2,104 research outputs found
Application of the Gaussian mixture model in pulsar astronomy -- pulsar classification and candidates ranking for {\it Fermi} 2FGL catalog
Machine learning, algorithms to extract empirical knowledge from data, can be
used to classify data, which is one of the most common tasks in observational
astronomy. In this paper, we focus on Bayesian data classification algorithms
using the Gaussian mixture model and show two applications in pulsar astronomy.
After reviewing the Gaussian mixture model and the related
Expectation-Maximization algorithm, we present a data classification method
using the Neyman-Pearson test. To demonstrate the method, we apply the
algorithm to two classification problems. Firstly, it is applied to the well
known period-period derivative diagram, where we find that the pulsar
distribution can be modeled with six Gaussian clusters, with two clusters for
millisecond pulsars (recycled pulsars) and the rest for normal pulsars. From
this distribution, we derive an empirical definition for millisecond pulsars as
. The two
millisecond pulsar clusters may have different evolutionary origins, since the
companion stars to these pulsars in the two clusters show different chemical
composition. Four clusters are found for normal pulsars. Possible implications
for these clusters are also discussed. Our second example is to calculate the
likelihood of unidentified \textit{Fermi} point sources being pulsars and rank
them accordingly. In the ranked point source list, the top 5% sources contain
50% known pulsars, the top 50% contain 99% known pulsars, and no known active
galaxy (the other major population) appears in the top 6%. Such a ranked list
can be used to help the future follow-up observations for finding pulsars in
unidentified \textit{Fermi} point sources.Comment: 9 pages, 4 figures, accepted by MNRA
Thyroid cell irradiation by radioiodines: a new Monte Carlo electron track-structure code
The characteristics of electrical trees in the inner and outer layers of different voltage rating XLPE cable insulation
Convexity criteria and uniqueness of absolutely minimizing functions
We show that absolutely minimizing functions relative to a convex Hamiltonian
are uniquely determined by their boundary
values under minimal assumptions on Along the way, we extend the known
equivalences between comparison with cones, convexity criteria, and absolutely
minimizing properties, to this generality. These results perfect a long
development in the uniqueness/existence theory of the archetypal problem of the
calculus of variations in Comment: 34 page
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Photoluminescence, recombination induced luminescence and electroluminescence in epoxy resin
Dielectric breakdown of epoxies is preceded by light emission, or so-called electroluminescence, from the solid-state material. Very little is known about the luminescence properties of epoxies. The aim of this paper is to derive information that can be used as a basis to understand the nature of the excited states and their involvement in electrical degradation processes. Three different kinds of stimulation were used to excite the material luminescence. Photoluminescence was performed on the base resin, the hardener and the cured resin. Luminescence excited by a silent discharge has been analysed to identify which of the luminescent centres are optically active upon the recombination of electrical charges and could therefore act as charge traps. Finally, the electroluminescence