78 research outputs found
Gravitational waves from Sco X-1: A comparison of search methods and prospects for detection with advanced detectors
The low-mass X-ray binary Scorpius X-1 (Sco X-1) is potentially the most
luminous source of continuous gravitational-wave radiation for interferometers
such as LIGO and Virgo. For low-mass X-ray binaries this radiation would be
sustained by active accretion of matter from its binary companion. With the
Advanced Detector Era fast approaching, work is underway to develop an array of
robust tools for maximizing the science and detection potential of Sco X-1. We
describe the plans and progress of a project designed to compare the numerous
independent search algorithms currently available. We employ a mock-data
challenge in which the search pipelines are tested for their relative
proficiencies in parameter estimation, computational efficiency, robust- ness,
and most importantly, search sensitivity. The mock-data challenge data contains
an ensemble of 50 Scorpius X-1 (Sco X-1) type signals, simulated within a
frequency band of 50-1500 Hz. Simulated detector noise was generated assuming
the expected best strain sensitivity of Advanced LIGO and Advanced VIRGO ( Hz). A distribution of signal amplitudes was then
chosen so as to allow a useful comparison of search methodologies. A factor of
2 in strain separates the quietest detected signal, at
strain, from the torque-balance limit at a spin frequency of 300 Hz, although
this limit could range from (25 Hz) to (750 Hz) depending on the unknown frequency of Sco X-1. With future
improvements to the search algorithms and using advanced detector data, our
expectations for probing below the theoretical torque-balance strain limit are
optimistic.Comment: 33 pages, 11 figure
Robust Filtering and Smoothing with Gaussian Processes
We propose a principled algorithm for robust Bayesian filtering and smoothing
in nonlinear stochastic dynamic systems when both the transition function and
the measurement function are described by non-parametric Gaussian process (GP)
models. GPs are gaining increasing importance in signal processing, machine
learning, robotics, and control for representing unknown system functions by
posterior probability distributions. This modern way of "system identification"
is more robust than finding point estimates of a parametric function
representation. In this article, we present a principled algorithm for robust
analytic smoothing in GP dynamic systems, which are increasingly used in
robotics and control. Our numerical evaluations demonstrate the robustness of
the proposed approach in situations where other state-of-the-art Gaussian
filters and smoothers can fail.Comment: 7 pages, 1 figure, draft version of paper accepted at IEEE
Transactions on Automatic Contro
Some remarks on the size of tubular neighborhoods in contact topology and fillability
The well-known tubular neighborhood theorem for contact submanifolds states
that a small enough neighborhood of such a submanifold N is uniquely determined
by the contact structure on N, and the conformal symplectic structure of the
normal bundle. In particular, if the submanifold N has trivial normal bundle
then its tubular neighborhood will be contactomorphic to a neighborhood of
Nx{0} in the model space NxR^{2k}.
In this article we make the observation that if (N,\xi_N) is a 3-dimensional
overtwisted submanifold with trivial normal bundle in (M,\xi), and if its model
neighborhood is sufficiently large, then (M,\xi) does not admit an exact
symplectic filling.Comment: 19 pages, 2 figures; added example of manifold that is not fillable
by neighborhood criterium; typo
Reset-free Trial-and-Error Learning for Robot Damage Recovery
The high probability of hardware failures prevents many advanced robots
(e.g., legged robots) from being confidently deployed in real-world situations
(e.g., post-disaster rescue). Instead of attempting to diagnose the failures,
robots could adapt by trial-and-error in order to be able to complete their
tasks. In this situation, damage recovery can be seen as a Reinforcement
Learning (RL) problem. However, the best RL algorithms for robotics require the
robot and the environment to be reset to an initial state after each episode,
that is, the robot is not learning autonomously. In addition, most of the RL
methods for robotics do not scale well with complex robots (e.g., walking
robots) and either cannot be used at all or take too long to converge to a
solution (e.g., hours of learning). In this paper, we introduce a novel
learning algorithm called "Reset-free Trial-and-Error" (RTE) that (1) breaks
the complexity by pre-generating hundreds of possible behaviors with a dynamics
simulator of the intact robot, and (2) allows complex robots to quickly recover
from damage while completing their tasks and taking the environment into
account. We evaluate our algorithm on a simulated wheeled robot, a simulated
six-legged robot, and a real six-legged walking robot that are damaged in
several ways (e.g., a missing leg, a shortened leg, faulty motor, etc.) and
whose objective is to reach a sequence of targets in an arena. Our experiments
show that the robots can recover most of their locomotion abilities in an
environment with obstacles, and without any human intervention.Comment: 18 pages, 16 figures, 3 tables, 6 pseudocodes/algorithms, video at
https://youtu.be/IqtyHFrb3BU, code at
https://github.com/resibots/chatzilygeroudis_2018_rt
Towards a funded system of social security: Design and implications ; the case of Germany
What would a feasible system of social security in Germany have looked like in the year of 1995 and beyond? In order to find an answer we describe three base systems: ( l ) a purely funded system of social security, (2) a fully mandatory funded system of social security, and (3) a partially mandatory funded system. It is argued that - neglecting problems of transition - a purely funded system would be the best in economic terms; a fully mandatory funded system would need almost as many controls as the currently prevailing system (often labelled pay-asyou- go system). A partially mandatory funded system, assuring some kind of basic income, would need less controls and less governmental authority than the fully mandatory system but more than a funded system. After quantification of two scenarios which represent components of the three base systems, a system of taxation with respect to contributions and/or benefits is discussed which is at the same time simple in terms of costs of bureaucracy and does not tax economic growth more than necessary.
Model-Based Cross-Correlation Search for Gravitational Waves from Scorpius X-1
We consider the cross-correlation search for periodic GWs and its potential
application to the LMXB Sco X-1. This method coherently combines data from
different detectors at the same time, as well as different times from the same
or different detectors. By adjusting the maximum time offset between a pair of
data segments to be coherently combined, one can tune the method to trade off
sensitivity and computing costs. In particular, the detectable signal amplitude
scales as the inverse fourth root of this coherence time. The improvement in
amplitude sensitivity for a search with a coherence time of 1hr, compared with
a directed stochastic background search with 0.25Hz wide bins is about a factor
of 5.4. We show that a search of 1yr of data from Advanced LIGO and Advanced
Virgo with a coherence time of 1hr would be able to detect GWs from Sco X-1 at
the level predicted by torque balance over a range of signal frequencies from
30-300Hz; if the coherence time could be increased to 10hr, the range would be
20-500Hz. In addition, we consider several technical aspects of the
cross-correlation method: We quantify the effects of spectral leakage and show
that nearly rectangular windows still lead to the most sensitive search. We
produce an explicit parameter-space metric for the cross-correlation search in
general and as applied to a neutron star in a circular binary system. We
consider the effects of using a signal template averaged over unknown amplitude
parameters: the search is sensitive to a combination of the intrinsic signal
amplitude and the inclination of the neutron star rotation axis, and the peak
of the expected detection statistic is systematically offset from the true
signal parameters. Finally, we describe the potential loss of SNR due to
unmodelled effects such as signal phase acceleration within the Fourier
transform timescale and gradual evolution of the spin frequency.Comment: 27 pages, 12 figures, 4 tables, pdflatex; synchronized to final
version published in Phys Rev
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