Session X16: Astrophysics with Gravitational Waves I

Live

Gravitational waves emitted by neutron star black hole mergers encode key properties of neutron stars -- such as their size, maximum mass and spins -- and black holes. However, the presence of matter together with singularity makes generating long and accurate waveforms from these systems hard with numerical relativity, and not much is known about systematic uncertainties due to waveform modeling. We simulate gravitational waves from neutron star black hole mergers by hybridizing numerical relativity waveforms with a recent numerical relativity surrogate and analyze these signals with a range of available waveform families. We find that at a network signal-to-noise ratio (SNR) of 30, statistical uncertainties are usually larger than systematic offsets, while at an SNR of 70 the two become comparable. The individual black hole and neutron star masses, as well as the mass ratios, are typically measured very precisely, though not always accurately at high SNR. At a SNR of 30, the neutron star tidal deformability can only be bound from above, while for louder sources it may be measured and constrained away from zero. All neutron stars in our simulations are non-spinning, but in no case we can constrain the neutron star spin to be smaller than $\sim$ 0.4 (90\% credible interval).   

Live

Binary neutron star mergers provide a unique probe of the neutron star equation of state (EoS) across a wide range of parameter space, from the zero-temperature EoS during the inspiral to the finite-temperature EoS following the merger. In this talk, I will present the results of new numerical simulations of binary neutron star mergers, using a parameterized framework for calculating the finite-temperature EoS. I will begin with a summary of the EoS framework, which is based on a two-parameter approximation of the particle effective mass and includes the leading-order effects of degeneracy. I will show that including the effects of degeneracy can significantly impact the outcomes of merger simulations, compared to ideal-fluid (or “hybrid”) approximations. Using a parameter study to explore this new EoS model, I will discuss how different assumptions about the particle effective mass can affect the post-merger gravitational wave signal and the amount of merger ejecta. Finally, I will comment on the prospects for extracting cold neutron star properties (such as the radius) from the post-merger gravitational wave spectrum, in light of this dependence on finite-temperature effects.   

Live

Existing theories of gravitational-wave (GW)-matter coupling are not directly applicable to GWs that are inhomogeneous in space and have more general polarization than those in vacuum, thus leaving some important observable physical phenomena out of their scope. The standard approach to this problem has been to solve Einstein equations with matter and fields as source terms, but this has proven to be prohibitively cumbersome. We use an alternative, variational formulation [PRD 102, 064012 (2020)] to derive the gauge-invariant wave equation for collective oscillations of the self-consistent metric with a general polarization. We also propose a new nonlinear hereditary effect, the ``gravitational ponderomotive effect.'' From a limiting case of our wave equation, we reproduce the Jeans instability on the same footing as vacuum GWs. Developing further on this equation, we also present corrections to the geometrical optics of GWs, which are of the same order as the GW-matter interaction term for near-vacuum waves.   

Live

The third LIGO-Virgo observing run turned up the first gravitational-wave observation of a compact object from the mass gap between known neutron stars and known black holes. With a mass of approximately 2.6 $M_{\odot}$, the nature of the secondary component of the compact binary merger GW190814 is unclear. Accounting for current uncertainty in the maximum neutron star mass and making reasonable assumptions about the compact-object mass distribution, we argue based on a hierarchical Bayesian analysis that the merger's secondary component is the lightest black hole observed to date, rather than an exceptionally massive neutron star.   

Live

Observations of black holes mergers from the LIGO-Virgo interferometers provide an unprecedented opportunity to glance into an unexplored regime of gravity, where spacetime curvature is several orders of magnitudes larger than the one probed by other experiments. First, the state of the art of black holes ringdown spectra observations will be reviewed. Next, we will show how requiring stringent, yet well-motivated, perturbative parametrisations of beyond-General Relativity effects, allows to extract observational constraints much stronger than those present in the literature on large classes of alternative theories of gravity. Such a boost brings observations close to the regime where corrections from Effective Field Theories of beyond-General Relativity gravity may start to leave a detectable imprint, and translates into a much smaller number of signals needed to detect violations due to an alternative theory of gravity. Finally, we will show what constraints can be placed on a few specific theories, where a self-consistent, non-perturbative prediction can be tested against the data.   

Live

A significant number of stellar-mass black-hole (BH) binaries may merge in galactic nuclei or in the surrounding gas disks. With purposed space-borne gravitational-wave observatories, we may use such a binary as a signal carrier to probe modulations induced by a central supermassive BH (SMBH), which further allows us to place constraints on the SMBH’s properties. We show in particular the de Sitter precession of the inner stellar-mass binary’s orbital angular momentum (AM) around the AM of the outer orbit will be detectable if the precession period is comparable to the duration of observation, typically a few years. Once detected, the precession can be combined with the Doppler shift arising from the outer orbital motion to determine the mass of the SMBH and the outer orbital separation individually and each with percent-level accuracy. If we further assume a joint detection by space-borne and ground-based detectors, the detectability threshold could be extended to a precession period of $\sim 100\, {\rm yr}$.   

Live

We study the gravitational waves from eccentric sub-solar mass black hole binaries which due to their mass would have to be primordial in origin instead of stellar evolution. Soon after formation in the early Universe, primordial black holes may form binaries. Alternatively, primordial black holes as dark matter can also form binaries in the late Universe due to dynamical encounter and gravitational-wave braking. We investigate the eccentricity distribution of binary primordial black holes from this late Universe formation channel at gravitational-wave frequency 10 Hz and find that the signals can retain non-negligible eccentricity. We use simulated gravitational wave data to study the ability to search for eccentric gravitational wave signals using a circular waveform template bank. A targeted search on LIGO/Virgo open data is also performed with the gravitational waveform for eccentric compact binary coalescence. The null results place new limits on astrophysical models for primordial black hole binary formation.   

Live

Strong gravitational lensing of gravitational waves can produce duplicate signals that are separated in time with different amplitudes. We consider the case in which strong lensing produces identifiable gravitational-wave events together with weaker sub-threshold signals that are hidden in the noise background. We present a search method for the sub-threshold signals using reduced template banks targeting specific confirmed gravitational-wave events. We apply the method to an event from Advanced LIGO's first observing run O1, GW150914. We show that the method is effective in reducing the noise background and hence raising the significance of (near-) sub-threshold triggers. In the case of GW150914, we are able to improve the sensitive distance by $2.0\% - 14.8\%$. Finally, we present a list of possible lensed candidates for O1/O2 events with significant sky location overlap with the original events.   

Not Participating

The masses and spins of binary black holes (BBHs) that form from the collapsed cores of isolated high-mass binary stars are determined by the interplay of phenomena such as tides, winds, accretion, common-envelope evolution, natal kicks, and stellar core-envelope coupling. The possibility for these BBHs to experience spin precession, which modulates the gravitational waves that are emitted during inspiral and merger, also depends on these astrophysical phenomena. If BBHs from the isolated channel generally have negligible spin-orbit misalignments, then their spin precession would be greatly suppressed. In previous work, we used a simplified model of stellar-binary evolution to identify regions of the parameter space that may produce BBHs with large, misaligned spins. Here, we use five new parameters (see D. Gangardt's talk), which describe the evolution of the direction of the orbital angular momentum, to explore the spin precession of such highly spinning and misaligned BBHs. We compare this behavior with the expected precession of BBHs originating from the dynamical channel and find that in the absence of alignment mechanisms, such as tides or accretion, isotropic natal kicks can produce precessing BBHs with appreciable nutation of the orbital angular momentum.