**Constraining the neutron star equation of state with gravitational waves and nuclear theory**

# Parallel Session: GRAVITATIONAL WAVES - MODELLING AND SOURCES (K)

## Location: Park 2.07

#### K3 - Tuesday 14:00-15:40 (Kostas Kokkotas)

**14:00-14:25**

**Collin Capano (Albert Einstein Institute Hannover)**

The properties of neutron stars are determined by the nature of the matter that they contain. These properties can be constrained by measurements of the star’s size. We construct possible neutron star equations of state constrained by chiral effective field theory and marginalize over these using gravitational-wave observations of GW170817. Combining this with the electromagnetic observations of the merger remnant that imply the presence of a short-lived hyper-massive neutron star, we are able to obtain stringent constraints on neutron-star radii. I discuss implications of these constraints, and prospects for future detections.

**Gravitational-wave-driven tidal secular instability in neutron star binaries**

**14:25-14:50**

**Pantelis Pnigouras (School of Mathematical Sciences, University of Southampton)**

We report the existence of a gravitational-wave-driven secular instability in neutron star binaries, acting on the equilibrium tide. The instability is similar to the classic Chandrasekhar-Friedman-Schutz (CFS) instability of normal modes and is active when the spin of the primary star exceeds the orbital frequency of the companion. Modeling the neutron star as a Newtonian n=1 polytrope, we calculate the instability time scale, which can be as low as a few seconds at small orbital separations but still larger than the inspiral time scale. The implications for orbital and spin evolution are also briefly explored, where it is found that the instability slows down the inspiral and decreases the stellar spin.

**Constraining f-Modes in Binary Neutron Star Inspirals with Gravitational Waves**

**14:50-15:15**

**Patricia Schmidt (University of Birmingham)**

Gravitational waves (GWs) from colliding neutron star binaries provide a unique means to probe matter at supranuclear densities. Whilst adiabatic tidal effects leave the strongest imprint on the GW phase, dynamical tidal effects due to the fundamental (f-) mode modify the GW phase above ~ 800 Hz. We present an approximate closed-form model for the GW phase from dynamical tides for circular, nonspinning binaries in the frequency domain. This additional tidal phase depends explicitly on the f-mode frequency of each star, making f-mode asteroseismology accessible through gravitational-wave detections from inspirals alone. Measuring the tidal deformability and f-mode frequency simultaneously enables novel tests of GR and would allow us to differentiate nuclear matter from exotic compact objects. Without invoking empirical universal relations, in a fully Bayesian re-analysis of GW170817 we find a lower bound of the f-mode frequencies of the two companions consistent with the predictions from universal relations. Finally, we present prospects for measuring the f-mode frequency from inspiraling neutron star binaries with future GW observatories.

**Searching for Continuous and Long-Duration Gravitational Waves from Neutron Stars**

**15:15-15:40**

**David Keitel (Institute of Cosmology and Gravitation, University of Portsmouth)**

Individual spinning neutron stars are expected to emit gravitational waves, but at much weaker levels than the spectacular binary coalescences observed so far. No such signals have been detected so far, but they remain one of the primary targets of LIGO-Virgo search efforts. I will give an overview of the computational challenge that searches for persistent “Continuous Waves” pose, how these are addressed in different regimes from targeted searches for known pulsars to blind all-sky surveys, and of the results from the advanced detector era so far. I will also cover recent advances in adapting continuous wave analysis methods to long-duration transient signals from newborn or glitching neutron stars.

#### K4 - Tuesday 16:10-17:50 (Kostas Kokkotas)

**Improving the measurement of binary black-holes parameters by tuning higher harmonics**

**16:10-16:35**

**Roberto Cotesta (Max Planck Institute (AEI Potsdam))**

Accurate models of gravitational waves are crucial to take full advantage of the sensitivity’s increase of LIGO and Virgo detectors. The gravitational waveform of binary black-hole coalescences is usually approximated by the dominant quadrupole mode. Several studies have shown that this approximation degrades with increasing total mass of the system, mass ratio, and binary-observer orientation. In these cases, neglecting higher-order modes in the waveform could lead to biases in the parameter estimation or, worst, to the loss of interesting detections.

