Session G11: Characterization and Instrumentation for Gravitational Wave Detection

Modeling the Chi-square veto in the inspiral searches

Chi-square discriminatory veto is a computationally expensive signal based veto as it requires lots of computing time. This veto is applied to the triggers surviving the second stage of the pipeline in the inspiral searches. We shall make an attempt to understand the nature of this veto and whether we can predict the chi-square values of triggers surviving this computationally expensive veto. This veto helps us to discriminate real signals from those arising due to noise transients by measuring the goodness of fit quantity. We discuss here two separate approaches. One is purely analytical which is based on the power accumulated in a given frequency bin due to the presence of non-stationary and non-Gaussian noise. We know that sum of squares of Gaussian random variables follow a central chi-square distribution whereas in case on non-Gaussian noise they obey a non-central chi-square distribution and hence the mean value has a contribution from central part non-central parameter $\lambda$. We shall show that this non-central parameter $\lambda$ is a function of signal to noise ratio (SNR) of the triggers. Thus enabling us to model the expected chi-square values of the triggers using the SNR. The second approach I will talk involves trying to obtain a parabolic fit to the measured chi-squared distribution for a range of values as a function of signal to noise ratio.   

Multi-baseline signal consistency tests in searches for gravitational-wave signals in LIGO and Virgo detectors from compact binary coalescences

The non-Gaussian and non-stationary nature of real data is known to hurt the performance of gravitational wave signal searches. Incorporating signal-based discriminators that exploit the differences between the time-frequency structure of signals and noise artifacts has been shown to improve their performance for modeled sources. However, the power of these discriminators varies across the signal parameter space. Here we study how the performance of the null-stream statistic for detecting compact binary coalescence signals in a multi-baseline network varies as a function of the sky. We report results on simulated Gaussian data with LIGO sensitivities, with and without signal injections. We compare them with those expected theoretically. These results serve as benchmarks for subsequent studies in real data, and can help in formulating data-analysis strategies for reducing the gap in performance of search pipelines in real and Gaussian data.   

Multivariate classification of signal versus background in the LIGO-Virgo search for high-mass compact binary coalescences

The LIGO-Virgo collaboration searches for gravitational waves (GWs) from astrophysical sources such as compact binary coalescences (CBCs). Unfortunately, the expected GW signals from high-mass CBCs have waveforms similar to those from glitches in the detectors. Thus, our goal is to develop robust methods for separating rare GW signals from the background of glitches. We will discuss why multivariate statistical classification methods are not only naturally suited for this problem, but also can give insight into our understanding of the background.   

Environmental Influences in the Sixth Science Run of LIGO

The LIGO gravitational wave detectors at the Hanford and Livingston Observatories are very sensitive to environmental effects, stationary or transient, which are a subtle and important source of noise and false positives. We present seismic, acoustic and magnetic influences in the sixth LIGO science run and explain some of the coupling mechanisms. The methods are general enough to be employed in the future, and will be used to measure the environmental influence in the more sensitive Advanced LIGO detectors currently being installed.   

Critical Coupling Likelihood: A new approach to integrate LIGO data quality with searches for gravitational waves from compact binary coalescence

As part of the current LIGO search for compact binary coalescence (CBC) gravitational waves (GW) we find ourselves trying to determine if noise is coupling into the instrument indirectly using our data quality knowledge. The Critical Coupling Likelihood (CCL) method will allow us to directly fold information about potential GW triggers directly into our search efforts. Using this technique we can quantitatively inspect the data quality of each individual CBC candidate, and make more rigorous cuts improving the quality of our search. The CCL method will give the CBC search the potential to integrate all of LIGO's physical and environmental monitors (PEM) into a search for gravitational waves. Using the CCL approach and assuming required models of uncoupled (background) and coupled (foreground) instrument states, we should be able to increase our ability to discriminate between noise sources, that can hamper our current search efforts, and, high quality gravitational wave signal candidates. We illustrate how the method works by demonstrating environmental coupling in LIGO S6 data. An approach like CCL will become increasingly important as we move into the Advanced LIGO era, as we go from the first GW detection to gravitational wave astronomy.   

Reducing the effects of noise transients in gravitational-wave searches

Non-Gaussian transient noise in gravitational-wave detectors can increase the background in searches for short-duration and un-modeled gravitational-wave sources. This presentation describes the methods used to identify and reduce the effects of noise transients in the LIGO detectors during their latest data run.   

Cooling by evanescent-wave heat transfer in a parallel plane geometry

To increase their reach into the universe, future gravitational-wave interferometers may operate at cryogenic temperatures. Novel methods will be needed to cool the test masses without introducing displacement noise. Evanescent-wave heat transfer is the process by which near-field radiation effects are used to transfer heat from one body to another. These evanescent waves allow a thermal transmission across a small vacuum gap. With decreasing gap size, the heat transfer rises exponentially above the far-field blackbody radiation limit. Although this process was first theoretically explained in the early 1970's by Polder and Van Hove, experimental testing of this theory has been limited. We experimentally demonstrate evanescent heat transfer in a bulk geometry~between two parallel plates of sapphire. ~Our experiments are in good agreement with theoretical predictions. ~We discuss possible ways to apply our method to future gravitational-wave interferometers.~   

Error Sources for Gravitational Wave Sensors using Atom Interferometry

Proposals were made in 2008 for using two atom interferometers with laser links between them as gravitational wave sensors (S. Dimopoulos et al., PRD 78, 122002 (2008)). However, the importance of fluctuations in the laser wavefront aberrations over periods of seconds to minutes was overlooked in these proposals. For the AGIS-Sat 3 proposal, the total path length would be 10,000 km, and the laser wavefront aberration fluctuations would have to be attenuated to a level of 2x10$^{-9}$ wavelengths in order to achieve the quoted gravitational wave sensitivity. For the AGIS-Sat 2 proposal, the requirement would be a factor 10 less severe. In addition, the atom cloud temperature fluctuations from cloud to cloud would have to be less than 0.2 pK and 2 pK for the two proposed missions. More recently, a proposal for a gravitational wave mission called AGIS-LEO at 1000 km altitude in Earth orbit has been made (J. M. Hogan et al., arXiv:1009.2702v1, 14 Sept. 2010). For this mission, the atom interferometer separation would be 30 km, and the wavefront aberration jitter would need to be reduced to 2x10$^{-8}$ wavelengths. Some steps toward mitigation of the wavefront aberration jitter problem are mentioned, including possible use of an extra initial laser propagation path and a mode-scrubbing cavity. However, the requirements on such filtering systems appear to be severe.