on the average CR nuclei density in the SMC of ~15% of the value
measured locally in the Milky Way. The population of high-energy pulsars of the
SMC may account for a substantial fraction of the gamma-ray flux, which would
make the inferred CR nuclei density even lower. The average density of CR
electrons derived from radio synchrotron observations is consistent with the
same reduction factor but the uncertainties are large. From our current
knowledge of the SMC, such a low CR density does not seem to be due to a lower
rate of CR injection and rather indicates a smaller CR confinement volume
characteristic size.Comment: 14 pages, 6 figures, accepted for publication in A&
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