34,966 research outputs found
boosting in kernel regression
In this paper, we investigate the theoretical and empirical properties of
boosting with kernel regression estimates as weak learners. We show that
each step of boosting reduces the bias of the estimate by two orders of
magnitude, while it does not deteriorate the order of the variance. We
illustrate the theoretical findings by some simulated examples. Also, we
demonstrate that boosting is superior to the use of higher-order kernels,
which is a well-known method of reducing the bias of the kernel estimate.Comment: Published in at http://dx.doi.org/10.3150/08-BEJ160 the Bernoulli
(http://isi.cbs.nl/bernoulli/) by the International Statistical
Institute/Bernoulli Society (http://isi.cbs.nl/BS/bshome.htm
Regularization, Renormalization and Range: The Nucleon-Nucleon Interaction from Effective Field Theory
Regularization and renormalization is discussed in the context of low-energy
effective field theory treatments of two or more heavy particles (such as
nucleons). It is desirable to regulate the contact interactions from the outset
by treating them as having a finite range. The low energy physical observables
should be insensitive to this range provided that the range is of a similar or
greater scale than that of the interaction. Alternative schemes, such as
dimensional regularization, lead to paradoxical conclusions such as the
impossibility of repulsive interactions for truly low energy effective theories
where all of the exchange particles are integrated out. This difficulty arises
because a nonrelativistic field theory with repulsive contact interactions is
trivial in the sense that the matrix is unity and the renormalized coupling
constant zero. Possible consequences of low energy attraction are also
discussed. It is argued that in the case of large or small scattering lengths,
the region of validity of effective field theory expansion is much larger if
the contact interactions are given a finite range from the beginning.Comment: 7 page
An augmented moment method for stochastic ensembles with delayed couplings: II. FitzHugh-Nagumo model
Dynamics of FitzHugh-Nagumo (FN) neuron ensembles with time-delayed couplings
subject to white noises, has been studied by using both direct simulations and
a semi-analytical augmented moment method (AMM) which has been proposed in a
recent paper [H. Hasegawa, E-print: cond-mat/0311021]. For -unit FN neuron
ensembles, AMM transforms original -dimensional {\it stochastic} delay
differential equations (SDDEs) to infinite-dimensional {\it deterministic} DEs
for means and correlation functions of local and global variables.
Infinite-order recursive DEs are terminated at the finite level in the
level- AMM (AMM), yielding -dimensional deterministic DEs. When a
single spike is applied, the oscillation may be induced if parameters of
coupling strength, delay, noise intensity and/or ensemble size are appropriate.
Effects of these parameters on the emergence of the oscillation and on the
synchronization in FN neuron ensembles have been studied. The synchronization
shows the {\it fluctuation-induced} enhancement at the transition between
non-oscillating and oscillating states. Results calculated by AMM5 are in
fairly good agreement with those obtained by direct simulations.Comment: 15 pages, 3 figures; changed the title with correcting typos,
accepted in Phys. Rev. E with some change
Numerical Study on GRB-Jet Formation in Collapsars
Two-dimensional magnetohydrodynamic simulations are performed using the
ZEUS-2D code to investigate the dynamics of a collapsar that generates a GRB
jet, taking account of realistic equation of state, neutrino cooling and
heating processes, magnetic fields, and gravitational force from the central
black hole and self gravity. It is found that neutrino heating processes are
not so efficient to launch a jet in this study. It is also found that a jet is
launched mainly by B_\phi fields that are amplified by the winding-up effect.
However, since the ratio of total energy relative to the rest mass energy in
the jet is not so high as several hundred, we conclude that the jets seen in
this study are not be a GRB jet. This result suggests that general relativistic
effects, which are not included in this study, will be important to generate a
GRB jet. Also, the accretion disk with magnetic fields may still play an
important role to launch a GRB jet, although a simulation for much longer
physical time (\sim 10-100 s) is required to confirm this effect. It is shown
that considerable amount of 56Ni is synthesized in the accretion disk. Thus
there will be a possibility for the accretion disk to supply sufficient amount
of 56Ni required to explain the luminosity of a hypernova. Also, it is shown
that neutron-rich matter due to electron captures with high entropy per baryon
is ejected along the polar axis. Moreover, it is found that the electron
fraction becomes larger than 0.5 around the polar axis near the black hole by
\nu_e capture at the region. Thus there will be a possibility that r-process
and r/p-process nucleosynthesis occur at these regions. Finally, much neutrons
will be ejected from the jet, which suggests that signals from the neutron
decays may be observed as the delayed bump of afterglow or gamma-rays.Comment: 54 pages with 19 postscript figures. Accepted for publication in ApJ.
