5,910 research outputs found
Interference of stochastic resonances: Splitting of Kramers' rate
We consider the escape of particles located in the middle well of a symmetric
triple well potential driven sinusoidally by two forces such that the potential
wells roll as in stochastic resonance and the height of the potential barrier
oscillates symmetrically about a mean as in resonant activation. It has been
shown that depending on their phase difference the application of these two
synchronized signals may lead to a splitting of time averaged Kramers' escape
rate and a preferential product distribution in a parallel chemical reaction in
the steady state
Bayesian reconstruction of binary media with unresolved fine-scale spatial structures
We present a Bayesian technique to estimate the fine-scale properties of a binary medium from multiscale observations. The binary medium of interest consists of spatially varying proportions of low and high permeability material with an isotropic structure. Inclusions of one material within the other are far smaller than the domain sizes of interest, and thus are never explicitly resolved. We consider the problem of estimating the spatial distribution of the inclusion proportion, F(x), and a characteristic length-scale of the inclusions, δ, from sparse multiscale measurements. The observations consist of coarse-scale (of the order of the domain size) measurements of the effective permeability of the medium (i.e., static data) and tracer breakthrough times (i.e., dynamic data), which interrogate the fine scale, at a sparsely distributed set of locations. This ill-posed problem is regularized by specifying a Gaussian process model for the unknown field F(x) and expressing it as a superposition of Karhunen–Loève modes. The effect of the fine-scale structures on the coarse-scale effective permeability i.e., upscaling, is performed using a subgrid-model which includes δ as one of its parameters. A statistical inverse problem is posed to infer the weights of the Karhunen–Loève modes and δ, which is then solved using an adaptive Markov Chain Monte Carlo method. The solution yields non-parametric distributions for the objects of interest, thus providing most probable estimates and uncertainty bounds on latent structures at coarse and fine scales. The technique is tested using synthetic data. The individual contributions of the static and dynamic data to the inference are also analyzed.United States. Dept. of Energy. National Nuclear Security Administration (Contract DE-AC04_94AL85000
Tomography of the Quark Gluon Plasma by Heavy Quarks
Using the recently published model \cite{Gossiaux:2008jv,goss2} for the
collisional energy loss of heavy quarks in a Quark Gluon Plasma (QGP), based on
perturbative QCD (pQCD), we study the centrality dependence of and
, %= \frac{dN_{AA}/dp_T}{ dN_{pp}/dp_T}$
measured by the Phenix collaboration, and compare our model with other
approaches based on pQCD and on Anti de Sitter/ Conformal Field Theory
(AdS/CFT)Comment: proceedings for SQM0
Measuring Active-Sterile Neutrino Oscillations with a Stopped Pion Neutrino Source
The question of the existence of light sterile neutrinos is of great interest
in many areas of particle physics, astrophysics, and cosmology. Furthermore,
should the MiniBooNE experiment at Fermilab confirm the LSND oscillation
signal, then new measurements are required to identify the mechanism
responsible for these oscillations. Possibilities include sterile neutrinos, CP
or CPT violation, variable mass neutrinos, Lorentz violation, and extra
dimensions. In this paper, we consider an experiment at a stopped pion neutrino
source to determine if active-sterile neutrino oscillations with delta-m
greater than 0.1 eV2 can account for the signal. By exploiting stopped pi+
decay to produce a monoenergetic nu_mu source, and measuring the rate of the
neutral current reaction nu_x + 12C -> nu_x +12C* as a function of distance
from the source, we show that a convincing test for active-sterile neutrino
oscillations can be performed.Comment: 10 pages, 9 figure
Model selection in High-Dimensions: A Quadratic-risk based approach
In this article we propose a general class of risk measures which can be used
for data based evaluation of parametric models. The loss function is defined as
generalized quadratic distance between the true density and the proposed model.
These distances are characterized by a simple quadratic form structure that is
adaptable through the choice of a nonnegative definite kernel and a bandwidth
parameter. Using asymptotic results for the quadratic distances we build a
quick-to-compute approximation for the risk function. Its derivation is
analogous to the Akaike Information Criterion (AIC), but unlike AIC, the
quadratic risk is a global comparison tool. The method does not require
resampling, a great advantage when point estimators are expensive to compute.
