8,134 research outputs found
Network recovery from massive failures under uncertain knowledge of damages
This paper addresses progressive network recovery under uncertain knowledge of damages. We formulate the problem as a mixed integer linear programming (MILP), and show that it is NP-Hard. We propose an iterative stochastic recovery algorithm (ISR) to recover the network in a progressive manner to satisfy the critical services. At each optimization step, we make a decision to repair a part of the network and gather more information iteratively, until critical services are completely restored. Three different algorithms are used to find a feasible set and determine which node to repair, namely, 1) an iterative shortest path algorithm (ISR-SRT), 2) an approximate branch and bound (ISR-BB) and 3) an iterative multi-commodity LP relaxation (ISR-MULT). Further, we have modified the state-of-the-Art iterative split and prune (ISP) algorithm to incorporate the uncertain failures. Our results show that ISR-BB and ISR- MULT outperform the state-of-the-Art 'progressive ISP' algorithm while we can configure our choice of trade-off between the execution time, number of repairs (cost) and the demand loss. We show that our recovery algorithm, on average, can reduce the total number of repairs by a factor of about 3 with respect to ISP, while satisfying all critical deman
The Discovery of an X-ray/UV Stellar Flare from the Late-K/Early-M Dwarf LMC 335
We report the discovery of an X-ray/UV stellar flare from the source LMC 335,
captured by XMM-Newton in the field of the Large Magellanic Cloud. The flare
event was recorded continuously in X-ray for its first 10 hours from the
precursor to the late decay phases. The observed fluxes increased by more than
two orders of magnitude at its peak in X-ray and at least one in the UV as
compared to quiescence. The peak 0.1-7.0 keV X-ray flux is derived from the
two-temperature APEC model to be ~(8.4 +/- 0.6) x 10^-12 erg cm-2 s-1.
Combining astrometric information from multiple X-ray observations in the
quiescent and flare states, we identify the NIR counterpart of LMC 335 as the
2MASS source J05414534-6921512. The NIR color relations and spectroscopic
parallax characterize the source as a Galactic K7-M4 dwarf at a foreground
distance of (100 - 264) pc, implying a total energy output of the entire event
of ~(0.4 - 2.9) x 10^35 erg. This report comprises detailed analyses of this
late-K / early-M dwarf flare event that has the longest time coverage yet
reported in the literature. The flare decay can be modeled with two exponential
components with timescales of ~28 min and ~4 hours, with a single component
decay firmly ruled out. The X-ray spectra during flare can be described by two
components, a dominant high temperature component of ~40-60MK and a low
temperature component of ~10MK, with a flare loop length of about 1.1-1.3
stellar radius.Comment: 35 pages, 6 figures, 5 tables, accepted for publication in Ap
Estimating probabilities from experimental frequencies
Estimating the probability distribution 'q' governing the behaviour of a
certain variable by sampling its value a finite number of times most typically
involves an error. Successive measurements allow the construction of a
histogram, or frequency count 'f', of each of the possible outcomes. In this
work, the probability that the true distribution be 'q', given that the
frequency count 'f' was sampled, is studied. Such a probability may be written
as a Gibbs distribution. A thermodynamic potential, which allows an easy
evaluation of the mean Kullback-Leibler divergence between the true and
measured distribution, is defined. For a large number of samples, the
expectation value of any function of 'q' is expanded in powers of the inverse
number of samples. As an example, the moments, the entropy and the mutual
information are analyzed.Comment: 10 pages, 3 figures, to be published in Physical Review
Scheme for teleportation of quantum states onto a mechanical resonator
We propose an experimentally feasible scheme to teleport an unkown quantum
state onto the vibrational degree of freedom of a macroscopic mirror. The
quantum channel between the two parties is established by exploiting radiation
pressure effects.Comment: 5 pages, 2 figures, in press on PR
Gamma Rays from Clusters and Groups of Galaxies: Cosmic Rays versus Dark Matter
Clusters of galaxies have not yet been detected at gamma-ray frequencies;
however, the recently launched Fermi Gamma-ray Space Telescope, formerly known
as GLAST, could provide the first detections in the near future. Clusters are
expected to emit gamma rays as a result of (1) a population of high-energy
primary and re-accelerated secondary cosmic rays (CR) fueled by structure
formation and merger shocks, active galactic nuclei and supernovae, and (2)
particle dark matter (DM) annihilation. In this paper, we ask the question of
whether the Fermi telescope will be able to discriminate between the two
emission processes. We present data-driven predictions for a large X-ray flux
limited sample of galaxy clusters and groups. We point out that the gamma ray
signals from CR and DM can be comparable. In particular, we find that poor
clusters and groups are the systems predicted to have the highest DM to CR
emission at gamma-ray energies. Based on detailed Fermi simulations, we study
observational handles that might enable us to distinguish the two emission
mechanisms, including the gamma-ray spectra, the spatial distribution of the
signal and the associated multi-wavelength emissions. We also propose optimal
hardness ratios, which will help to understand the nature of the gamma-ray
emission. Our study indicates that gamma rays from DM annihilation with a high
particle mass can be distinguished from a CR spectrum even for fairly faint
sources. Discriminating a CR spectrum from a light DM particle will be instead
much more difficult, and will require long observations and/or a bright source.
