14,025 research outputs found
Gamma Rays From The Galactic Center and the WMAP Haze
Recently, an analysis of data from the Fermi Gamma Ray Space Telescope has
revealed a flux of gamma rays concentrated around the inner ~0.5 degrees of the
Milky Way, with a spectrum that is sharply peaked at 2-4 GeV. If interpreted as
the products of annihilating dark matter, this signal implies that the dark
matter consists of particles with a mass between 7.3 and 9.2 GeV annihilating
primarily to charged leptons. This mass range is very similar to that required
to accommodate the signals reported by CoGeNT and DAMA/LIBRA. In addition to
gamma rays, the dark matter is predicted to produce energetic electrons and
positrons in the Inner Galaxy, which emit synchrotron photons as a result of
their interaction with the galactic magnetic field. In this letter, we
calculate the flux and spectrum of this synchrotron emission assuming that the
gamma rays from the Galactic Center originate from dark matter, and compare the
results to measurements from the WMAP satellite. We find that a sizable flux of
hard synchrotron emission is predicted in this scenario, and that this can
easily account for the observed intensity, spectrum, and morphology of the
"WMAP Haze".Comment: 5 pages, 4 figure
Peri-abelian categories and the universal central extension condition
We study the relation between Bourn's notion of peri-abelian category and
conditions involving the coincidence of the Smith, Huq and Higgins commutators.
In particular we show that a semi-abelian category is peri-abelian if and only
if for each normal subobject , the Higgins commutator of with
itself coincides with the normalisation of the Smith commutator of the
denormalisation of with itself. We show that if a category is peri-abelian,
then the condition (UCE), which was introduced and studied by Casas and the
second author, holds for that category. In addition we show, using amongst
other things a result by Cigoli, that all categories of interest in the sense
of Orzech are peri-abelian and therefore satisfy the condition (UCE).Comment: 14 pages, final version accepted for publicatio
Continuous Histories and the History Group in Generalised Quantum Theory
We treat continuous histories within the histories approach to generalised
quantum mechanics. The essential tool is the `history group': the analogue,
within the generalised history scheme, of the canonical group of single-time
quantum mechanics.Comment: 25 page
Preoperative predictors of knee range of motion during stair walking after total knee replacement
This paper discusses the preoperative predictors of knee range of motion during stair walking after total knee replacement. It was presented at the 17th Annual Meeting of the European Society of Movement Analysis for Adults and Children (ESMAC) in 2008
On the normality of Higgins commutators
In a semi-abelian context, we study the condition (NH) asking that Higgins
commutators of normal subobjects are normal subobjects. We provide examples of
categories that do or do not satisfy this property. We focus on the
relationship with the "Smith is Huq" condition (SH) and characterise those
semi-abelian categories in which both (NH) and (SH) hold in terms of reflection
and preservation properties of the change of base functors of the fibration of
points.Comment: 15 pages; final published versio
Why current-carrying magnetic flux tubes gobble up plasma and become thin as a result
It is shown that if a current-carrying magnetic flux tube is bulged at its
axial midpoint z=0 and constricted at its axial endpoints z=+h,-h, then plasma
will be accelerated from z=+h,-h towards z=0 resulting in a situation similar
to two water jets pointed at each other. The ingested plasma convects embedded,
frozen-in toroidal magnetic flux from z=+h,-h to z=0. The counter-directed
flows collide and stagnate at z=0 and in so doing (i) convert their
translational kinetic energy into heat, (ii) increase the plasma density at
z~0, and (iii) increase the embedded toroidal flux density at z~0. The increase
in toroidal flux density at z~0 increases the toroidal field Bphi and hence
increases the magnetic pinch force at z~0 and so causes a reduction of the flux
tube radius at z~0. Thus, the flux tube develops an axially uniform
cross-section, a decreased volume, an increased density, and an increased
temperature. This model is proposed as a likely hypothesis for the
long-standing mystery of why solar coronal loops are observed to be axially
uniform, hot, and bright.Comment: to appear in Physics of Plasmas 24 pages, 5 figure
Unstable coronal loops : numerical simulations with predicted observational signatures
We present numerical studies of the nonlinear, resistive magnetohydrodynamic
(MHD) evolution of coronal loops. For these simulations we assume that the
loops carry no net current, as might be expected if the loop had evolved due to
vortex flows. Furthermore the initial equilibrium is taken to be a cylindrical
flux tube with line-tied ends. For a given amount of twist in the magnetic
field it is well known that once such a loop exceeds a critical length it
becomes unstableto ideal MHD instabilities. The early evolution of these
instabilities generates large current concentrations. Firstly we show that
these current concentrations are consistent with the formation of a current
sheet. Magnetic reconnection can only occur in the vicinity of these current
concentrations and we therefore couple the resistivity to the local current
density. This has the advantage of avoiding resistive diffusion in regions
where it should be negligible. We demonstrate the importance of this procedure
by comparison with simulations based on a uniform resistivity. From our
numerical experiments we are able to estimate some observational signatures for
unstable coronal loops. These signatures include: the timescale of the loop
brightening; the temperature increase; the energy released and the predicted
observable flow speeds. Finally we discuss to what extent these observational
signatures are consistent with the properties of transient brightening loops.Comment: 13 pages, 9 figure
Spectral properties of coupled cavity arrays in one dimension
Spectral properties of coupled cavity arrays in one dimension are
investigated by means of the variational cluster approach. Coupled cavity
arrays consist of two distinct "particles," namely, photons and atomiclike
excitations. Spectral functions are evaluated and discussed for both particle
types. In addition, densities of states, momentum distributions and spatial
correlation functions are presented. Based on this information, polariton
"quasiparticles" are introduced as appropriate, wave vector and filling
dependent linear combinations of photon and atomiclike particles. Spectral
functions and densities of states are evaluated for the polariton
quasiparticles, and the weights of their components are analyzed.Comment: 17 pages, 16 figures, version as publishe
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