1,075,591 research outputs found
Low frequency measurements of synchrotron absorbing HII regions and modeling of observed synchrotron emissivity
Cosmic rays (CRs) and magnetic fields are dynamically important components in
the Galaxy, and their energy densities are comparable to that of the turbulent
interstellar gas. The interaction of CRs and Galactic magnetic fields produces
synchrotron radiation clearly visible in the radio regime. Detailed
measurements of synchrotron radiation averaged over the line-of-sight (LOS),
so-called synchrotron emissivities, can be used as a tracer of the CR density
and Galactic magnetic field (GMF) strength. Our aim is to model the synchrotron
emissivity in the Milky Way using a 3 dimensional dataset instead of
LOS-integrated intensity maps on the sky. Using absorbed HII regions we can
measure the synchrotron emissivity over a part of the LOS through the Galaxy,
changing from a 2 dimensional to a 3 dimensional view. Performing these
measurements on a large scale is one of the new applications of the window
opened by current low frequency arrays. Using various simple axisymmetric
emissivity models and a number of GMF-based emissivity models we can simulate
the synchrotron emissivities and compare them to the observed values in the
catalog. We present a catalog of low-frequency absorption measurements of HII
regions, their distances and electron temperatures, compiled from literature.
These data show that the axisymmetric emissivity models are not complex enough,
but the GMF-based emissivity models deliver a reasonable fit. These models
suggest that the fit can be improved by either an enhanced synchrotron
emissivity in the outer reaches of the Milky Way, or an emissivity drop near
the Galactic center. State-of-the-art GMF models plus a constant CR density
model cannot explain low-frequency absorption measurements, but the fits
improved with slight (ad-hoc) adaptations. It is clear that more detailed
models are needed, but the current results are very promising.Comment: 14 pages, 9 figures, accepted for publication in A&
Exponential versus linear amplitude decay in damped oscillators
We comment of the widespread belief among some undergraduate students that
the amplitude of any harmonic oscillator in the presence of any type of
friction, decays exponentially in time. To dispel that notion, we compare the
amplitude decay for a harmonic oscillator in the presence of (i) viscous
friction and (ii) dry friction. It is shown that, in the first case, the
amplitude decays exponentially with time while in the second case, it decays
linearly with time.Comment: 3 pages, 1 figure, accepted in Phys. Teac
On the infeasibility of entanglement generation in Gaussian quantum systems via classical control
This paper uses a system theoretic approach to show that classical linear
time invariant controllers cannot generate steady state entanglement in a
bipartite Gaussian quantum system which is initialized in a Gaussian state. The
paper also shows that the use of classical linear controllers cannot generate
entanglement in a finite time from a bipartite system initialized in a
separable Gaussian state. The approach reveals connections between system
theoretic concepts and the well known physical principle that local operations
and classical communications cannot generate entangled states starting from
separable states.Comment: 6 pages, 3 figures. To appear in IEEE Transactions on Automatic
Control, 201
Sensitivity to the pion-nucleon coupling constant in partial-wave analyses of elastic pi-N and NN scattering and pion photoproduction
We summarize results obtained in our studies of the pion-nucleon coupling
constant. Several different techniques have been applied to pi-N and NN elastic
scattering data, and the existing database for single-pion photoproduction. The
most reliable determination comes from pi-N elastic scattering. The sensitivity
in this reaction was found to be greater, by at least a factor of 3, when
compared with analyses of NN elastic scattering or single-pion photoproduction.Comment: 10 pages, 1 figure. Talk given at the Uppsala workshop on the
pion-nucleon coupling constan
A New Class of Path Equations in AP-Geometry
In the present work, it is shown that, the application of the Bazanski
approach to Lagrangians, written in AP-geometry and including the basic vector
of the space, gives rise to a new class of path equations. The general equation
representing this class contains four extra terms, whose vanishing reduces this
equation to the geodesic one. If the basic vector of the
AP-geometry is considered as playing the role of the electromagnetic
potential, as done in a previous work, then the second term
(of the extra terms) will represent Lorentz force while the fourth term gives
a direct effect of the electromagnetic potential on the motion of the charged
particle. This last term may give rise to an effect similar to the
Aharanov-Bohm effect. It is to be considered that all extra terms will vanish
if the space-time used is torsion-less.Comment: 11 pages, LaTeX fil
Optical orientation of electron spins in GaAs quantum wells
We present a detailed experimental and theoretical analysis of the optical
orientation of electron spins in GaAs/AlAs quantum wells. Using time and
polarization resolved photoluminescence excitation spectroscopy, the initial
degree of electron spin polarization is measured as a function of excitation
energy for a sequence of quantum wells with well widths between 63 Ang and 198
Ang. The experimental results are compared with an accurate theory of excitonic
absorption taking fully into account electron-hole Coulomb correlations and
heavy-hole light-hole coupling. We find in wide quantum wells that the measured
initial degree of polarization of the luminescence follows closely the spin
polarization of the optically excited electrons calculated as a function of
energy. This implies that the orientation of the electron spins is essentially
preserved when the electrons relax from the optically excited high-energy
states to quasi-thermal equilibrium of their momenta. Due to initial spin
relaxation, the measured polarization in narrow quantum wells is reduced by a
constant factor that does not depend on the excitation energy.Comment: 12 pages, 9 figure
The two types of Cherenkov gluons at LHC energies
Beside comparatively low energy Cherenkov gluons observed at RHIC, there
could be high energy gluons at LHC, related to the high energy region of
positive real part of the forward scattering amplitude. In both cases they give
rise to particles emitted along some cone. The characteristics of the cones
produced by these two types of gluons are different. Therefore different
experiments are needed to detect them. The cosmic ray event which initiated
this idea is described in detail.Comment: 6 pages, talk at IWCF2006, Hangzhou, Chin
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