In this talk I will show how the inclusion of the higher-order modes in our effective-one-body waveform model impact the infererence of the astrophysical properties of the gravitational wave signals. Typically these analysis require the evaluation of O(10^7) waveforms thus they can be prohibitive for the more accurate and computationally demanding waveform models. For this reason here they have been performed using our newly developed surrogate of the spin-aligned waveform model with higher-order modes. In fact the surrogate waveform model is O(100) times faster with respect to the original without degrading in accuracy.

**Extracting physics from neutron star black hole binaries with gravitational waves**

**16:35-17:00**

**Andrew Matas (AEI Potsdam)**

The discovery of gravitational waves emitted by binary black hole and binary neutron star systems by Advanced LIGO and Advanced Virgo has opened a new window to explore the Universe. Neutron star black hole (NSBH) binaries are a natural candidate for the next source class to be detected. Phenomena such as tidal disruption of the neutron star by the black hole can be imprinted on the gravitational waveform and can be probed observationally. In this talk, I will discuss the construction of an effective one body (EOB) model of the gravitational waveform produced by NSBH systems, based on numerical relativity simulations and incorporating recent theoretical advances in the understanding of these systems. I will then show how this model can be applied in Bayesian parameter estimation to extract information about source properties, such as the neutron star tidal deformability, which contains information about the nuclear equation of state.

**Quasi-circular inspirals and plunges from non-spinning effective-one-body Hamiltonians with gravitational self-force information**

**17:00-17:25**

**Andrea Antonelli (Max Planck Institute for Gravitational Physics (AEI Potsdam))**

The relativistic dynamics of compact binaries coalescences can be described by approximations to the Einstein’s equations that are applicable to different domains of validity. In order to construct waveforms to be used by the LIGO-Virgo collaboration, synergistic approaches between such approximations must be pursued: one example is the effective-one-body (EOB) framework, which includes physical information from the post-Newtonian (PN) approximation as well as the test-body limit. In this talk, I present recent work (arXiv:1907.11597) aimed at including in EOB theory the self-force (SF) effects of a small body in a circular orbit around a much more massive Schwarzschild black hole. We show that, when compared to standard PN results, inspiral EOB waveforms with both SF and PN information improve the agreement with numerical relativity predictions, for all mass ratios for which numerical results are available. This result is very encouraging and suggests that such waveforms, once completed with more physical effects, may become the baseline for future accurate inference studies of gravitational-wave sources with a small as well as comparable mass ratio.

**A fully precessing higher-mode surrogate model of effective-one-body waveforms**

**17:25-17:50**

**Bhooshan Gadre (MPI for Gravitational Physics, Potsdam)**

We can recover the masses and spins of binary black hole mergers using Bayesian inference and a model of the emitted gravitational waveform. Accurate generic waveform models can be used to infer the binaries’ spins and thus shed light on astrophysical formation scenarios. However, for accurate time-domain models inference runs are typically very computationally demanding and can take from weeks to months. Here, we present a surrogate model of fully precessing time-domain effective-one-body waveforms including sub-dominant modes. This surrogate is two orders of magnitude faster than the underlying time-domain model. We follow an approach similar to that used to build recent numerical relativity surrogate models. Our surrogate is 5000M in duration and covers mass ratios up to 20 and dimensionless spins up to 0.99. The surrogate errors are less than 1%.

#### K5 - Wednesday 14:00-15:40 (Kostas Kokkotas)

**Gravitational Waves: the theorist’s swiss knife**

**14:00-14:30**

**Mairi Sakellariadou (Physics, King’s College London)**

The stochastic gravitational-wave background (SGWB) is formed from the incoherent superposition of many GW sources throughout cosmic history. I will summarise the astrophysical and cosmological sources that contribute to the SGWB and the ongoing searches by cross-correlating data between multiple GW detectors. I will review the current limits on the SGWB and briefly discuss directional searches. I will then discuss how one can use gravitational waves not only to learn about compact binaries and the large-scale-structure of our universe, but also to constrain particle physics models beyond the Standard Model, modified gravity proposals, and even quantum gravity theories.