While the gamma-ray emission from our simulated clusters is extended,
determining the spatial distribution with Fermi will be a challenging task
requiring an optimal control of the backgrounds.Comment: revised to match resubmitted version, 35 pages, 16 figures: results
unchanged, some discussion added and unnecessary text and figures remove
Magneto-mechanical interplay in spin-polarized point contacts
We investigate the interplay between magnetic and structural dynamics in
ferromagnetic atomic point contacts. In particular, we look at the effect of
the atomic relaxation on the energy barrier for magnetic domain wall migration
and, reversely, at the effect of the magnetic state on the mechanical forces
and structural relaxation. We observe changes of the barrier height due to the
atomic relaxation up to 200%, suggesting a very strong coupling between the
structural and the magnetic degrees of freedom. The reverse interplay is weak,
i.e. the magnetic state has little effect on the structural relaxation at
equilibrium or under non-equilibrium, current-carrying conditions.Comment: 5 pages, 4 figure
Non-Markovian Decay and Lasing Condition in an Optical Microcavity Coupled to a Structured Reservoir
The decay dynamics of the classical electromagnetic field in a leaky optical
resonator supporting a single mode coupled to a structured continuum of modes
(reservoir) is theoretically investigated, and the issue of threshold condition
for lasing in presence of an inverted medium is comprehensively addressed.
Specific analytical results are given for a single-mode microcavity resonantly
coupled to a coupled resonator optical waveguide (CROW), which supports a band
of continuous modes acting as decay channels. For weak coupling, the usual
exponential Weisskopf-Wigner (Markovian) decay of the field in the bare
resonator is found, and the threshold for lasing increases linearly with the
coupling strength. As the coupling between the microcavity and the structured
reservoir increases, the field decay in the passive cavity shows non
exponential features, and correspondingly the threshold for lasing ceases to
increase, reaching a maximum and then starting to decrease as the coupling
strength is further increased. A singular behavior for the "laser phase
transition", which is a clear signature of strong non-Markovian dynamics, is
found at critical values of the coupling between the microcavity and the
reservoir.Comment: to appear in Phys. Rev. A (December 2006 issue
Interference effects in f-deformed fields
We show how the introduction of an algeabric field deformation affects the
interference phenomena. We also give a physical interpretation of the developed
theory.Comment: 6 pages, Latex file, no figures, accepted by Physica Script
Discrete Breathers in a Realistic Coarse-Grained Model of Proteins
We report the results of molecular dynamics simulations of an off-lattice
protein model featuring a physical force-field and amino-acid sequence. We show
that localized modes of nonlinear origin (discrete breathers) emerge naturally
as continuations of a subset of high-frequency normal modes residing at
specific sites dictated by the native fold. In the case of the small
-barrel structure that we consider, localization occurs on the turns
connecting the strands. At high energies, discrete breathers stabilize the
structure by concentrating energy on few sites, while their collapse marks the
onset of large-amplitude fluctuations of the protein. Furthermore, we show how
breathers develop as energy-accumulating centres following perturbations even
at distant locations, thus mediating efficient and irreversible energy
transfers. Remarkably, due to the presence of angular potentials, the breather
induces a local static distortion of the native fold. Altogether, the
combination of this two nonlinear effects may provide a ready means for
remotely controlling local conformational changes in proteins.Comment: Submitted to Physical Biolog
Lossy data compression with random gates
We introduce a new protocol for a lossy data compression algorithm which is
based on constraint satisfaction gates. We show that the theoretical capacity
of algorithms built from standard parity-check gates converges exponentially
fast to the Shannon's bound when the number of variables seen by each gate
increases. We then generalize this approach by introducing random gates. They
have theoretical performances nearly as good as parity checks, but they offer
the great advantage that the encoding can be done in linear time using the
Survey Inspired Decimation algorithm, a powerful algorithm for constraint
satisfaction problems derived from statistical physics
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