High resolution version is available at
http://www2.yukawa.kyoto-u.ac.jp/~nagataki/collapsar.pd
Fermi-LAT Detection of a Break in the Gamma-Ray Spectrum of the Supernova Remnant Cassiopeia A
We report on observations of the supernova remnant Cassiopeia A in the energy
range from 100 MeV to 100 GeV using 44 months of observations from the Large
Area Telescope on board the Fermi Gamma-ray Space Telescope. We perform a
detailed spectral analysis of this source and report on a low-energy break in
the spectrum at GeV. By comparing the results with
models for the gamma-ray emission, we find that hadronic emission is preferred
for the GeV energy range.Comment: 18 pages, 5 figures, 2 tables, to be published in Ap
Spatial Distribution of Metal Emissions in SNR 3C 397 Viewed with Chandra and XMM
We present X-ray equivalent width imaging of the supernova remnant (SNR) 3C
397 for Mg He\alpha, Si He\alpha, S He\alpha, and Fe K\alpha complex lines with
the Chandra and XMM-Newton observations. The images reveal that the heavier the
element is, the smaller the extent of the element distribution is. The Mg
emission is evidently enhanced in the southeastern blow-out region, well along
the radio boundary there, and appears to partially envelope the eastern Fe
knot. Two bilateral hat-like Si line-emitting structures are along the northern
and southern borders, roughly symmetric with respect to the southeast-northwest
elongation axis. An S line-emitting shell is located just inner to the northern
radio and IR shell, indicating of a layer of reversely shocked sulphur in the
ejecta. A few enhanced Fe features are basically aligned along the diagonal of
the rectangular shape of the SNR, which implicates an early asymmetric SN
explosion.Comment: 4 pages, 4 figures, appears in Science China Physics, Mechanics &
Astronomy, 2010, 53 (Suppl.1), 267-27
Rhapsody. I. Structural Properties and Formation History From a Statistical Sample of Re-simulated Cluster-size Halos
We present the first results from the Rhapsody cluster re-simulation project:
a sample of 96 "zoom-in" simulations of dark matter halos of 10^14.8 +- 0.05
Msun/h, selected from a 1 (Gpc/h)^3 volume. This simulation suite is the first
to resolve this many halos with ~5x10^6 particles per halo in the cluster-mass
regime, allowing us to statistically characterize the distribution of and
correlation between halo properties at fixed mass. We focus on the properties
of the main halos and how they are affected by formation history, which we
track back to z=12, over five decades in mass. We give particular attention to
the impact of the formation history on the density profiles of the halos. We
find that the deviations from the Navarro-Frenk-White (NFW) model and the
Einasto model depend on formation time. Late-forming halos tend to have
considerable deviations from both models, partly due to the presence of massive
subhalos, while early-forming halos deviate less but still significantly from
the NFW model and are better described by the Einasto model. We find that the
halo shapes depend only moderately on formation time. Departure from spherical
symmetry impacts the density profiles through the anisotropic distribution of
massive subhalos. Further evidence of the impact of subhalos is provided by
analyzing the phase-space structure. A detailed analysis of the properties of
the subhalo population in Rhapsody is presented in a companion paper.Comment: 20 pages, 13 figures, replaced to match published versio
Stirring the Embers: High Sensitivity VLBI Observations of GRB030329
We present high sensitivity Very Long Baseline Interferometry (VLBI)
observations 806 days after the gamma-ray burst of 2003 March 29 (GRB030329).
The angular diameter of the radio afterglow is measured to be 0.347 +- 0.09
mas, corresponding to 0.99 +- 0.26 pc at the redshift of GRB030329 (z =
0.1685). The evolution of the image size favors a uniform external density over
an R^-2 wind-like density profile (at distances of R >~10^18 cm from the
source), although the latter cannot be ruled out yet. The current apparent
expansion velocity of the image size is only mildly relativistic, suggesting a
non-relativistic transition time of t_NR ~ 1 yr. A rebrightening, or at least a
significant flattening in the flux decay, is expected within the next several
years as the counter-jet becomes visible (this has not yet been observed). An
upper limit of <1.9c is set on the proper motion of the flux centroid.Comment: 16 pages, 5 figures, accepted for publication in Astrophysical
Journa
- …