4,978 research outputs found
Where Are the r-modes? Chandra Observations of Millisecond Pulsars
We present the results of {\it Chandra} observations of two non-accreting
millisecond pulsars, PSRs J16402224 (J1640) and J17092313 (J1709), with
low inferred magnetic fields and spin-down rates in order to constrain their
surface temperatures, obtain limits on the amplitude of unstable -modes in
them, and make comparisons with similar limits obtained for a sample of
accreting low-mass X-ray binary (LMXB) neutron stars. We detect both pulsars in
the X-ray band for the first time. They are faint, with inferred soft X-ray
fluxes ( keV) of and erg
cm s for J1640 and J1709, respectively. Spectral analysis
assuming hydrogen atmosphere emission gives global effective temperature upper
limits ( confidence) of K for J1640 and K for J1709, where the low end of the range corresponds to
canonical neutron stars (), and the upper end corresponds to
higher-mass stars (). Under the assumption that -mode
heating provides the thermal support, we obtain dimensionless -mode
amplitude upper limits of and for J1640 and J1709, respectively, where again the low end of the
range corresponds to lower-mass, canonical neutron stars ().
These limits are about an order of magnitude lower than those we derived
previously for a sample of LMXBs, except for the accreting millisecond X-ray
pulsar (AMXP) SAX J1808.43658, which has a comparable amplitude limit to
J1640 and J1709.Comment: 9 pages, 4 figures, published in Ap
Examining the opportunities of importing LED Lighting from China to New Zealand
The aim of my research was to undertake a feasibility study on how to diversify a China LED business in New Zealand. This research focuses on the feasibility study of associating with the LED lighting market. The primary analysis consisted of interviewing the owner of the particular business. Because it was a feasibility study it was not necessary to interview customers; I used a qualitative method to gain information.
The overall results outline that the company has a professional team in their group, but they do not have a good foreign platform that can sell their products. The owner has sufficient funding himself to open up a branch in New Zealand., He will use his money to invest in prospective properties with a high return. The discussion was about the artificial lighting bulb which is taking up roughly 20% of the world’s energy consumption. However, more energy efficient LED lights can be an alternative to replace that 70 % of other lights.
The conclusion was that most lighting companies have started manufacturing LED lights, and certain Chinese manufacturers are entering the market with cheaper LED products. LED does contain some poisonous substances such as gallium arsenide. The quantity of this substance is very small and thus LED can be considered as a greener alternative to fluorescent lamps.
The recommendations could be people should pay more attention to LED bulbs in that they are energy saving and environmentally sound, which it is a good way to protect the environment
X-ray Burst Oscillations: From Flame Spreading to the Cooling Wake
Type I X-ray bursts are thermonuclear flashes observed from the surfaces of
accreting neutron stars (NSs) in Low Mass X-ray Binaries. Oscillations have
been observed during the rise and/or decay of some of these X-ray bursts. Those
seen during the rise can be well explained by a spreading hot spot model, but
large amplitude oscillations in the decay phase remain mysterious because of
the absence of a clear-cut source of asymmetry. To date there have not been any
quantitative studies that consistently track the oscillation amplitude both
during the rise and decay (cooling tail) of bursts. Here we compute the light
curves and amplitudes of oscillations in X-ray burst models that realistically
account for both flame spreading and subsequent cooling. We present results for
several such "cooling wake" models, a "canonical" cooling model where each
patch on the NS surface heats and cools identically, or with a
latitude-dependent cooling timescale set by the local effective gravity, and an
"asymmetric" model where parts of the star cool at significantly different
rates. We show that while the canonical cooling models can generate
oscillations in the tails of bursts, they cannot easily produce the highest
observed modulation amplitudes. Alternatively, a simple phenomenological model
with asymmetric cooling can achieve higher amplitudes consistent with the
observations.Comment: 8 pages, 7 figures, Accepted for publication in ApJ, Additional
calculations and discussion compared to v
A Taxonomy for Management and Optimization of Multiple Resources in Edge Computing
Edge computing is promoted to meet increasing performance needs of
data-driven services using computational and storage resources close to the end
devices, at the edge of the current network. To achieve higher performance in
this new paradigm one has to consider how to combine the efficiency of resource
usage at all three layers of architecture: end devices, edge devices, and the
cloud. While cloud capacity is elastically extendable, end devices and edge
devices are to various degrees resource-constrained. Hence, an efficient
resource management is essential to make edge computing a reality. In this
work, we first present terminology and architectures to characterize current
works within the field of edge computing. Then, we review a wide range of
recent articles and categorize relevant aspects in terms of 4 perspectives:
resource type, resource management objective, resource location, and resource
use. This taxonomy and the ensuing analysis is used to identify some gaps in
the existing research. Among several research gaps, we found that research is
less prevalent on data, storage, and energy as a resource, and less extensive
towards the estimation, discovery and sharing objectives. As for resource
types, the most well-studied resources are computation and communication
resources. Our analysis shows that resource management at the edge requires a
deeper understanding of how methods applied at different levels and geared
towards different resource types interact. Specifically, the impact of mobility
and collaboration schemes requiring incentives are expected to be different in
edge architectures compared to the classic cloud solutions. Finally, we find
that fewer works are dedicated to the study of non-functional properties or to
quantifying the footprint of resource management techniques, including
edge-specific means of migrating data and services.Comment: Accepted in the Special Issue Mobile Edge Computing of the Wireless
Communications and Mobile Computing journa
Formal Verification of Input-Output Mappings of Tree Ensembles
Recent advances in machine learning and artificial intelligence are now being
considered in safety-critical autonomous systems where software defects may
cause severe harm to humans and the environment. Design organizations in these
domains are currently unable to provide convincing arguments that their systems
are safe to operate when machine learning algorithms are used to implement
their software.
In this paper, we present an efficient method to extract equivalence classes
from decision trees and tree ensembles, and to formally verify that their
input-output mappings comply with requirements. The idea is that, given that
safety requirements can be traced to desirable properties on system
input-output patterns, we can use positive verification outcomes in safety
arguments. This paper presents the implementation of the method in the tool
VoTE (Verifier of Tree Ensembles), and evaluates its scalability on two case
studies presented in current literature.
We demonstrate that our method is practical for tree ensembles trained on
low-dimensional data with up to 25 decision trees and tree depths of up to 20.
Our work also studies the limitations of the method with high-dimensional data
and preliminarily investigates the trade-off between large number of trees and
time taken for verification
Omnidirectionally Bending to the Normal in epsilon-near-Zero Metamaterials
Contrary to conventional wisdom that light bends away from the normal at the
interface when it passes from high to low refractive index media, here we
demonstrate an exotic phenomenon that the direction of electromagnetic power
bends towards the normal when light is incident from arbitrary high refractive
index medium to \epsilon-near-zero metamaterial. Moreover, the direction of the
transmitted beam is close to the normal for all angles of incidence. In other
words, the electromagnetic power coming from different directions in air or
arbitrary high refractive index medium can be redirected to the direction
almost parallel to the normal upon entering the \epsilon-near-zero
metamaterial. This phenomenon is counterintuitive to the behavior described by
conventional Snell's law and resulted from the interplay between
\epsilon-near-zero and material loss. This property has potential applications
in communications to increase acceptance angle and energy delivery without
using optical lenses and mechanical gimbals
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