First narrow-band search for continuous gravitational waves from known pulsars in advanced detector data

Spinning neutron stars asymmetric with respect to their rotation axis are potential sources of continuous gravitational waves for ground-based interferometric detectors. In the case of known pulsars a fully coherent search, based on matched filtering, which uses the position and rotational parameters obtained from electromagnetic observations, can be carried out. Matched filtering maximizes the signalto- noise (SNR) ratio, but a large sensitivity loss is expected in case of even a very small mismatch between the assumed and the true signal parameters. For this reason, narrow-band analysis methods have been developed, allowing a fully coherent search for gravitational waves from known pulsars over a fraction of a hertz and several spin-down values. In this paper we describe a narrow-band search of 11 pulsars using data from Advanced LIGO’s first observing run. Although we have found several initial outliers, further studies show no significant evidence for the presence of a gravitational wave signal. Finally, we have placed upper limits on the signal strain amplitude lower than the spin-down limit for 5 of the 11 targets over the bands searched; in the case of J1813-1749 the spin-down limit has been beaten for the first time. For an additional 3 targets, the median upper limit across the search bands is below the spin-down limit. This is the most sensitive narrow-band search for continuous gravitational waves carried out so far

Detectability and parameter estimation of GWTC-3 events with LISA

Multiband observations of coalescing stellar-mass black holes binaries could deliver valuable information on the formation of those sources and potential deviations from General Relativity. Some of these binaries might be first detected by the space-based detector LISA and, then, several years later, observed with ground-based detectors. Due to large uncertainties in astrophysical models, it is hard to predict the population of such binaries that LISA could observe. In this work, we assess the ability of LISA to detect the events of the third catalogue of gravitational wave sources released by the LIGO/Virgo/KAGRA collaboration. We consider the possibility of directly detecting the source with LISA and performing archival searches in the LISA data stream, after the event has been observed with ground-based detectors. We also assess how much could LISA improve the determination of source parameters. We find that it is not guaranteed that any event other than GW150914 would have been detected. Nevertheless, if any event is detected by LISA, even with a very low signal-to-noise ratio, the measurement of source parameters would improve by combining observations of LISA and ground based detectors, in particular for the chirp mass

Fast post-adiabatic waveforms in the time domain: Applications to compact binary coalescences in LIGO and Virgo

We present a computationally efficient (time-domain) multipolar waveform model for quasi-circular spin-aligned compact binary coalescences. The model combines the advantages of the numerical-relativity informed, effective-one-body (EOB) family of models with a post-adiabatic solution of the equations of motion for the inspiral part of the two-body dynamics. We benchmark this model against other state-of-the-art waveforms in terms of efficiency and accuracy. We find a speed-up of one to two orders of magnitude compared to the underlying time-domain EOB model for the total mass range $2 - 100 M_{\odot}$. More specifically, for a low total-mass system, such as a binary neutron star with equal masses of $1.4 M_{\odot}$, like GW170817, the computational speedup is around 100 times; for an event with total mass $\sim 40 M_\odot$ and mass ratio $\sim 3$, like GW190412, the speedup is by a factor of $\sim 20$, while for a binary system of comparable masses and total mass of $\sim 70 M_{\odot}$, like GW150914, it is by a factor of $\sim 10$. We demonstrate that the new model is extremely faithful to the underlying EOB model with unfaithfulness less than $0.01\%$ across the entire applicable region of parameter space. Finally, we present successful applications of this new waveform model to parameter estimation studies and tests of general relativity

Effective-one-body multipolar waveforms for eccentric binary black holes with nonprecessing spins

We construct an inspiral-merger-ringdown eccentric gravitational-wave (GW) model for binary black holes with non-precessing spins within the effective-one-body formalism. This waveform model, SEOBNRv4EHM, extends the accurate quasi-circular SEOBNRv4HM model to eccentric binaries by including recently computed eccentric corrections up to 2PN order in the gravitational waveform modes, notably the $(l,|m|)=(2,2),(2,1),(3,3),(4,4),(5,5)$ multipoles. The waveform model reproduces the zero eccentricity limit with an accuracy comparable to the underlying quasi-circular model, with the unfaithfulness of $\lesssim1\%$ against quasi-circular numerical-relativity (NR) simulations. When compared against 28 public eccentric NR simulations from the Simulating eXtreme Spacetimes catalog with initial orbital eccentricities up to $e\simeq0.3$ and dimensionless spin magnitudes up to $+0.7$, the model provides unfaithfulness $<1\%$, showing that both the $(2,|2|)$-modes and the higher-order modes are reliably described without calibration to NR datasets in the eccentric sector. The waveform model SEOBNRv4EHM is able to qualitatively reproduce the phenomenology of dynamical captures, and can be extended to include spin-precession effects. It can be employed for upcoming observing runs with the LIGO-Virgo-KAGRA detectors and used to re-analyze existing GW catalogs to infer the eccentricity parameters for binaries with $e\lesssim0.3$ (at 20 Hz or lower) and spins up to $\lesssim 0.9-0.95$. The latter is a promising region of the parameter space where some astrophysical formation scenarios of binaries predict mild eccentricity in the ground-based detectors' bandwidth. Assessing the accuracy and robustness of the eccentric waveform model SEOBNRv4EHM for larger eccentricities and spins will require comparisons with, and, likely, calibration to eccentric NR waveforms in a larger region of the parameter space

