Session C15: Gravitational Wave Astrophysics II

The Loudest Gravitational Wave Events

Compact binary coalescences are likely to be the source of the first gravitational wave (GW) detections. While most Advanced LIGO-Virgo detections are expected to have signal-to-noise ratios (SNR) near the detection threshold, there will be a distribution of events to higher SNR. Assuming the space density of the sources is uniform in the nearby Universe, we derive the universal distribution of SNR in an arbitrary GW network, as well as the SNR distribution of the loudest event. These distributions only depend on the detection threshold and the number of detections; they are independent of the detector network, sensitivity, and the distribution of source variables such as the binary masses and spins. We also derive the SNR distribution for each individual detector within a network as a function of the detector orientation. We find that, in 90\% of cases, the loudest event out of the first four Advanced LIGO-Virgo detections should be louder than SNR of 15.8 (for a threshold of 12), increasing to an SNR of 31 for 40 detections. We expect these loudest events to provide the best constraints on their source parameters, and therefore play an important role in extracting astrophysics from GW sources.   

The NINJA-2 project: Detecting and characterizing gravitational waveforms modelled using numerical binary black hole simulations

The Numerical INJection Analysis (NINJA) project is a collaborative effort between members of the numerical relativity and gravitational-wave astrophysics communities. The purpose of NINJA is to study the ability to detect gravitational waves emitted from merging binary black holes and recover their parameters with next-generation gravitational-wave observatories. We report here on the results of the second NINJA project, NINJA-2, which employs 60 complete binary black hole hybrid waveforms consisting of a numerical portion modelling the late inspiral, merger, and ringdown stitched to a post-Newtonian portion modelling the early inspiral.   

Impact of higher-order modes on parameter recovery from binary black hole coalescences

Thus far modeled searches for the gravitational waves produced by the coalescence of compact binaries have used templates that include only the 2,2 mode. However, it is known that there can be significant power in higher-order modes -- indeed there are parameters for which these modes become dominant. Numerical relativity can now produce waveforms that are accurate though late inspiral, merger, and ringdown including many higher-order modes. We present recent work using waveforms produced at Georgia Tech to determine how the inclusion of higher modes in model waveforms can increase the accuracy with which the parameters of the system can be recovered from a detected signal in Advanced LIGO. We consider a variety of binary black hole systems, including systems that precess.   

The gravitational-wave signature of binary black holes in spin-orbit resonances

Mass transfer and tidal alignment during the evolution of their stellar progenitors can induce an asymmetry in the misalignment of binary black-hole spins with the orbital angular momentum. If binaries preferentially form with the the spins of the more massive black hole more (less) aligned with the orbital angular momentum than that of the less massive black hole, the components of the spin in the orbital plane will preferentially align (anti-align) during the gravitational-wave induced inspiral. Once trapped in these spin-orbit resonances, the orbital angular momentum and both spins jointly precess in a common plane during the remainder of the inspiral. We examine the gravitational waves emitted by binary black holes in these resonant configurations, and find that binaries with aligned spin components in the orbital plane can be distinguished by the greater precession of the orbital plane. This precession leaves a distinctive signature in the gravitational waveform which can be identified by ground-based gravitational-wave detectors in sources with suficcient signal-to-noise ratios.   

Estimating parameters of BH-NS binaries with gravitational waves

Ground-based gravitational wave detectors will soon identify the gravitational wave signal from merging stellar-mass compact binaries, including black hole-neutron star (BH-NS) binaries. With their mass ratio and spin, BH-NS binaries produce an intrinsically complicated multimodal signal. In this talk, we examine how well gravitational wave detectors can estimate the parameters of fiducial nonprecessing and precessing binaries. We compare our detailed Markov-chain Monte Carlo simulations against analytic (Fisher matrix) calculations.   

Techniques for high-frequency searches for Gravitational Waves associated with Gamma-ray Bursts

In the next few years, the global network of advanced gravitational-wave detectors will begin observing the universe with unprecedented sensitivity. Some of the most promising sources of transient gravitational-wave (GW) signals are the central engines of gamma-ray bursts (GRBs), which are expected to emit GWs across a wide frequency band. In this talk, I will describe the methods of a search for high-frequency ($>$1kHz) GWs in data from the LIGO and Virgo experiments, associated with GRBs detected by the Fermi Gamma-ray Burst Monitor (GBM). I will discuss the challenges for GW searches that arise from the GBM sky localization, and techniques to make these searches computationally feasible. I will also describe the ability of GRB-triggered GW searches to refine the localization a detectable GW signal within the Fermi-GBM error region.   

An all-sky search for unmodeled long-duration transient gravitational-wave signals

A number of astrophysical models predict transient emission of gravitational waves (GWs) on relatively long time-scales, lasting from seconds to days. These GWs are often not accompanied by a detectable electromagnetic counterpart, e.g., a gamma-ray burst.~ In order to search for gravitational waves produced by these mechanisms, we are developing an un-triggered all-sky extension of an excess cross-power search pipeline that has recently been used to study long-lived signals associated with gamma-ray bursts. Here, we give an overview of this all-sky search pipeline, focusing on GW sources of interest, data analysis methods, and the expected sensitivity of such a search using LIGO data.   

NANOGrav Limits on Continuous Gravitational Waves from Supermassive Black Hole Binaries

Gravitational Waves (GWs) are tiny ripples in the fabric of space-time predicted by Einstein's theory of General Relativity. Pulsar timing arrays (PTAs) offer a unique opportunity to detect low frequency GWs in the near future. Such a detection would be complementary to both LIGO and future space based GW efforts. In the low (1e-9 - 1e-7 Hz) frequency band, the expected source of GWs is a stochastic background resulting from the ensemble of supermassive black hole binaries (SMBHBs) formed during the merger of galaxies, and possibly a few nearby/massive sources that will be individually resolvable. In this talk we will discuss continuous wave search efforts using the current NANOGrav data releases. We will briefly outline both Bayesian and Frequentist search and upper limit pipelines and present preliminary results for sky averaged and targeted upper limits on the strain amplitude using both techniques. Finally we present sensitivity projections for future NANOGrav observations and comment on the plausibility of detection based on current simulations of SMBHBs.   

NANOGrav: prospects for the detection of a stochastic background of low-frequency gravitational waves

For the better part of the last decade, the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) has been using the Green Bank and Arecibo radio telescopes to monitor millisecond pulsars. NANOGrav aims to directly detect low-frequency gravitational waves which cause small changes to the times of arrival of radio pulses. In this talk I will discuss the work of the NANOGrav collaboration and recent progress made toward realistic simulations of our sensitivity to a stochastic background of gravitational waves. I will show that a detection is possible as early as 2017.