Topics in Broadband Gravitational-Wave Astronomy

The direct detection of gravitational waves promises to open a new observational window onto the universe, and a number of large scale efforts are underway worldwide to make such a detection a reality. In this work, we attack some of the current problems in gravitational-wave detection over a wide range of frequencies. In the first part of this work, low frequency gravitational-wave detection is considered using pulsar timing arrays (PTAs). PTAs are a promising tool for probing the universe through gravitational radiation. Supermassive black hole binaries (SMBHBs), cosmic strings, relic gravitational waves from inflation, and first order phase transitions in the early universe are expected to contribute to a stochastic background of gravitational waves in the PTA frequency band of 1 nHz-100 nHz. The detection of low-frequency stochastic backgrounds of gravitational waves in the PTA band is considered in the context of constructing an optimal cross-correlation statistic in the time domain. Also presented are some useful applications of this statistic, and discussion on its limitations in actual gravitational-wave searches. Also considered are methods by which gravitational waves in the PTA frequency band can serve as a mechanism for testing general relativity (GR). In addition to providing a new paradigm for exploring the universe, the direct detection of gravitational waves will allow general relativity to be tested against other metric theories of gravity in the regime of strong gravitational fields. This work involves the analysis of the overlap reduction function (ORF), a geometrical factor that appears in the expected cross correlation of signals, for general metric theories of gravity. The ORF characterizes the loss of sensitivity due to detectors not being co-located or coaligned, and it is an important element in defining the optimal cross-correlation statistic. It is shown that PTA detection sensitivity increases for non-transverse gravitational waves. Additionally, the ORFs for a subset of the NANOGrav PTA are described, and are used to show that sensitivity to vector and longitudinal modes can increase dramatically for pulsar pairs with small angular separations. Implications of these results are discussed. In the second part of this work, the detection of gravitational-wave bursts in the 10 Hz-1000 Hz frequency band is considered using ground-based laser interferometers. An excess power method for conducting unmodeled searches for gravitational-wave bursts is discussed, and its implementation into a search pipeline is described in detail. The performance of this pipeline is probed using software injections. Also discussed are potential applications of the ExcessPower pipeline to detector characterization efforts, which aim to improve interferometric searches by characterizing and mitigating non-Gaussian noise transients in the detectors

Stochastic backgrounds in alternative theories of gravity: overlap reduction functions for pulsar timing arrays

In the next decade gravitational waves might be detected using a pulsar timing array. In an effort to develop optimal detection strategies for stochastic backgrounds of gravitational waves in generic metric theories of gravity, we investigate the overlap reduction functions for these theories and discuss their features. We show that the sensitivity to non-transverse gravitational waves is greater than the sensitivity to transverse gravitational waves and discuss the physical origin of this effect. We calculate the overlap reduction functions for the current NANOGrav Pulsar Timing Array (PTA) and show that the sensitivity to the vector and scalar-longitudinal modes can increase dramatically for pulsar pairs with small angular separations. For example, the J1853+1303-J1857+0943 pulsar pair, with an angular separation of about 3 degrees, is about 10^4 times more sensitive to the longitudinal component of the stochastic background, if it is present, than the transverse components.Comment: 13 pages, 7 figures, published in Physical Review D 85 (082001), 201

Time-domain Implementation of the Optimal Cross-Correlation Statistic for Stochastic Gravitational-Wave Background Searches in Pulsar Timing Data

Supermassive black hole binaries, cosmic strings, relic gravitational waves from inflation, and first order phase transitions in the early universe are expected to contribute to a stochastic background of gravitational waves in the 10^(-9) Hz-10^(-7) Hz frequency band. Pulsar timing arrays (PTAs) exploit the high precision timing of radio pulsars to detect signals at such frequencies. Here we present a time-domain implementation of the optimal cross-correlation statistic for stochastic background searches in PTA data. Due to the irregular sampling typical of PTA data as well as the use of a timing model to predict the times-of-arrival of radio pulses, time-domain methods are better suited for gravitational wave data analysis of such data. We present a derivation of the optimal cross-correlation statistic starting from the likelihood function, a method to produce simulated stochastic background signals, and a rigorous derivation of the scaling laws for the signal-to-noise ratio of the cross-correlation statistic in the two relevant PTA regimes: the weak signal limit where instrumental noise dominates over the gravitational wave signal at all frequencies, and a second regime where the gravitational wave signal dominates at the lowest frequencies.Comment: 12 pages, 3 figures, submitted to Physical Review

The GstLAL Search Analysis Methods for Compact Binary Mergers in Advanced LIGO's Second and Advanced Virgo's First Observing Runs

After their successful first observing run (September 12, 2015 - January 12, 2016), the Advanced LIGO detectors were upgraded to increase their sensitivity for the second observing run (November 30, 2016 - August 26, 2017). The Advanced Virgo detector joined the second observing run on August 1, 2017. We discuss the updates that happened during this period in the GstLAL-based inspiral pipeline, which is used to detect gravitational waves from the coalescence of compact binaries both in low latency and an offline configuration. These updates include deployment of a zero-latency whitening filter to reduce the over-all latency of the pipeline by up to 32 seconds, incorporation of the Virgo data stream in the analysis, introduction of a single-detector search to analyze data from the periods when only one of the detectors is running, addition of new parameters to the likelihood ratio ranking statistic, increase in the parameter space of the search, and introduction of a template mass-dependent glitch-excision thresholding method.Comment: 12 pages, 7 figures, to be submitted to Phys. Rev. D, comments welcom

The GstLAL template bank for spinning compact binary mergers in the second observation run of Advanced LIGO and Virgo

We describe the methods used to construct the aligned-spin template bank of gravitational waveforms used by the GstLAL-based inspiral pipeline to analyze data from the second observing run of Advanced LIGO and Virgo. The bank expands upon the parameter space covered during the first observing run, including coverage for merging compact binary systems with total mass between 2 $\mathrm{M}_{\odot}$ and 400 $\mathrm{M}_{\odot}$ and mass ratios between 1 and 97.989. Thus the systems targeted include merging neutron star-neutron star systems, neutron star-black hole binaries, and black hole-black hole binaries expanding into the intermediate-mass range. Component masses less than 2 $\mathrm{M}_{\odot}$ have allowed (anti-)aligned spins between $\pm0.05$ while component masses greater than 2 $\mathrm{M}_{\odot}$ have allowed (anti-)aligned between $\pm0.999$. The bank placement technique combines a stochastic method with a new grid-bank method to better isolate noisy templates, resulting in a total of 677,000 templates.Comment: 9 pages, 13 figure