Session B15: Gravitational Wave Astrophysics I

What can we learn about the neutron-star equation of state from gravitational-wave observations of inspiralling binary neutron stars?

Gravitational-wave observations of inspiralling binary neutron star systems can provide information about the neutron-star equation of state (EOS) through the tidally induced shift in the waveform phase which depends on the tidal deformability parameter $\Lambda$. Previous work has shown that $\Lambda$, a function of the neutron-star EOS and mass, is marginally measurable by Advanced LIGO for a single event when including the tidal information up to the frequency of merger. In this work, we describe a method for stacking measurements of $\Lambda$ from multiple inspiral events to measure the EOS. Specifically, we use Markov Chain Monte Carlo simulations to estimate the parameters of a 4-parameter piecewise polytrope EOS that matches theoretical EOS models to a few percent. We find that when 20--50 observations are combined with the constraints from causality and recent high mass neutron-star measurements, the EOS above nuclear density can be measured to better than a factor of two. We also find that quantities that describe the neutron-star structure such as the radius and tidal deformability can be measured to $\sim$10\% over a wide range of masses.   

Measuring the neutron star tidal Love number with inspiral waveforms

The tidal Love number parameterizes how easily a binary companion deforms a neutron star. This deformation modifies the gravitational field near the neutron star and imprints itself on the binary orbit and gravitational waveform. Measuring the Love number with LIGO or other detectors will help constrain the neutron star equation of state (which is uncertain at high densities). I will discuss an improved parameterization of the waveform's Love-number dependence. I will also discuss how systematic errors will make this number difficult to measure. These systematic errors could arise from unknown post-Newtonian terms that enter at lower orders than tidal effects, or from neglecting small neutron star spins or binary eccentricity.   

Cas A and friends: directed searches for continuous gravitational waves from isolated neutron stars

We present the status of searches for continuous gravitational waves from the central compact object in supernova remnant Cassiopeia A and eight other young suspected neutron stars whose positions are known well enough to use a single barycentric correction per object. All objects have age estimates less than a few thousand years, young enough that r-modes could still be active. The searches coherently integrate from five to twenty-five days of the LIGO S6 data run and cover gravitational wave frequency bands of varying widths from 140 Hz to 2 kHz so that each requires a similar computational cost, which is about 1/3 that of the published LIGO search for Cassiopeia A due to the use of SSE2 floating point extensions. The objects are chosen so that each search can detect a neutron star in the band if its (unknown) spin-down has been dominated by gravitational-wave emission since birth.   

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 has been undertaken 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.   

Rapidly extracting astrophysics from gravitational-wave observations in the Advanced Detector era

Coalescing compact binaries - consisting of neutron stars and/or black holes - are the most promising source of gravitational waves for the next-generation gravitational-wave detectors Advanced LIGO and Advanced Virgo. Accurately measuring the astrophysical parameters of these sources is crucial for precision astrophysics and astronomy with gravitational waves, but the computational times of such analyses can be prohibitively long. Here we present a new approach to parameter estimation based on ``reduced order modeling'' (see plenary overview talk by Tiglio: ``Reduced Order Modeling in General Relativity''). This approach can enable low latency parameter estimation on time scales of minutes to hours. We will discuss recent results of our approach for extracting the astrophysical parameters of binary neutron stars in mock Advanced LIGO/Virgo data. We will also discuss extensions of the approach to binary black hole parameter estimation where the spins of the black holes can be large.   

Constraining models of Population III stars using gravitational-wave observations

A stochastic gravitational-wave background arises from the superposition of many incoherent sources of gravitational waves, which may be cosmological or astrophysical in origin.~ Recent searches for the stochastic background using LIGO and Virgo data have placed upper limits on the energy density spectrum.~ Here, we present a method for using measurements of the stochastic background to constrain the parameters of theoretical models, focusing on a background produced by the core-collapse of Population III stars.~ Finally, we discuss what can be achieved with future generations of gravitational-wave detectors, including Advanced LIGO and the Einstein Telescope.   

Sco X-1 in LIGO: directed searches for continuous gravitational waves from neutron stars in binary systems

Scorpius X-1 and similar low-mass X-ray binary (LMXB) systems with neutron stars contain favorable conditions for the emission of continuous gravitational waves (GW). Companion star accretion is believed to recycle the neutron star, spinning it up to high rotational speeds. That accretion could also induce non-axisymmetries in the neutron star, leading to detectable GW emission. Advanced LIGO and other 2nd-generation interferometric observatories will permit searches for such gravitational waves using new algorithms, including the TwoSpect program, which was developed originally for all-sky binary searches. In this presentation we discuss an implementation of TwoSpect using fine templates in parameter space at the initial stage and optimized to search for LMXBs, such as Sco X-1, where some of the orbital parameters are known. Results from simulations will be shown.   

Gravitational and neutrino signatures from core-collapse supernovae

Core-collapse supernovae (CCSNe) as powerful sources of gravitational and neutrino radiation are among the prime candidates for multimessenger astronomy. Simultaneous detection of both gravitational and neutrino signals will provide invaluable information about dynamics of the supernova core and reveal details of the CCSN mechanism. We present the gravitational and neutrino signatures from the series of 2D and 3D \emph{ab initio} CCSN simulations performed with Chimera code. Chimera is a radiation hydrodynamics code that is developed specifically for simulation of CCSNe. It combines hydrodynamics, neutrino transport, and nuclear reaction network in one computational infrastructure that allows to model the evolution of supernova from first principles. I will compare gravitational waveforms and neutrino signals obtained in the 2D and 3D simulations with different progenitor masses and provide an estimation of their detectability by gravitational wave and neutrino observatories.   

Measuring the Angular Momentum Distribution of Core-Collapse Supernova Progenitors

The gravitational wave signature of core-collapse supernovae encodes important information on the physical characteristics of the associated progenitor stars, particularly the angular momentum distribution, in addition to the relevant fundamental physics, for example, the nuclear equation of state and electron capture during collapse. Neither of these aspects can be inferred via observations of core-collapse supernovae in the electromagnetic spectrum. We explore the dependence of the gravitational wave signals on the total angular momentum and its distribution in the progenitor. To this end we carry out a large set of axisymmetric (2D) general relativistic hydrodynamics simulations of rotating core collapse. We construct a numerical template bank from these waveforms, and apply a matched filtering analysis to infer the total angular momentum and its distribution in the progenitor's inner core, given an observed, previously unknown gravitational wave signal. In the context of Advanced LIGO, we show that the total angular momentum can be inferred to within $\sim20-30\%$, for galactic supernovae with rapidly rotating cores at a fiducial distance of $10\,\mathrm{kpc}$.