34,413 research outputs found
Low Frequency Gravitational Waves from Black Hole MACHO Binaries
Nakamura, Sasaki, Tanaka, and Thorne have recently estimated the initial
distribution of binary MACHOs in the galactic halo assuming that the MACHOs are
primordial half solar mass black holes, and considered their coalescence as a
possible source for ground-based interferometer gravitational wave detectors
such as LIGO. Evolving their binary distribution forward in time to the
present, the low-frequency (10^{-5} < f < 10^{-1} Hz) spectrum of gravitational
waves associated with such a population of compact binaries is calculated. The
resulting gravitational waves would form a strong stochastic background in
proposed space interferometers such as LISA and OMEGA. Low frequency
gravitational waves are likely to become a key tool for determining the
properties of binaries within the dark MACHO population.Comment: 8 pages + 2 ps figures; AASTe
Wet Ontologies, Fluid Spaces: Giving Depth to Volume through Oceanic Thinking
This paper expands on recent attempts to destabilise the static, bordered, and linear framings that typify human geographical studies of place, territory, and time. In a world conceptualised as open, immanent, and ever-becoming, scholars have turned away from notions of fixity towards fluidity and flow, and, in so doing, have developed networked, ‘flat’ ontologies. Recent attempts have gone further, challenging the horizontalism inherent in such approaches by opening up a vertical world of volume. In this paper we contend that such approaches are still somewhat lacking. The vertical element of volume is all too often abstract and dematerialised; the emphasis on materiality that is typically used to rectify this excess of abstraction tends to reproduce a sense of matter as fixed and grounded; and the temporality that is employed to reintroduce ‘motion’ to matter has the unintended effect of signalling a periodised sense of time that minimises the chaotic underpinnings and experiences of place. We argue that the ocean is an ideal spatial foundation for addressing these challenges since it is indisputably voluminous, stubbornly material, and unmistakably undergoing continual reformation, and that a ‘wet ontology’ can reinvigorate, redirect, and reshape debates that are all too often restricted by terrestrial limits
The Shape of Dark Matter Haloes IV. The Structure of Stellar Discs in Edge-on Galaxies
We present optical and near-infrared archival observations of eight edge-on
galaxies. These observations are used to model the stellar content of each
galaxy using the FitSKIRT software package. Using FitSKIRT, we can
self-consistently model a galaxy in each band simultaneously while treating for
dust. This allows us to accurately measure both the scale length and scale
height of the stellar disc, plus the shape parameters of the bulge. By
combining this data with the previously reported integrated magnitudes of each
galaxy, we can infer their true luminosities. We have successfully modelled
seven out of the eight galaxies in our sample. We find that stellar discs can
be modelled correctly, but have not been able to model the stellar bulge
reliably. Our sample consists for the most part of slow rotating galaxies, and
we find that the average dust layer is much thicker than what is reported for
faster rotating galaxies.Comment: Accepted for publication by Monthly Notices RAS. Hi-res. version
available at www.astro.rug.nl/~vdkruit/Petersetal-IV.pd
The Shape of Dark Matter Haloes II. The Galactus HI Modelling & Fitting Tool
We present a new HI modelling tool called \textsc{Galactus}. The program has
been designed to perform automated fits of disc-galaxy models to observations.
It includes a treatment for the self-absorption of the gas. The software has
been released into the public domain. We describe the design philosophy and
inner workings of the program. After this, we model the face-on galaxy NGC2403,
using both self-absorption and optically thin models, showing that
self-absorption occurs even in face-on galaxies. It is shown that the maximum
surface brightness plateaus seen in Paper I of this series are indeed signs of
self-absorption. The apparent HI mass of an edge-on galaxy can be drastically
lower compared to that same galaxy seen face-on. The Tully-Fisher relation is
found to be relatively free from self-absorption issues.Comment: Accepted for publication by Monthly Notices RAS. Hi-res. version
available at www.astro.rug.nl/~vdkruit/Petersetal-II.pd
Unconventional carrier-mediated ferromagnetism above room temperature in ion-implanted (Ga, Mn)P:C
Ion implantation of Mn ions into hole-doped GaP has been used to induce
ferromagnetic behavior above room temperature for optimized Mn concentrations
near 3 at.%. The magnetism is suppressed when the Mn dose is increased or
decreased away from the 3 at.% value, or when n-type GaP substrates are used.
