2,319 research outputs found
Decentralised Coordination of Low-Power Embedded Devices Using the Max-Sum Algorithm
This paper considers the problem of performing decentralised coordination of low-power embedded devices (as is required within many environmental sensing and surveillance applications). Specifically, we address the generic problem of maximising social welfare within a group of interacting agents. We propose a novel representation of the problem, as a cyclic bipartite factor graph, composed of variable and function nodes (representing the agentsâ states and utilities respectively). We show that such representation allows us to use an extension of the max-sum algorithm to generate approximate solutions to this global optimisation problem through local decentralised message passing. We empirically evaluate this approach on a canonical coordination problem (graph colouring), and benchmark it against state of the art approximate and complete algorithms (DSA and DPOP). We show that our approach is robust to lossy communication, that it generates solutions closer to those of DPOP than DSA is able to, and that it does so with a communication cost (in terms of total messages size) that scales very well with the number of agents in the system (compared to the exponential increase of DPOP). Finally, we describe a hardware implementation of our algorithm operating on low-power Chipcon CC2431 System-on-Chip sensor nodes
On the nature of the first transient Z-source XTE J1701-462: its accretion disk structure, neutron star magnetic field strength, and hard tail
Using the data from the Rossi X-Ray Timing Explorer satellite, we investigate
the spectral evolution along a "Z" track and a "v" track on the
hardness-intensity diagrams of the first transient Z source XTE J1701-462. The
spectral analyses suggest that the inner disk radius depends on the mass
accretion rate, in agreement with the model prediction, R_in \propto
((dM/dt)_disk)^{2/7}, for a radiation pressure dominated accretion disk
interacting with the magnetosphere of a neutron star (NS). The changes in the
disk mass accretion rate (dM/dt)_disk are responsible for the evolution of the
"Z" or "v" track. The radiation pressure thickens the disk considerably, and
also produces significant outflows. The NS surface magnetic field strength,
derived from the interaction between the magnetosphere and the radiation
pressure dominated accretion disk, is ~(1--3)X10^9 G, which is possibly between
normal atoll and Z sources. A significant hard tail is detected in the
horizontal branches and we discuss several possible origins of the hard tail
The transient hard X-ray tail of GX 17+2: new BeppoSAX results
We report on results of two BeppoSAX observations of the Z source GX 17+2. In
both cases the source is in the horizontal branch of the colour-intensity
diagram. The persistent continuum can be fit by two-component models consisting
of a blackbody plus a Comptonization spectrum. With one of these models, two
solutions for the blackbody temperature of both the observed and seed photons
for Comptonization are equally accepted by the data. In the first observation,
when the source is on the left part of the horizontal branch, we observe a hard
tail extending up to 120 keV, while in the second observation, when the source
moves towards right in the same branch, the tail is no longer detected. The
hard (\ga 30 keV) X-ray emission can be modeled either by a simple power-law
with photon index 2.7, or assuming Comptonization of 1 keV
soft photons off a hybrid thermal plus non-thermal electron plasma. The
spectral index of the non-thermal injected electrons is . The
observation of hard {X-ray} emission only in the left part of the horizontal
branch could be indicative of the presence of a threshold in the accretion rate
above which the hard tail disappears. An emission line at 6.7 keV with
equivalent width eV is also found in both observations. We discuss
these results and their physical implicationsComment: 11 pages, 10 figures, 4 tables. Accepted for publication in A&
On the Spectral Evolution of Cygnus X-2 along its Color-Color Diagram
We report on the results of a broad band (0.1-200 keV) spectral study of Cyg X-2 using two BeppoSAX observations taken in 1996 and 1997, respectively, for a total effective on-source time of ~100 ks. The color-color (CD) and hardness-intensity (HID) diagrams show that the source was in the horizontal branch (HB) and normal branch (NB) during the 1996 and 1997 observation, respectively. Five spectra were selected around different positions of the source in the CD/HID, two in the HB and three in the NB. These spectra are fit to a model consisting of a disk blackbody, a Comptonization component, and two Gaussian emission lines at ~1 keV and ~6.6 keV, respectively. The addition of a hard power-law tail with photon index ~2, contributing ~1.5% of the source luminosity, improves the fit of the spectra in the HB. We interpret the soft component as the emission from the inner accretion disk, with inner temperature, k T_in, varying between 0.8 and 1.7 keV and inner radius, R_in, varying between 26 and 11 km (assuming an inclination angle of the system of 60 degrees). The Comptonization component is probably emitted by hot plasma (electron temperature k T_e varying between 3 and ~20 keV, optical depth \tau ~ 11-0.4, seed-photon temperature k T_W ~ 1-2.4 keV) surrounding the NS. The changes in the parameters of the blackbody component indicate that the inner rim of the disk approaches the NS surface when the source moves from the HB to the NB, i.e. as the (inferred) mass accretion rate increases. The parameters of the Comptonized component also change significantly when the source moves from the HB to the NB. We discuss possible scenarios which can explain these changes
High-resolution tracking in a GEM-Emulsion detector
SHiP (Search for Hidden Particles) is a beam dump experiment proposed at the
CERN SPS aiming at the observation of long lived particles very weakly coupled
with ordinary matter mostly produced in the decay of charmed hadrons. The beam
dump facility of SHiP is also a copious factory of neutrinos of all three kinds
and therefore a dedicated neutrino detector is foreseen in the SHiP apparatus.
The neutrino detector exploits the Emulsion Cloud Chamber technique with a
modular structure, alternating walls of target units and planes of electronic
detectors providing the time stamp to the event. GEM detectors are one of the
possible choices for this task. This paper reports the results of the first
exposure to a muon beam at CERN of a new hybrid chamber, obtained by coupling a
GEM chamber and an emulsion detector. Thanks to the micrometric accuracy of the
emulsion detector, the position resolution of the GEM chamber as a function of
the particle inclination was evaluated in two configurations, with and without
the magnetic fiel
A Hard X-ray View on Scorpius X-1 with INTEGRAL: non-Thermal Emission ?
We present here simultaneous INTEGRAL/RXTE observations of Sco X-1, and in
particular a study of the hard X-ray emission of the source and its correlation
with the position in the Z-track of the X-ray color-color diagram. We find that
the hard X-ray (above about 30 keV) emission of Sco X-1 is dominated by a
power-law component with a photon index of ~3. The flux in the power-law
component slightly decreases when the source moves in the color-color diagram
in the sense of increasing inferred mass accretion rate from the horizontal
branch to the normal branch/flaring branch vertex. It becomes not significantly
detectable in the flaring branch, where its flux has decreased by about an
order of magnitude. These results present close analogies to the behavior of GX
17+2, one of so-called Sco-like Z sources. Finally, the hard power law in the
spectrum of Sco X-1 does not show any evidence of a high energy cutoff up to
100 - 200 keV, strongly suggesting a non-thermal origin of this component.Comment: 5 pages including 3 figures. Accepted for publication by ApJ Letter
On a spin conformal invariant on manifolds with boundary
On a n-dimensional connected compact manifold with non-empty boundary
equipped with a Riemannian metric, a spin structure and a chirality operator,
we study some properties of a spin conformal invariant defined from the first
eigenvalue of the Dirac operator under the chiral bag boundary condition. More
precisely, we show that we can derive a spinorial analogue of Aubin's
inequality.Comment: 26 page
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