14,640 research outputs found
Three episodes of jet activity in the FRII radio galaxy B0925+420
We present Very Large Array images of a "Double-Double Radio Galaxy", a class
of objects in which two pairs of lobes are aligned either side of the nucleus.
In this object, B0925+420, we discover a third pair of lobes, close to the core
and again in alignment with the other lobes. This first-known "Triple-Double"
object strongly increases the likelihood that these lobes represent mutiple
episodes of jet activity, as opposed to knots in an underlying jet. We model
the lobes in terms of their dynamical evolution. We find that the inner pair of
lobes is consistent with the outer pair having been displaced buoyantly by the
ambient medium. The middle pair of lobes is more problematic - to the extent
where an alternative model interpreting the middle and inner "lobes" as
additional bow shocks within the outer lobes may be more appropriate - and we
discuss the implications of this on our understanding of the density of the
ambient medium.Comment: Accepted for publication in MNRAS. Figure 2 is best viewed in colou
Double-double radio galaxies: further insights into the formation of the radio structures
Double-double radio galaxies (DDRGs) offer a unique opportunity for us to
study multiple episodes of jet activity in large-scale radio sources. We use
radio data from the Very Large Array and the literature to model two DDRGs,
B1450+333 and B1834+620, in terms of their dynamical evolution. We find that
the standard Fanaroff-Riley II model is able to explain the properties of the
two outer lobes of each source, whereby the lobes are formed by ram-pressure
balance of a shock at the end of the jet with the surrounding medium. The inner
pairs of lobes, however, are not well-described by the standard model. Instead
we interpret the inner lobes as arising from the emission of relativistic
electrons within the outer lobes, which are compressed and re-accelerated by
the bow-shock in front of the restarted jets and within the outer lobes. The
predicted rapid progression of the inner lobes through the outer lobes requires
the eventual development of a hotspot at the edge of the outer lobe, causing
the DDRG ultimately to resemble a standard Fanaroff-Riley II radio galaxy. This
may suggest that DDRGs are a brief, yet normal, phase of the evolution of
large-scale radio galaxies.Comment: Accepted for publication in MNRAS. 17 pages; 15 figures, 2 of which
are in colou
Multiplexed holographic transmission gratings recorded in holographic polymer-dispersed liquid crystals: static and dynamic studies
The optimization of the experimental parameters of two multiplexed holographic transmission gratings recorded in holographic polymer-dispersed liquid crystals is investigated. Two methods are used to record the holograms: simultaneous and sequential multiplexing. These two processes are optimized to produce two multiplexed Bragg gratings that have the same and the highest possible diffraction efficiencies in the first order. The two methods show similar results when suitable recording parameters are used. The parameters of the recorded gratings (mainly the refractive-index modulation) are retrieved by use of an extension of the rigorous coupled-wave theory to multiplexed gratings. Finally, the response of the holograms to an electric field is studied. We demonstrate few coupling effects between the behavior of both gratings, and we expect a possibility of switching from one grating to the other
The Low-Energy Theorem of Pion Photoproduction in Soliton Models of the Nucleon
We derive an analytic expression for the Kroll-Ruderman amplitude up to the
order 1/N_C for general Skyrme-type models of the nucleon. Due to the
degeneracy of intermediate N- and Delta-states we find deviations from the
standard low-energy theorem for the photoproduction of neutral pions.Comment: 17 pages, LATEX, SI-93-TP3S
Fast compression of a cold atomic cloud using a blue detuned crossed dipole trap
We present the experimental realization of a compressible blue detuned
crossed dipole trap for cold atoms allowing for fast dynamical compression (~ 5
- 10 ms) of 5x10^7 Rubidium atoms up to densities of ~ 10^13 cm^-3. The dipole
trap consists of two intersecting tubes of blue-detuned laser light. These
tubes are formed using a single, rapidly rotating laser beam which, for
sufficiently fast rotation frequencies, can be accurately described by a
quasi-static potential. The atomic cloud is compressed by dynamically reducing
the trap volume leading to densities close to the Ioffe-Reggel criterion for
light localization.Comment: 14 pages, 15 figures, 2 table
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Diversity in the Digital Age: Integrating Pedagogy and Technology for Equity and Inclusion
This qualitative dissertation explores the perceptions of how and why teachers might integrate technology to support their goals of equity and inclusion with a group of teachers who identify as culturally responsive in their pedagogy and describe themselves as fluent in the use of technology in school. Teachers working with students of diverse backgrounds were chosen purposively using an "extreme case sampling" method in order to interview experienced and pedagogically aligned participants. Drawing on in-depth interviews, a review of class artifacts and documents, and a focus group, this study provides critical insights into how self-identified culturally relevant teachers use technology. Discussion of the findings focused on two areas. The first examined how the unique affordances of technology lend themselves as a critical resource for teachers engaged in culturally responsive pedagogy. The second looked at how the self-directed approach of participant teachers led them to seek learning opportunities through informal means, in particular with peers they saw as aligned with their own thoughtful practice in service of their beliefs and values for equity and inclusion. Participants provided evidence that technology can be an active dimension of their work toward equity and inclusion. Thus, this research expands upon existing literature on pedagogical practice in both technology in education and diverse classrooms
Temporal intensity correlation of light scattered by a hot atomic vapor
We present temporal intensity correlation measurements of light scattered by
a hot atomic vapor. Clear evidence of photon bunching is shown at very short
time-scales (nanoseconds) imposed by the Doppler broadening of the hot vapor.
Moreover, we demonstrate that relevant information about the scattering
process, such as the ratio of single to multiple scattering, can be deduced
from the measured intensity correlation function. These measurements confirm
the interest of temporal intensity correlation to access non-trivial spectral
features, with potential applications in astrophysics
Determination of the chiral coupling constants c3 and c4 in new pp and np partial-wave analyses
As a first result of two new partial-wave analyses, one of the pp and another
one of the np scattering data below 500 MeV, we report a study of the
long-range chiral two-pion exchange interaction which contains the chiral
coupling constants c1, c3, and c4. By using as input a theoretical value for c1
we are able to determine in pp as well as in np scattering accurate values for
c3 and c4. The values determined from the pp data and independently from the np
data are in very good agreement, indicating the correctness of the chiral
two-pion exchange interaction. The weighted averages are c3 = -4.78(10) / GeV
and c4 = 3.96(22) / GeV, where the errors are statistical. The value of c3 is
best determined from the pp data and that of c4 from the np data.Comment: 9 pages, 1 figure. Accepted for publication in Phys. Rev.
A study of the parity-odd nucleon-nucleon potential
We investigate the parity-violating nucleon-nucleon potential as obtained in
chiral effective field theory. By using resonance saturation we compare the
chiral potential to the more traditional one-meson exchange potential. In
particular, we show how parameters appearing in the different approaches can be
compared with each other and demonstrate that analyses of parity violation in
proton-proton scattering within the different approaches are in good agreement.
In the second part of this work, we extend the parity-violating potential to
next-to-next-to-leading order. We show that generally it includes both
one-pion- and two-pion-exchange corrections, but the former play no significant
role. The two-pion-exchange corrections depend on five new low-energy constants
which only become important if the leading-order weak pion-nucleon constant
turns out to be very small.Comment: Published versio
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