**Supermassive Black Hole Demographics In The Era Of Multimessenger Pulsar Timing Array Detection**

**14:30-14:55**

**Stephen Taylor (Department of Physics & Astronomt, Vanderbilt University)**

Supermassive black holes lurk at the centers of massive galaxies, and are themselves the most massive compact objects in the Universe. Over cosmic time, galaxies grow through accretion and mergers, such that in the post-merger phase they harbor two supermassive black holes that spiral toward coalescence through a variety of dynamical processes. The subset of these with 1e8 – 1e10 solar masses and orbital periods of several years form the target population for pulsar-timing array (PTA) experiments such as the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) and the International Pulsar Timing Array (IPTA). PTAs search for nanohertz gravitational-wave signals through induced Doppler shifts to the arrival rate of radio-pulses from millisecond pulsars. Many candidate binaries have been found through traditional electromagnetic means, although the only system with confidently detected multiple radio cores is too widely separated for PTAs to detect. Likewise, the quasi-variability of AGN in various photometric surveys (e.g. CRTS, PTF, and PanSTARRS) has produced many candidates, but none whose variability is unambiguously tied to the presence of a binary. I will review current efforts to find binary supermassive black holes through PTA searches and targeted multi-messenger campaigns, then discuss what current constraints and future detections can unveil about massive black hole demographics and the growth of galaxies.

**Transition from Inspiral to Plunge: A complete near-extremal waveform**

**14:55-15:20**

**Ollie Burke (Astrophysical and Cosmological Relativity, Albert Einstein Institute)**

We discuss an algorithm to compute the transition from an adiabatic inspiral into a geodesic plunge for any spin. In particular, we model the full trajectory of a compact object into a near-extremal supermassive black hole. Our analysis revisits the validity of the approximations made in Ori and Thorne (OT). We find three different scaling regimes according to whether the mass ratio is much smaller, of the same order or much larger than the near extremal parameter describing how fast the primary black hole rotates. Eccentricity and non-geodesic past-history corrections are always sub-leading, indicating that the quasi-circular approximation applies throughout the transition regime. However, we show that the OT assumption that the energy and angular momentum evolve linearly with proper time must be modified in the near-extremal regime. Using our transition equations, we describe an algorithm to compute the full world-line in proper time for an extreme mass ratio inspiral (EMRI) and the full gravitational waveform in the high spin limit. If time, we will discuss the detectability of these near-extremal systems by the space-based gravitational wave detector LISA.

**Anisotropies in the stochastic gravitational-wave background: a new cosmological probe**

**15:20-15:40**

**Alexander Jenkins (Theoretical Particle Physics and Cosmology (TPPC), King’s College London)**

In the new era of gravitational-wave (GW) astronomy, one of the most exciting targets for future observations is the stochastic gravitational-wave background (SGWB). While we have yet to detect the SGWB, we expect that by studying the angular power spectrum of its anisotropies, we may learn about the large-scale structure of the Universe (analogous to studies of the CMB), probing early- and late-time cosmology in a completely new way. With this in mind, we develop detailed models of the SGWB anisotropies from two important stochastic GW sources: unresolved compact binaries, and cosmic strings. We calculate, for the first time, the shot noise in these observations due to the finite number of sources, and develop optimal data-analysis methods for mitigating this problem.

#### K6 - Wednesday 16:10-17:50 (Kostas Kokkotas)

**The origin of spin in binary black holes: predicting the distributions of the main observables of Advanced LIGO**

**16:10-16:35**

**Simone Bavera ( The Astronomy Department, University of Geneva)**

After years of scientific progress, the origin of stellar binary black holes is still a great mystery. Several formation channels for merging black holes have been proposed in the literature. We study the formation of coalescing binary black holes via the evolution of isolated field binaries that go through the common envelope phase in order to obtain the combined distributions of observables such as black-holes spins, masses and cosmological redshift of mergers. We achieve this aim using a hybrid technique that combines the parametric binary population synthesis code COMPAS with detailed binary evolution simulations performed with the MESA code, in order to track accurately the angular momentum content of the second-born black hole progenitor. We then convolve our binary evolution calculations with the redshift- and metallicity-dependent star-formation rate and the selection effects of gravitational-wave detectors to obtain predictions of observable properties for the current O3 and future design sensitivity LIGO runs.

