75,960 research outputs found
Fundamental limitations of high contrast imaging set by small sample statistics
In this paper, we review the impact of small sample statistics on detection
thresholds and corresponding confidence levels (CLs) in high contrast imaging
at small angles. When looking close to the star, the number of resolution
elements decreases rapidly towards small angles. This reduction of the number
of degrees of freedom dramatically affects CLs and false alarm probabilities.
Naively using the same ideal hypothesis and methods as for larger separations,
which are well understood and commonly assume Gaussian noise, can yield up to
one order of magnitude error in contrast estimations at fixed CL. The
statistical penalty exponentially increases towards very small inner working
angles. Even at 5-10 resolution elements from the star, false alarm
probabilities can be significantly higher than expected. Here we present a
rigorous statistical analysis which ensures robustness of the CL, but also
imposes a substantial limitation on corresponding achievable detection limits
(thus contrast) at small angles. This unavoidable fundamental statistical
effect has a significant impact on current coronagraphic and future high
contrast imagers. Finally, the paper concludes with practical recommendations
to account for small number statistics when computing the sensitivity to
companions at small angles and when exploiting the results of direct imaging
planet surveys.Comment: 12 pages, 10 figures, accepted to Ap
Large-Scale Sunyaev-Zel'dovich Effect: Measuring Statistical Properties with Multifrequency Maps
We study the prospects for extracting detailed statistical properties of the
Sunyaev-Zel'dovich (SZ) effect associated with large scale structure using
upcoming multifrequency CMB experiments. The greatest obstacle to detecting the
large-angle signal is the confusion noise provided by the primary anisotropies
themselves, and to a lesser degree galactic and extragalactic foregrounds. We
employ multifrequency subtraction techniques and the latest foregrounds models
to determine the detection threshold for the Boomerang, MAP (several microK)
and Planck CMB (sub microK) experiments. Calibrating a simplified biased-tracer
model of the gas pressure off recent hydrodynamic simulations, we estimate the
SZ power spectrum, skewness and bispectrum through analytic scalings and N-body
simulations of the dark matter. We show that the Planck satellite should be
able to measure the SZ effect with sufficient precision to determine its power
spectrum and higher order correlations, e.g. the skewness and bispectrum.
Planck should also be able to detect the cross correlation between the SZ and
gravitational lensing effect in the CMB. Detection of these effects will help
determine the properties of the as yet undetected gas, including the manner in
which the gas pressure traces the dark matter.Comment: 13 ApJ pages, 11 figures; typos and figure 5 revised; submitted to
Ap
3-pt Statistics of Cosmological Stochastic Gravitational Waves
We consider the 3-pt function (i.e. the bispectrum or non-Gaussianity) for
stochastic backgrounds of gravitational waves. We estimate the amplitude of
this signal for the primordial inflationary background, gravitational waves
generated during preheating, and for gravitational waves produced by
self-ordering scalar fields following a global phase transition. To assess
detectability, we describe how to extract the 3-pt signal from an idealized
interferometric experiment and compute the signal to noise ratio as a function
of integration time. The 3-pt signal for the stochastic gravitational wave
background generated by inflation is unsurprisingly tiny. For gravitational
radiation generated by purely causal, classical mechanisms we find that, no
matter how non-linear the process is, the 3-pt correlations produced vanish in
direct detection experiments. On the other hand, we show that in scenarios
where the B-mode of the CMB is sourced by gravitational waves generated by a
global phase transition, a strong 3-pt signal among the polarization modes
could also be produced. This may provide another method of distinguishing
inflationary B-modes. To carry out this computation, we have developed a
diagrammatic approach to the calculation of stochastic gravitational waves
sourced by scalar fluids, which has applications beyond the present scenario.Comment: 16 pages, 5 figure
Robust statistics for deterministic and stochastic gravitational waves in non-Gaussian noise I: Frequentist analyses
Gravitational wave detectors will need optimal signal-processing algorithms
to extract weak signals from the detector noise. Most algorithms designed to
date are based on the unrealistic assumption that the detector noise may be
modeled as a stationary Gaussian process. However most experiments exhibit a
non-Gaussian ``tail'' in the probability distribution. This ``excess'' of large
signals can be a troublesome source of false alarms. This article derives an
optimal (in the Neyman-Pearson sense, for weak signals) signal processing
strategy when the detector noise is non-Gaussian and exhibits tail terms. This
strategy is robust, meaning that it is close to optimal for Gaussian noise but
far less sensitive than conventional methods to the excess large events that
form the tail of the distribution. The method is analyzed for two different
signal analysis problems: (i) a known waveform (e.g., a binary inspiral chirp)
and (ii) a stochastic background, which requires a multi-detector signal
processing algorithm. The methods should be easy to implement: they amount to
truncation or clipping of sample values which lie in the outlier part of the
probability distribution.Comment: RevTeX 4, 17 pages, 8 figures, typos corrected from first version
Improving and Assessing Planet Sensitivity of the GPI Exoplanet Survey with a Forward Model Matched Filter
We present a new matched filter algorithm for direct detection of point
sources in the immediate vicinity of bright stars. The stellar Point Spread
Function (PSF) is first subtracted using a Karhunen-Lo\'eve Image Processing
(KLIP) algorithm with Angular and Spectral Differential Imaging (ADI and SDI).
The KLIP-induced distortion of the astrophysical signal is included in the
matched filter template by computing a forward model of the PSF at every
position in the image. To optimize the performance of the algorithm, we conduct
extensive planet injection and recovery tests and tune the exoplanet spectra
template and KLIP reduction aggressiveness to maximize the Signal-to-Noise
Ratio (SNR) of the recovered planets. We show that only two spectral templates
are necessary to recover any young Jovian exoplanets with minimal SNR loss. We
also developed a complete pipeline for the automated detection of point source
candidates, the calculation of Receiver Operating Characteristics (ROC), false
positives based contrast curves, and completeness contours. We process in a
uniform manner more than 330 datasets from the Gemini Planet Imager Exoplanet
Survey (GPIES) and assess GPI typical sensitivity as a function of the star and
the hypothetical companion spectral type. This work allows for the first time a
comparison of different detection algorithms at a survey scale accounting for
both planet completeness and false positive rate. We show that the new forward
model matched filter allows the detection of fainter objects than a
conventional cross-correlation technique with a Gaussian PSF template for the
same false positive rate.Comment: ApJ accepte
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