The method is illustrated using the problem of selecting the number of
components in a mixture model, where it is shown that, by using an appropriate
kernel, the method is computationally straightforward in arbitrarily high data
dimensions. In this same context it is shown that the method has some clear
advantages over AIC and BIC.Comment: Updated with reviewer suggestion
The long-term evolution of the spin, pulse shape, and orbit of the accretion-powered millisecond pulsar SAX J1808.4-3658
We present a 7 yr timing study of the 2.5 ms X-ray pulsar SAX J1808.4-3658,
an X-ray transient with a recurrence time of ~2 yr, using data from the Rossi
X-ray Timing Explorer covering 4 transient outbursts (1998-2005). We verify
that the 401 Hz pulsation traces the spin frequency fundamental and not a
harmonic. Substantial pulse shape variability, both stochastic and systematic,
was observed during each outburst. Analysis of the systematic pulse shape
changes suggests that, as an outburst dims, the X-ray "hot spot" on the pulsar
surface drifts longitudinally and a second hot spot may appear. The overall
pulse shape variability limits the ability to measure spin frequency evolution
within a given X-ray outburst (and calls previous nudot measurements of this
source into question), with typical upper limits of |nudot| < 2.5x10^{-14} Hz/s
(2 sigma). However, combining data from all the outbursts shows with high (6
sigma) significance that the pulsar is undergoing long-term spin down at a rate
nudot = (-5.6+/-2.0)x10^{-16} Hz/s, with most of the spin evolution occurring
during X-ray quiescence. We discuss the possible contributions of magnetic
propeller torques, magnetic dipole radiation, and gravitational radiation to
the measured spin down, setting an upper limit of B < 1.5x10^8 G for the
pulsar's surface dipole magnetic field and and Q/I < 5x10^{-9} for the
fractional mass quadrupole moment. We also measured an orbital period
derivative of Pdot = (3.5+/-0.2)x10^{-12} s/s. This surprising large Pdot is
reminiscent of the large and quasi-cyclic orbital period variation observed in
the so-called "black widow" millisecond radio pulsars, supporting speculation
that SAX J1808.4-3658 may turn on as a radio pulsar during quiescence. In an
appendix we derive an improved (0.15 arcsec) source position from optical data.Comment: 22 pages, 10 figures; accepted for publication in Ap
Simultaneous X-ray and Radio Monitoring of the Unusual Binary LSI+61 303: Measurements of the Lightcurve and High-Energy Spectrum
The binary system, LSI+61 303, is unusual both because of the dramatic,
periodic, radio outbursts, and because of its possible association with the 100
MeV gamma-ray source, 2CG135+01. We have performed simultaneous radio and Rossi
X-ray Timing Explorer X-ray observations at eleven intervals over the 26.5 day
orbit, and in addition searched for variability on timescales ranging from
milliseconds to hours. We confirm the modulation of the X-ray emission on
orbital timescales originally reported by Taylor et al. (1996), and in addition
we find a significant offset between the peak of the X-ray and radio flux. We
argue that based on these results, the most likely X-ray emission mechanism is
inverse Compton scattering of stellar photons off of electrons accelerated at
the shock boundary between the relativistic wind of a young pulsar and the Be
star wind. In these observations we also detected 2 -- 150 keV flux from the
nearby low-redshift quasar QSO~0241+622. Comparing these measurements to
previous hard X-ray and gamma-ray observations of the region containing both
LSI+61 303 and QSO~0241+622, it is clear that emission from the QSO dominates.Comment: 23 pages, 6 figures, Accepted for publication in the Astrophysical
Journa
Clustering between high-mass X-ray binaries and OB associations in the Milky Way
We present the first direct measurement of the spatial cross-correlation
function of high-mass X-ray binaries (HMXBs) and active OB star-forming
complexes in the Milky Way. This result relied on a sample containing 79 hard
X-ray selected HMXBs and 458 OB associations. Clustering between the two
populations is detected with a significance above 7-sigmas for distances < 1
kpc. Thus, HMXBs closely trace the underlying distribution of the massive
star-forming regions that are expected to produce the progenitor stars of
HMXBs. The average offset of 0.4+-0.2 kpc between HMXBs and OB associations is
consistent with being due to natal kicks at velocities of the order of 100+-50
km/s. The characteristic scale of the correlation function suggests an average
kinematical age (since the supernova phase) of ~4 Myr for the HMXB population.
Despite being derived from a global view of our Galaxy, these signatures of
HMXB evolution are consistent with theoretical expectations as well as
observations of individual objects.Comment: 18 pages, 10 figures, 3 tables, accepted for publication in Ap
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