Our model is capable of predicting simultaneously the three main gravitational-wave observables: the effective inspiral spin parameter, the chirp mass and the cosmological redshift of merger. We find a very good agreement between our model and the ten events from the first two LIGO/Virgo science observing runs. The favorable comparison of the existing observations with our model predictions gives support to the idea that the majority, if not all of the observed mergers, originate from the evolution of isolated binaries. In this scenario, the first-born black hole has negligible spin because it lost its envelope after it expanded to become a giant star, while the spin of the second-born black hole is determined by the tidal spin up of its naked helium star progenitor by the first-born black hole companion after the binary finished the common-envelope phase. Finally, we explore the implications regarding the origin of the observed population of long Gamma-ray burst, as a subset of the second-born black holes progenitors in our model, acquire enough angular momentum that can produce a long gamma-ray burst under the collapsar model.

**Search for gravitational-wave signals associated with gamma-ray bursts during the second observing run of Advanced LIGO and Advanced Virgo **

**16:35-17:00**

**Andrew Williamson (ICG, University of Portsmouth)**

We present the results of targeted searches for gravitational wave transients associated with gamma-ray bursts during the second observing run of Advanced LIGO and Advanced Virgo, which took place from November 2016 to August 2017. We analyzed 98 gamma-ray bursts using an unmodeled search method that searches for generic transient gravitational waves and 42 with a modeled search method that targets compact-binary mergers as progenitors of short gamma-ray bursts. Both methods clearly detect the previously reported binary merger signal GW170817, with p-values of <9.38×10−6 (modeled) and 3.1×10−4 (unmodeled). We do not find any significant evidence for gravitational-wave signals associated with the other gamma-ray bursts analyzed, and therefore report lower bounds on the distance to each of these, assuming various source types and signal morphologies. Using our final modeled search results, short gamma-ray burst observations, and assuming binary neutron star progenitors, we place bounds on the rate of short gamma-ray bursts as a function of redshift for z ≤ 1. We estimate 0.07-1.80 joint detections with Fermi-GBM per year for the 2019-20 LIGO-Virgo observing run and 0.15-3.90 per year when current gravitational-wave detectors are operating at their design sensitivities.

**The method for targeted search for long-duration transients from glitching pulsars**

**17:00-17:25**

**Liudmila Fesik (Max Planck Institute for Gravitational Physics (Albert Einstein Institute) )**

We propose a method for identifying continuous waves (CWs) fromspinning neutron stars. We focus on glitching pulsars with abrupt spin-ups and long term spin-down, which imprint in CWs as long-duration transients from weeks to months. The main principle of the method is the combination of a coherent detection statistics over time intervals of different duration. We characterise the method by determining the false alarm and false dismissal probabilities for different signal strengths, and appropriate choices of the relative detection thresholds. We compare the sensitivity of this method with previously proposed methods for transient search.

**Constraining the Mass Density of Cosmic Strings Piercing Black Holes through the Analysis of Ringdown Signals**

**17:25-17:50**

**Ho Yeuk Cheung (Physics, the Chinese University of Hong Kong)**

Multiple gauge theories predict the presence of cosmic strings with different mass densities μ, and

a number of ways to constrain μ for the cosmic string network have been developed. In this paper,

we derived an equation governing the perturbation of a rotating black hole pierced by a straight,

infinitely long cosmic string along its axis of rotation, from which the quasinormal mode frequencies

(QNMFs) were calculated. Injected ringdown waveforms simulating those generated by such Kerr-

string black holes were used to test how well we can constrain μ for different sets of parameters

characterizing the black holes. Parameter estimation was also done on the GW150914 signal by

treating the remnant black hole as a Kerr-string black hole, yielding a constrain of Gμ < 3.5 × 10 −3

at the 90% confidence interval. Such a constraint only considers particular threads of cosmic strings

that might be piercing through the black hole in question, in contrast to other works where μ is

constrained for the whole cosmic string network.

