16,103 research outputs found
External forward shock origin of high energy emission for three GRBs detected by Fermi
We analyze the >100MeV data for 3 GRBs detected by Fermi (GRBs 080916C,
090510, 090902B) and find that these photons were generated via synchrotron
emission in the external forward shock. We arrive at this conclusion by four
different methods as follows. (1) We check the light curve and spectral
behavior of the >100MeV data, and late time X-ray and optical data, and find
them consistent with the closure relations for the external forward shock
radiation. (2) We calculate the expected external forward shock synchrotron
flux at 100MeV, and it matches the observed flux value. (3) We determine the
external forward shock model parameters using the >100MeV data, and with these
we calculate the expected X-ray and optical fluxes at late times (hours to days
after the burst) and find these to be in good agreement with the observed data.
(4) We calculate the external forward shock model parameters using only the
late time X-ray, optical and radio data and from these estimate the expected
flux at >100 MeV at the end of the sub-MeV burst (and at subsequent times) and
find that to be entirely consistent with the high energy data obtained by
Fermi/LAT. The ability of a simple external forward shock, to fit the entire
data from the end of the burst (1-50s) to about a week, covering more than
eight-decades in photon frequency provides compelling confirmation of the
external forward shock synchrotron origin of the >100MeV radiation from these
Fermi GRBs. Moreover, the parameters determined in points (3) and (4) show that
the magnetic field required in these GRBs is consistent with shock-compressed
magnetic field in the circum-stellar medium with pre-shocked values of a few
tens of micro-Gauss.Comment: 12 pages, 7 figures, 2 tables. Accepted for publication in MNRAS.
Analytical estimates include
The 3-dimensional random walk with applications to overstretched DNA and the protein titin
We study the three-dimensional persistent random walk with drift. Then we
develop a thermodynamic model that is based on this random walk without
assuming the Boltzmann-Gibbs form for the equilibrium distribution. The
simplicity of the model allows us to perform all calculations in closed form.
We show that, despite its simplicity, the model can be used to describe
different polymer stretching experiments. We study the reversible
overstretching transition of DNA and the static force-extension relation of the
protein titin.Comment: 9 pages, 10 figure
Distances in random graphs with finite variance degrees
In this paper we study a random graph with nodes, where node has
degree and are i.i.d. with \prob(D_j\leq x)=F(x). We
assume that for some and some constant
. This graph model is a variant of the so-called configuration model, and
includes heavy tail degrees with finite variance.
The minimal number of edges between two arbitrary connected nodes, also known
as the graph distance or the hopcount, is investigated when . We
prove that the graph distance grows like , when the base of the
logarithm equals \nu=\expec[D_j(D_j -1)]/\expec[D_j]>1. This confirms the
heuristic argument of Newman, Strogatz and Watts \cite{NSW00}. In addition, the
random fluctuations around this asymptotic mean are
characterized and shown to be uniformly bounded. In particular, we show
convergence in distribution of the centered graph distance along exponentially
growing subsequences.Comment: 40 pages, 2 figure
Managing Supply Chain Events to Build Sense-and-Respond Capability
As supply chains become more dynamic, there is a need for a sense-and-respond capability to react to events in a real-time manner. In this paper, we propose Petri nets extended with time and color (for case data) as a formalism for doing so. Hence, we describe seven basic patterns that are used to capture modeling concepts that arise commonly in supply chains. These basic patterns may be used by themselves and also be combined to create new patterns. Next, we show how to use the patterns as building blocks to model a complete supply chain and analyze it using dependency graphs and simulation. Dependency graphs can be used to analyze the various events and their causes. Simulation was, in addition, used to analyze various performance indicators (e.g. fill rates, replenishment times, and lead times) under different supply chain strategies. We performed sensitivity analysis to study the effect of changing parameter values on the performance indicators. In the experiments, by cutting resolution time for production delays in half (strategy 1), we were able to increase order fill rate from 89% to 95%. Similarly, upon raising the probability of successful alternative sourcing (strategy 2) from 0.5 to 0.7 the order fill rate again increased from 89% to 95%. We show that by modeling timing and causality issues accurately, it is possible to improve supply chain performance