spectrum has been acquired and compared with the previous ones. Although the identification of the origin of these emissions is far from being complete, it has been found that the photoluminescence from the cured resin is due to in-chain chromophores, which acts as trapping centres. The excited states involved in photoluminescence also seems to be involved in electroluminescence, but other components are detected as well, which could be due to the degradation of the resin molecule under the effect of the electric stress
Mechanical behavior of Zr-based bulk metallic glasses
Bulk metallic glasses have a very high corrosion resistance and mechanical strength. Bulk metallic glasses show elastic-perfectly plastic behavior with an extended region of elastic strain (β 2%). But at room temperature their macroscopic plasticity is weak even though a local plastic strain is observed in shear bands. A relaxation analysis allowed studying micro-mechanisms of plastic deformation and estimating the apparent activation volume (β 2000 ΓΒ³).ΠΡΡΠΎΠΊΠΎΠΏΡΠΎΡΠ½ΡΠ΅ ΡΡΠ΅ΠΊΠ»Π° Π½Π° ΠΎΡΠ½ΠΎΠ²Π΅ ΠΌΠ΅ΡΠ°Π»Π»ΠΎΠ² ΠΈΠΌΠ΅ΡΡ ΠΎΡΠ΅Π½Ρ Π²ΡΡΠΎΠΊΠΈΠ΅ Ρ
Π°ΡΠ°ΠΊΡΠ΅ΡΠΈΡΡΠΈΠΊΠΈ ΠΊΠΎΡΡΠΎΠ·ΠΈΠΎΠ½Π½ΠΎΠΉ ΡΡΡΠΎΠΉΡΠΈΠ²ΠΎΡΡΠΈ ΠΈ ΠΌΠ΅Ρ
Π°Π½ΠΈΡΠ΅ΡΠΊΠΎΠΉ ΠΏΡΠΎΡΒΠ½ΠΎΡΡΠΈ. ΠΡ
Π΄Π΅ΡΠΎΡΠΌΠΈΡΠΎΠ²Π°Π½ΠΈΠ΅ ΡΠ²Π»ΡΠ΅ΡΡΡ Π°Π±ΡΠΎΠ»ΡΡΠ½ΠΎ ΡΠΏΡΡΠ³ΠΎΠΏΠ»Π°ΡΡΠΈΡΠ΅ΡΠΊΠΈΠΌ Ρ ΠΏΡΠΎΡΡΠΆΠ΅Π½Π½ΡΠΌ ΡΡΠ°ΡΡΠΊΠΎΠΌ ΡΠΏΡΡΠ³ΠΎΡΡΠΈ (β 2%). ΠΠ΄Π½Π°ΠΊΠΎ ΠΏΡΠΈ ΠΊΠΎΠΌΠ½Π°ΡΠ½ΠΎΠΉ ΡΠ΅ΠΌΠΏΠ΅ΡΠ°ΡΡΡΠ΅ ΠΈΡ
ΠΌΠ°ΠΊΡΠΎΠΏΠ»Π°ΡΡΠΈΡΠ½ΠΎΡΡΡ ΠΏΡΠΎΡΠ²Π»ΡΠ΅ΡΡΡ ΡΠ»Π°Π±ΠΎ, Π½Π΅ΡΠΌΠΎΡΡΡ Π½Π° Π½Π°Π»ΠΈΡΠΈΠ΅ Π»ΠΎΠΊΠ°Π»ΡΠ½ΡΡ
ΠΏΠ»Π°ΡΡΠΈΡΠ΅ΡΠΊΠΈΡ
Π΄Π΅ΡΠΎΡΠΌΠ°ΡΠΈΠΉ Π² ΠΏΠΎΠ»ΠΎΒΡΠ°Ρ
ΡΠΊΠΎΠ»ΡΠΆΠ΅Π½ΠΈΡ. ΠΠ½Π°Π»ΠΈΠ· ΡΠ΅Π»Π°ΠΊΡΠ°ΡΠΈΠΈ Π½Π°ΠΏΡΡΠΆΠ΅Π½ΠΈΠΉ ΠΏΠΎΠ·Π²ΠΎΠ»ΠΈΠ» ΠΈΡΡΠ»Π΅Π΄ΠΎΠ²Π°ΡΡ ΠΌΠΈΠΊΡΠΎΠΌΠ΅Ρ
Π°Π½ΠΈΠ·ΒΠΌΡ ΠΏΠ»Π°ΡΡΠΈΡΠ΅ΡΠΊΠΎΠ³ΠΎ Π΄Π΅ΡΠΎΡΠΌΠΈΡΠΎΠ²Π°Π½ΠΈΡ ΠΈ ΠΎΡΠ΅Π½ΠΈΡΡ Π·Π½Π°ΡΠ΅Π½ΠΈΠ΅ ΠΎΠ±ΡΠ΅ΠΌΠ° Π°ΠΊΡΠΈΠ²Π°ΡΠΈΠΈ (β 2000 ΓΒ³)
Development of a pulsar-based timescale
Using observations of pulsars from the Parkes Pulsar Timing Array (PPTA)
project we develop the first pulsar-based timescale that has a precision
comparable to the uncertainties in international atomic timescales. Our
ensemble of pulsars provides an Ensemble Pulsar Scale (EPS) analogous to the
free atomic timescale Echelle Atomique Libre (EAL). The EPS can be used to
detect fluctuations in atomic timescales and therefore can lead to a new
realisation of Terrestrial Time, TT(PPTA11). We successfully follow features
known to affect the frequency of the International Atomic Timescale (TAI) and
we find marginally significant differences between TT(PPTA11) and TT(BIPM11).
We discuss the various phenomena that lead to a correlated signal in the pulsar
timing residuals and therefore limit the stability of the pulsar timescale.Comment: Accepted for publication in MNRA
Understanding Paramagnetic Spin Correlations in the Spin-Liquid Pyrochlore Tb2Ti2O7
Recent elastic and inelastic neutron scattering studies of the highly
frustrated pyrochlore antiferromagnet Tb2Ti2O7 have shown some very intriguing
features that cannot be modeled by the local classical Ising model,
naively expected to describe this system at low temperatures. Using the random
phase approximation to take into account fluctuations between the ground state
doublet and the first excited doublet, we successfully describe the elastic
neutron scattering pattern and dispersion relations in Tb2Ti2O7,
semi-quantitatively consistent with experimental observations.Comment: revtex4, 4 pages, 1 Color+ 2 BW figure
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