70,775 research outputs found
Modeling association between DNA copy number and gene expression with constrained piecewise linear regression splines
DNA copy number and mRNA expression are widely used data types in cancer
studies, which combined provide more insight than separately. Whereas in
existing literature the form of the relationship between these two types of
markers is fixed a priori, in this paper we model their association. We employ
piecewise linear regression splines (PLRS), which combine good interpretation
with sufficient flexibility to identify any plausible type of relationship. The
specification of the model leads to estimation and model selection in a
constrained, nonstandard setting. We provide methodology for testing the effect
of DNA on mRNA and choosing the appropriate model. Furthermore, we present a
novel approach to obtain reliable confidence bands for constrained PLRS, which
incorporates model uncertainty. The procedures are applied to colorectal and
breast cancer data. Common assumptions are found to be potentially misleading
for biologically relevant genes. More flexible models may bring more insight in
the interaction between the two markers.Comment: Published in at http://dx.doi.org/10.1214/12-AOAS605 the Annals of
Applied Statistics (http://www.imstat.org/aoas/) by the Institute of
Mathematical Statistics (http://www.imstat.org
The noise properties of 42 millisecond pulsars from the European Pulsar Timing Array and their impact on gravitational wave searches
The sensitivity of Pulsar Timing Arrays to gravitational waves depends on the
noise present in the individual pulsar timing data. Noise may be either
intrinsic or extrinsic to the pulsar. Intrinsic sources of noise will include
rotational instabilities, for example. Extrinsic sources of noise include
contributions from physical processes which are not sufficiently well modelled,
for example, dispersion and scattering effects, analysis errors and
instrumental instabilities. We present the results from a noise analysis for 42
millisecond pulsars (MSPs) observed with the European Pulsar Timing Array. For
characterising the low-frequency, stochastic and achromatic noise component, or
"timing noise", we employ two methods, based on Bayesian and frequentist
statistics. For 25 MSPs, we achieve statistically significant measurements of
their timing noise parameters and find that the two methods give consistent
results. For the remaining 17 MSPs, we place upper limits on the timing noise
amplitude at the 95% confidence level. We additionally place an upper limit on
the contribution to the pulsar noise budget from errors in the reference
terrestrial time standards (below 1%), and we find evidence for a noise
component which is present only in the data of one of the four used telescopes.
Finally, we estimate that the timing noise of individual pulsars reduces the
sensitivity of this data set to an isotropic, stochastic GW background by a
factor of >9.1 and by a factor of >2.3 for continuous GWs from resolvable,
inspiralling supermassive black-hole binaries with circular orbits.Comment: Accepted for publication by the Monthly Notices of the Royal
Astronomical Societ
Simulations of partially coherent focal plane imaging arrays: Fisher matrix approach to performance evaluation
Focal plane arrays of bolometers are increasingly employed in astronomy at
far--infrared to millimetre wavelengths. The focal plane fields and the
detectors are both partially coherent in these systems, but no account has
previously been taken of the effect of partial coherence on array performance.
In this paper, we use our recently developed coupled--mode theory of detection
together with Fisher information matrix techniques from signal processing to
characterize the behaviour of partially coherent imaging arrays. We investigate
the effects of the size and coherence length of both the source and the
detectors, and the packing density of the array, on the amount of information
that can be extracted from observations with such arrays.Comment: 14 pages, 7 figures, submitted to MNRAS 7th March 200
Diffusive Nested Sampling
We introduce a general Monte Carlo method based on Nested Sampling (NS), for
sampling complex probability distributions and estimating the normalising
constant. The method uses one or more particles, which explore a mixture of
nested probability distributions, each successive distribution occupying ~e^-1
times the enclosed prior mass of the previous distribution. While NS
technically requires independent generation of particles, Markov Chain Monte
Carlo (MCMC) exploration fits naturally into this technique. We illustrate the
new method on a test problem and find that it can achieve four times the
accuracy of classic MCMC-based Nested Sampling, for the same computational
effort; equivalent to a factor of 16 speedup. An additional benefit is that
more samples and a more accurate evidence value can be obtained simply by
continuing the run for longer, as in standard MCMC.Comment: Accepted for publication in Statistics and Computing. C++ code
available at http://lindor.physics.ucsb.edu/DNes
The NANOGrav 11 yr Data Set: Limits on Gravitational Wave Memory