State-to-state resolved differential cross sections for rotationally inelastic scattering of ND3 with He
Effect of long-range Coulomb interaction on shot-noise suppression in ballistic transport
We present a microscopic analysis of shot-noise suppression due to long-range
Coulomb interaction in semiconductor devices under ballistic transport
conditions. An ensemble Monte Carlo simulator self-consistently coupled with a
Poisson solver is used for the calculations. A wide range of injection-rate
densities leading to different degrees of suppression is investigated. A sharp
tendency of noise suppression at increasing injection densities is found to
scale with a dimensionless Debye length related to the importance of
space-charge effects in the structure.Comment: RevTex, 4 pages, 4 figures, minor correction
Structural phase transition and material properties of few-layer monochalcogenides
GeSe and SnSe monochalcogenide monolayers and bilayers undergo a
two-dimensional phase transition from a rectangular unit cell to a square unit
cell at a temperature well below the melting point. Its consequences on
material properties are studied within the framework of Car-Parrinello
molecular dynamics and density-functional theory. No in-gap states develop as
the structural transition takes place, so that these phase-change materials
remain semiconducting below and above . As the in-plane lattice transforms
from a rectangle onto a square at , the electronic, spin, optical, and
piezo-electric properties dramatically depart from earlier predictions. Indeed,
the and points in the Brillouin zone become effectively equivalent at
, leading to a symmetric electronic structure. The spin polarization at
the conduction valley edge vanishes, and the hole conductivity must display an
anomalous thermal increase at . The linear optical absorption band edge
must change its polarization as well, making this structural and electronic
evolution verifiable by optical means. Much excitement has been drawn by
theoretical predictions of giant piezo-electricity and ferroelectricity in
these materials, and we estimate a pyroelectric response of about here. These results uncover the fundamental role of
temperature as a control knob for the physical properties of few-layer group-IV
monochalcogenidesComment: Supplementary information included. Published versio
XRL/Woflan : verification of an XML/Petri-net-based language for inter-organizational workflows
Overview of the EU FP7-project HISTORIC
HISTORIC aims to develop and test complex photonic integrated circuits containing a relatively large number of digital photonic elements for use in e.g. all-optical packet switching. These photonic digital units are all-optical flip-flops based on ultra compact laser diodes, such as microdisk lasers and photonic crystal lasers. These lasers are fabricated making use of the heterogeneous integration of InP membranes on top of silicon on insulator (SOI) passive optical circuits. The very small dimensions of the lasers are, at least for some approaches, possible because of the high index contrast of the InP membranes and by making use of the extreme accuracy of CMOS processing.
All-optical flip-flops based on heterogeneously integrated microdisk lasers with diameter of 7.5 mu m have already been demonstrated. They operate with a CW power consumption of a few mW and can switch in 60ps with switching energies as low as 1.8 fJ. Their operation as all-optical gate has also been demonstrated. Work is also on-going to fabricate heterogeneously integrated photonic crystal lasers and all-optical flip-flops based on such lasers. A lot of attention is given to the electrical pumping of the membrane InP-based photonic crystal lasers and to the coupling to SOI wire waveguides. Optically pumped photonic crystal lasers coupled to SOI wires have been demonstrated already.
The all-optical flip-flops and gates will be combined into more complex photonic integrated circuits, implementing all-optical shift registers, D flip-flops, and other all-optical switching building blocks. The possibility to integrate a large number of photonic digital units together, but also to integrate them with compact passive optical routers such as AWGs, opens new perspectives for the design of integrated optical processors or optical buffers. The project therefore also focuses on designing new architectures for such optical processing or buffer chips
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