2,312 research outputs found
Gravitational waves and stalled satellites from massive galaxy mergers at z <= 1
We present a model for merger-driven evolution of the mass function for
massive galaxies and their central supermassive black holes at late times. We
discuss the current observational evidence in favor of merger-driven massive
galaxy evolution during this epoch, and demonstrate that the observed evolution
of the mass function can be reproduced by evolving an initial mass function
under the assumption of negligible star formation. We calculate the stochastic
gravitational wave signal from the resulting black-hole binary mergers in the
low redshift universe (z <= 1) implied by this model, and find that this
population has a signal-to-noise ratio as much as ~5x larger than previous
estimates for pulsar timing arrays, with an expectation value for the
characteristic strain h_c (f=1 yr^{-1}) = 4.1 x 10^{-15} that may already be in
tension with observational constraints, and a {2-sigma, 3-sigma} lower limit
within this model of h_c (f=1 yr^{-1}) = {1.1 x 10^{-15}, 6.8 x 10^{-16}}. The
strength of this signal is sufficient to make it detectable with high
probability under conservative assumptions within the next several years, if
the principle assumption of merger-driven galaxy evolution since z = 1 holds
true. For cases where a galaxy merger fails to lead to a black hole merger, we
estimate the probability for a given number of satellite unmerged black holes
to remain within a massive host galaxy, and interpret the result in light of
ULX observations. In particular, we find that the brightest cluster galaxies
should have 1-2 such sources with luminosities above 10^{39} erg/s, which is
consistent with the statistics of observed ULXs.Comment: 11 pages, 5 figures, submitted to ApJ, v2 includes the referee's
requested change
Stability of exact force-free electrodynamic solutions and scattering from spacetime curvature
Recently, a family of exact force-free electrodynamic (FFE) solutions was
given by Brennan, Gralla and Jacobson, which generalizes earlier solutions by
Michel, Menon and Dermer, and other authors. These solutions have been proposed
as useful models for describing the outer magnetosphere of conducting stars. As
with any exact analytical solution that aspires to describe actual physical
systems, it is vitally important that the solution possess the necessary
stability. In this paper, we show via fully nonlinear numerical simulations
that the aforementioned FFE solutions, despite being highly special in their
properties, are nonetheless stable under small perturbations. Through this
study, we also introduce a three-dimensional pseudospectral relativistic FFE
code that achieves exponential convergence for smooth test cases, as well as
two additional well-posed FFE evolution systems in the appendix that have
desirable mathematical properties. Furthermore, we provide an explicit analysis
that demonstrates how propagation along degenerate principal null directions of
the spacetime curvature tensor simplifies scattering, thereby providing an
intuitive understanding of why these exact solutions are tractable, i.e. why
they are not backscattered by spacetime curvature.Comment: 33 pages, 21 figures; V2 updated to match published versio
Random Projections For Large-Scale Regression
Fitting linear regression models can be computationally very expensive in
large-scale data analysis tasks if the sample size and the number of variables
are very large. Random projections are extensively used as a dimension
reduction tool in machine learning and statistics. We discuss the applications
of random projections in linear regression problems, developed to decrease
computational costs, and give an overview of the theoretical guarantees of the
generalization error. It can be shown that the combination of random
projections with least squares regression leads to similar recovery as ridge
regression and principal component regression. We also discuss possible
improvements when averaging over multiple random projections, an approach that
lends itself easily to parallel implementation.Comment: 13 pages, 3 Figure
Calibration Of The Advanced Ligo Detectors For The Discovery Of The Binary Black-Hole Merger Gw150914
In Advanced LIGO, detection and astrophysical source parameter estimation of the binary black hole merger GW150914 requires a calibrated estimate of the gravitational-wave strain sensed by the detectors. Producing an estimate from each detector\u27s differential arm length control loop readout signals requires applying time domain filters, which are designed from a frequency domain model of the detector\u27s gravitational-wave response. The gravitational-wave response model is determined by the detector\u27s opto-mechanical response and the properties of its feedback control system. The measurements used to validate the model and characterize its uncertainty are derived primarily from a dedicated photon radiation pressure actuator, with cross-checks provided by optical and radio frequency references. We describe how the gravitational-wave readout signal is calibrated into equivalent gravitational-wave-induced strain and how the statistical uncertainties and systematic errors are assessed. Detector data collected over 38 calendar days, from September 12 to October 20, 2015, contain the event GW150914 and approximately 16 days of coincident data used to estimate the event false alarm probability. The calibration uncertainty is less than 10% in magnitude and 10° in phase across the relevant frequency band, 20 Hz to 1 kHz
Gw150914: Implications For The Stochastic Gravitational-Wave Background From Binary Black Holes
Results Of The Deepest All-Sky Survey For Continuous Gravitational Waves On Ligo S6 Data Running On The Einstein@Home Volunteer Distributed Computing Project
Search For Continuous Gravitational Waves From Neutron Stars In Globular Cluster Ngc 6544
We describe a directed search for continuous gravitational waves in data from the sixth LIGO science run. The target was the nearby globular cluster NGC 6544 at a distance of 2.7 kpc. The search covered a broad band of frequencies along with first and second frequency derivatives for a fixed sky position. The search coherently integrated data from the two LIGO interferometers over a time span of 9.2 days using the matched-filtering F-statistic. We found no gravitational-wave signals and set 95% confidence upper limits as stringent as 6.0 X 10^{-25} on intrinsic strain and 8.5 X 10^{-6} on fiducial ellipticity. These values beat the indirect limits from energy conservation for stars with characteristic spindown ages older than 300 years and are within the range of theoretical predictions for possible neutron-star ellipticities. An important feature of this search was use of a barycentric resampling algorithm which substantially reduced computational cost; this method will be used extensively in searches of Advanced LIGO and Virgo detector data
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