At low temperatures the saturated moment is on the order of one Bohr magneton,
and the spin wave stiffness inferred from the Bloch-law T^3/2 dependence of the
magnetization provides an estimate Tc = 385K of the Curie temperature that
exceeds the experimental value, Tc = 270K. The presence of ferromagnetic
clusters and hysteresis to temperatures of at least 330K is attributed to
disorder and proximity to a metal-insulating transition.Comment: 4 pages, 4 figures (RevTex4
QCD Corrections to Electroweak Vector Boson Scattering at Small Scattering Angles
We investigate the role of a certain class of QCD corrections to electroweak
vector boson scattering at small scattering angles and large energies. These
are present since, from the perturbative analysis, the vector bosons may
dissociate into quark-antiquark pairs giving rise to colour dipoles interacting
through gluon exchanges. After the computation of the vector boson impact
factors, we present expressions for the lowest order QCD scattering amplitude
and for the leading logarithmic BFKL amplitude. Particularly we discuss
numerical results for the process . The QCD corrections to
the cross section resulting from the interference with the electroweak ones are
estimated and compared with the leading pure electroweak part. Corrections
resulting from the leading log BFKL amplitude are of the order of few percent
already at the TeV energy range.Comment: 21 pages,9 figures, discussion on the helicity-flip impact factors
added, typos correcte
Mathematical modelling of ethanol metabolism in normal subjects and chronic alcohol misusers
The time course of ethanol disappearance from the blood has been examined in normal males and females and in alcohol misusers. Blood alcohol estimations were made over a period of 3 hr, following an oral dose of ethanol (0.8 g/kg body weight) administered in the form of whisky. Attempts were made to fit the data to zero order, first order and mixed zero + first order kinetics. In the majority (75%) of normal females the blood ethanol concentration was still increasing at 30 min. This was only seen in 50% of normal males and in 50% of non-dependent alcohol misusers, but not in dependent alcohol misusers. In all of the normal females the disappearance of ethanol could be adequately described by zero order kinetics. However, in the normal male group only 20% could be described by zero order kinetics, 10% fitted first order kinetics and the remainder required a mixed model of zero + first order. The rate constant for the zero order component of the control male group was identical to zero order rate constant obtained for the female control group. In the female alcohol misuser group, 40% of the curves could not be described by zero order kinetics and fitted best to a mixed model. The zero order component of the entire group was significantly increased (by 35%) compared to that obtained for the female control group. In the male dependent and non-dependent alcohol misuser groups, all blood alcohol concentration curves fitted best to mixed zero and first order kinetics. However, no significant differences were noted in the values of the kinetic parameters when compared with the male control group. It is suggested that the zero order component of the blood alcohol concentration curves is due to the action of liver alcohol dehydrogenase and the first order component represents redistribution to the tissues. The presence or absence of a first order component is attributed to differences in absorption rates from the gut
A compact and robust diode laser system for atom interferometry on a sounding rocket
We present a diode laser system optimized for laser cooling and atom
interferometry with ultra-cold rubidium atoms aboard sounding rockets as an
important milestone towards space-borne quantum sensors. Design, assembly and
qualification of the system, combing micro-integrated distributed feedback
(DFB) diode laser modules and free space optical bench technology is presented
in the context of the MAIUS (Matter-wave Interferometry in Microgravity)
mission.
This laser system, with a volume of 21 liters and total mass of 27 kg, passed
all qualification tests for operation on sounding rockets and is currently used
in the integrated MAIUS flight system producing Bose-Einstein condensates and
performing atom interferometry based on Bragg diffraction. The MAIUS payload is
being prepared for launch in fall 2016.
We further report on a reference laser system, comprising a rubidium
stabilized DFB laser, which was operated successfully on the TEXUS 51 mission
in April 2015. The system demonstrated a high level of technological maturity
by remaining frequency stabilized throughout the mission including the rocket's
boost phase
PIPPS: Flexible model-based policy search robust to the curse of chaos
Previously, the exploding gradient problem has
been explained to be central in deep learning and
model-based reinforcement learning, because it
causes numerical issues and instability in optimization.
Our experiments in model-based reinforcement
learning imply that the problem is not
just a numerical issue, but it may be caused by
a fundamental chaos-like nature of long chains
of nonlinear computations. Not only do the magnitudes
of the gradients become large, the direction
of the gradients becomes essentially random.
We show that reparameterization gradients suffer
from the problem, while likelihood ratio gradients
are robust. Using our insights, we develop
a model-based policy search framework, Probabilistic
Inference for Particle-Based Policy Search
(PIPPS), which is easily extensible, and allows
for almost arbitrary models and policies, while
simultaneously matching the performance of previous
data-efficient learning algorithms. Finally,
we invent the total propagation algorithm, which
efficiently computes a union over all pathwise
derivative depths during a single backwards pass,
automatically giving greater weight to estimators
with lower variance, sometimes improving over
reparameterization gradients by 10^6 times
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