#### K7 - Thursday 14:00-15:40 (Kostas Kokkotas)

**Mergers of accreting multiple BHs leading to SMBH formation in galactic nuclei**

**14:00-14:25**

**Masayuki Umemura (Center for Computational Sciences, University of Tsukuba)**

Recent gravitational-wave (GW) measurement of BH spins by aLIGO favors misalignment with the orbital angular momentum, which prefers a formation scenario where no spin alignment process is present. As a possibility, we consider BH mergers promoted in a multiple BH system under the gas-rich environments in galactic nuclei. For the purpose, we simulate orbits of multiple BHs in gas-rich environments with a post-Newtonian N-body scheme, incorporating gas accretion, dynamical friction, and GW emission. As a result, we find possible paths that allow BH mergers concordantly with the GW events. Also, we estimate event rates of massive stellar-mass BH mergers in galactic nuclei. Based on the result, we propose a novel scenario for the formation of supermassive black holes in galactic nuclei.

**Second generation black holes, the pair-instability mass gap, and the escape speed of stellar clusters**

**14:25-14:50**

**Davide Gerosa (University of Birmingham)**

Pair instabilities in supernovae might prevent the formation of black holes with masses between ∼50 and ∼130 solar masses. Multiple generations of black-hole mergers provide a possible way to populate this “mass gap” from below. However, this requires an astrophysical environment with a sufficiently large escape speed to retain merger remnants and prevent them from being ejected by gravitational-wave recoils. We show that a single LIGO/Virgo observation of a black hole in the pair-instability mass gap implies that its progenitors grew in an environment with escape speed >50 km/s. This is larger than the escape speeds of most globular clusters, requiring denser and heavier environments such as nuclear star clusters or disks-assisted migration in galactic nuclei. A single detection in the mass gap would also hint at the existence of a much larger population of first-generation events from the same environment, thus providing a tool to disentangle the contribution of different formation channels to the observed merger rate. More on arxiv:1906.05295.

**Inferring how compact object binaries form using gravitational wave observations**

**14:50-15:15**

**Christopher Berry (CIERA, Northwestern University)**

LIGO and Virgo provide a new source of information regarding the end-points of stellar evolution. Multiple potential formation channels have been suggested for these binaries, and each of these have associated physical uncertainties, such as the kicks imparted during supernova explosions. The details of the formation channels leave imprints on the properties of the binaries, such as masses and spins. From these, we can infer how binaries form. While the current catalogue only provides weak constraints, over the next few years we will begin to pin down the details of compact binary formation. Adding complementary constraints from electromagnetic observations, such as short gamma-ray burst observations can further aid in inferences. I will explain how combining all the information from gravitational-wave observations, particular in the era of next-generation detectors, will provide a tough test of our best models of binary evolution.

**Double chirps from binary black hole mergers as probes of the final horizon geometry **

**15:15-15:40**

**Juan Calderon Bustillo (Monash University)**

The merger of a binary black hole gives birth to a highly distorted remnant black hole that relaxes to its final state by emitting gravitational waves. During this fraction of a second, space-time provides us with a unique opportunity to probe the most extreme regime of gravity by studying the behaviour of highly dynamical black hole horizons. In this talk we will discuss how a concrete geometrical feature of the evolving horizon imprints the gravitational-wave emission. By using state-of-the art numerical simulations of binary black holes, we find that the line-of-sight passage of a “cusp”-like defect on the remnant horizon of asymmetric binary black hole mergers correlates to post-merger “chirp” signatures in the gravitational-waves. In a time when the Event Horizon Telescope has recently provided the most direct image of a mature, relatively sedate black hole, our results suggest that the Advanced LIGO and Virgo detectors may soon examine the horizon of fully dynamical newborn ones.

#### K8 - Thursday 16:10-17:50 (Kostas Kokkotas)

**Is there a mass-ratio gap between numerical relativity and gravitational self-force?**

**16:10-16:35**

**Maarten van de Meent (AEI Potsdam)**

Future gravitational wave observatories — both in space and on the ground — will be sensitive to compact binary coalescences with mass-ratios between 1:10 and 1:1000. Numerical relativity simulations work very well for comparable mass binaries, but become increasingly challenging as the mass-ratio decreases, and are unlikely to cover this entire range. Gravitational self-force methods employ a systematic expansion in the mass-ratio to produce waveform models. Their natural regime of validity is therefore small mass-ratio binaries. We examine how accurate we can expect gravitational self-force models to be in the comparable mass regime. Is 1/4 a small number?