The mergers of supermassive black hole binaries (SMBHBs) promise to be incredible sources of gravitational waves (GWs). While the oscillatory part of the merger gravitational waveform will be outside the frequency sensitivity range of pulsar timing arrays, the nonoscillatory GW memory effect is detectable. Further, any burst of GWs will produce GW memory, making memory a useful probe of unmodeled exotic sources and new physics. We searched the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) 11 yr data set for GW memory. This data set is sensitive to very low-frequency GWs of ~3 to 400 nHz (periods of ~11 yr–1 month). Finding no evidence for GWs, we placed limits on the strain amplitude of GW memory events during the observation period. We then used the strain upper limits to place limits on the rate of GW memory causing events. At a strain of 2.5 × 10⁻¹⁴, corresponding to the median upper limit as a function of source sky position, we set a limit on the rate of GW memory events at <0.4 yr⁻¹. That strain corresponds to an SMBHB merger with reduced mass of ηM ~ 2 × 10¹⁰ M_⊙ and inclination of ι = π/3 at a distance of 1 Gpc. As a test of our analysis, we analyzed the NANOGrav 9 yr data set as well. This analysis found an anomolous signal, which does not appear in the 11 yr data set. This signal is not a GW, and its origin remains unknown
Sparsity-Cognizant Total Least-Squares for Perturbed Compressive Sampling
Solving linear regression problems based on the total least-squares (TLS)
criterion has well-documented merits in various applications, where
perturbations appear both in the data vector as well as in the regression
matrix. However, existing TLS approaches do not account for sparsity possibly
present in the unknown vector of regression coefficients. On the other hand,
sparsity is the key attribute exploited by modern compressive sampling and
variable selection approaches to linear regression, which include noise in the
data, but do not account for perturbations in the regression matrix. The
present paper fills this gap by formulating and solving TLS optimization
problems under sparsity constraints. Near-optimum and reduced-complexity
suboptimum sparse (S-) TLS algorithms are developed to address the perturbed
compressive sampling (and the related dictionary learning) challenge, when
there is a mismatch between the true and adopted bases over which the unknown
vector is sparse. The novel S-TLS schemes also allow for perturbations in the
regression matrix of the least-absolute selection and shrinkage selection
operator (Lasso), and endow TLS approaches with ability to cope with sparse,
under-determined "errors-in-variables" models. Interesting generalizations can
further exploit prior knowledge on the perturbations to obtain novel weighted
and structured S-TLS solvers. Analysis and simulations demonstrate the
practical impact of S-TLS in calibrating the mismatch effects of contemporary
grid-based approaches to cognitive radio sensing, and robust
direction-of-arrival estimation using antenna arrays.Comment: 30 pages, 10 figures, submitted to IEEE Transactions on Signal
Processin
Tiling strategies for optical follow-up of gravitational wave triggers by wide field of view telescopes
Binary neutron stars are among the most promising candidates for joint
gravitational-wave and electromagnetic astronomy. The goal of this work is to
investigate the strategy of using gravitational wave sky-localizations for
binary neutron star systems, to search for electromagnetic counterparts using
wide field of view optical telescopes. We examine various strategies of
scanning the gravitational wave sky-localizations on the mock 2015-16
gravitational-wave events. We propose an optimal tiling-strategy that would
ensure the most economical coverage of the gravitational wave sky-localization,
while keeping in mind the realistic constrains of transient optical astronomy.
Our analysis reveals that the proposed tiling strategy improves the
sky-localization coverage over naive contour-covering method. The improvement
is more significant for observations conducted using larger field of view
telescopes, or for observations conducted over smaller confidence interval of
gravitational wave sky-localization probability distribution. Next, we
investigate the performance of the tiling strategy for telescope arrays and
compare their performance against monolithic giant field of view telescopes. We
observed that distributing the field of view of the telescopes into arrays of
multiple telescopes significantly improves the coverage efficiency by as much
as 50% over a single large FOV telescope in 2016 localizations while scanning
around 100 sq. degrees. Finally, we studied the ability of optical counterpart
detection by various types of telescopes. In Our analysis for a range of wide
field-of-view telescopes we found improvement in detection upon sacrificing
coverage of localization in order to achieve greater observation depth for very
large field-of-view - small aperture telescopes, especially if the intrinsic
brightness of the optical counterparts are weak.Comment: Accepted for publication in A&A. 10 pages, 10 figure
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