Session H8: Gravitational Wave Astronomy

Measuring Accretion Impact Radii With Optical and Gravitational Wave Observations of Compact Binaries

One of the primary astrophysical sources for space-based gravitational wave observatories will be ultra-compact binary star systems in the Milky Way. Millions of these systems exist in the galaxy, and it is estimated that thousands will be observable to space-based gravitational wave observatories. Many ultra-compact binaries will be simultaneously observable in the electromagnetic and gravitational waves, opening the door for a synthesis of independent data sources known generically as \textit{Multi-Messenger Astronomy}. By considering both electromagnetic and gravitational wave data, we have developed a technique which can be used to estimate the radius of the accretion disc; a feat currently possible only for a few eclipsing systems. This method does not require that the observed system be eclipsing, allowing accretion disc radii to be measured for many more systems.   

Binary black hole mergers in magnetized disks: simulations in full general relativity

We present results from the first fully general relativistic, magnetohydrodynamic (GRMHD) simulations of an equal-mass black hole binary (BHBH) in a magnetized, circumbinary accretion disk. We simulate both the pre and post-decoupling phases of a BHBH-disk system and both ``cooling'' and ``no-cooling'' gas flows. Prior to decoupling, the competition between the binary tidal torques and the effective viscous torques due to MHD turbulence depletes the disk interior to the binary orbit. However, it also induces a two-stream accretion flow and mildly relativistic polar outflows from the BHs. Following decoupling, but before gas fills the low-density 'hollow' surrounding the remnant, the accretion rate is reduced, while there is a prompt electromagnetic (EM) luminosity enhancement following merger due to shock heating and accretion onto the spinning BH remnant. This investigation, though preliminary, previews more detailed GRMHD simulations we plan to perform in anticipation of future, simultaneous detections of gravitational and EM radiation from a merging BHBH-disk system.   

Constructing binary black hole template banks using numerical relativity waveforms

We present methods for constructing and validating template banks for gravitational waves from high mass binary black holes in advanced gravitational-wave detectors using waveforms from numerical relativity. We construct these template banks using numerical waveforms from the Simulating eXtreme Spacetimes (SXS) collaboration. We show how a template bank can be constructed using numerical waveforms for non-spinning black hole binaries and discuss how this can be extended into the aligned spin black hole binary space.   

Prospects for GW transients in early Advanced LIGO and Virgo science runs

Advanced LIGO and Advanced Virgo are kilometer-scale gravitational wave detectors that are expected to yield the first direct observations of gravitational waves. I will describe the currently projected schedule, sensitivity, and sky localization accuracy for the GW detector network in the next decade.   

Increasing LIGO sensitivity by feedforward subtraction of auxiliary length control noise

LIGO, the Laser Interferometer Gravitational-wave Observatory [Hanford, Washington and Livingston, Louisiana] measures the differential length of 4-km Michelson arms with Fabry-Perot cavities. Length changes could indicate strain caused by astrophysical sources of gravitational waves. Fundamentally limited by seismic noise, thermal suspension noise, and laser shot noise in different frequency bands, a LIGO interferometer's sensitivity can also be degraded by additional relative motion of the inner arm cavity mirrors due to imperfectly-servoed Michelson motion. In this project we seek to subtract the effects of this residual motion by feedforward correction of the gravitational-wave data channel. We divide data from LIGO's sixth science run into 1024-second time windows and numerically fit a filter representing the frequency-domain transfer function from Michelson servo noise to gravitational wave channel for each window. Finally, the Michelson servo channel is processed through the filter and is subtracted from the gravitational-wave signal channel. The algorithm used in this procedure will be described with a preliminary assessment of the achievable sensitivity improvement.   

Pulsar Timing Arrays: No longer a blunt instrument for Gravitational Wave Detectio

The limits that pulsar timing places on the energy density of gravitational waves in the universe are on the brink of limiting models of galaxy formation and have already placed limits on the tension of cosmic strings. Pulsar timing has traditionally focused on stochastic sources, but recent research has demonstrated that pulsar timing will (1) offer a rich variety of information on individual gravitational wave sources including waveform, direction and luminosity distance, (2) test alternative theories of gravity, (3) allow us to observe the same gravitational wave source at two different epochs separated by thousands of years. In other words, pulsar timing is a shrewd and versatile gravitational wave detection instrument   

Search for Gravitational Waves From Nearby Globular Clusters

Although globular clusters in our galaxy are composed primarily of very old stars, there is evidence of young pulsar formation, suggesting that binary formation or collisions take place in these stellar-dense environments. Such events could lead to detectable continuous gravitational radiation from rapidly rotating young neutron stars or from older neutron stars perturbed by collision with debris. A search for continuous gravitational waves from neutron stars in the neighboring globular cluster NGC 6544 is under way, using LIGO S6 data and a new barycentric resampling algorithm that permits deeper searching (a longer coherence time). The algorithm used will be described, and the current status of the search presented.   

Searching for Gravitational Wave Bursts via Bayesian Nonparametrics Data Analysis with Pulsar Timing Arrays

A pulsar timing array (PTA) acts to detect gravitational waves by observing the small, correlated effect the waves have on millisecond pulsar pulse arrival times at Earth. Gravitational wave bursts --- signals whose duration is shorter than the observation period --- are expected to be one of the candidate signals that would arise in the pulsar timing array. Sources generating such signals include the periapsis passage of compact objects in highly eccentric or unbound orbits about an supermassive black hole, gravitational wave memory with coalescence of supermassive black holes, cusps on cosmic strings, etc. It is the usual case that we do not know the exact analytical formulae of gravitational wave burst signals and we are not able to parameterize them. In order to detect such signals, we introduce a new method --- Bayesian nonparametrics --- to analyze pulsar timing array data, which takes advantage of the prior expectation we have rather than parameterized formulae to characterize the gravitational wave bursts. Our Bayesian nonparametrics analysis will investigate the odds that a gravitational wave burst is present in the data and also infer the sky location of the source and the shape of the signal induced by this burst.