Multi-messenger observations of a binary neutron star merger

On 2017 August 17 a binary neutron star coalescence candidate (later designated GW170817) with merger time 12:41:04 UTC was observed through gravitational waves by the Advanced LIGO and Advanced Virgo detectors. The Fermi Gamma-ray Burst Monitor independently detected a gamma-ray burst (GRB 170817A) with a time delay of ~1.7 s with respect to the merger time. From the gravitational-wave signal, the source was initially localized to a sky region of 31 deg2 at a luminosity distance of 40+8-8 Mpc and with component masses consistent with neutron stars. The component masses were later measured to be in the range 0.86 to 2.26 Mo. An extensive observing campaign was launched across the electromagnetic spectrum leading to the discovery of a bright optical transient (SSS17a, now with the IAU identification of AT 2017gfo) in NGC 4993 (at ~40 Mpc) less than 11 hours after the merger by the One- Meter, Two Hemisphere (1M2H) team using the 1 m Swope Telescope. The optical transient was independently detected by multiple teams within an hour. Subsequent observations targeted the object and its environment. Early ultraviolet observations revealed a blue transient that faded within 48 hours. Optical and infrared observations showed a redward evolution over ~10 days. Following early non-detections, X-ray and radio emission were discovered at the transient’s position ~9 and ~16 days, respectively, after the merger. Both the X-ray and radio emission likely arise from a physical process that is distinct from the one that generates the UV/optical/near-infrared emission. No ultra-high-energy gamma-rays and no neutrino candidates consistent with the source were found in follow-up searches. These observations support the hypothesis that GW170817 was produced by the merger of two neutron stars in NGC4993 followed by a short gamma-ray burst (GRB 170817A) and a kilonova/macronova powered by the radioactive decay of r-process nuclei synthesized in the ejecta

Fully precessing higher-mode surrogate model of effective-one-body waveforms

We present a surrogate model of \texttt{SEOBNRv4PHM}, a fully precessingtime-domain effective-one-body waveform model including subdominant modes. Wefollow an approach similar to that used to build recent numerical relativitysurrogate models. Our surrogate is 5000M in duration, covers mass-ratios up to1:20 and dimensionless spin magnitudes up to 0.8. Validating the surrogateagainst an independent test set we find that the bulk of the surrogate errorsis less than $\sim 1\%$ in mismatch, which is similar to the modelling error of\texttt{SEOBNRv4PHM} itself. At high total mass a few percent of configurationscan exceed this threshold if they are highly precessing and they exceed amass-ratio of 1:4. This surrogate is nearly two orders of magnitude faster thanthe underlying time-domain \texttt{SEOBNRv4PHM} model and can be evaluated in$\sim 50$ ms. Bayesian inference analyses with \texttt{SEOBNRv4PHM} aretypically very computationally demanding and can take from weeks to months tocomplete. The two order of magnitude speedup attained by our surrogate modelenables practical parameter estimation analyses with this waveform family. Thisis \emph{crucial} because Bayesian inference allows us to recover the massesand spins of binary black hole mergers given a model of the emittedgravitational waveform along with a description of the noise.<br

A detailed analysis of GW190521 with phenomenological waveform models

In this paper we present an extensive analysis of the GW190521 gravitational wave event with the current (fourth) generation of phenomenological waveform models for binary black hole coalescences. GW190521 stands out from other events since only a few wave cycles are observable. This leads to a number of challenges, one being that such short signals are prone to not resolve approximate waveform degeneracies, which may result in multi-modal posterior distributions. The family of waveform models we use includes a new fast time-domain model IMRPhenomTPHM, which allows us extensive tests of different priors and robustness with respect to variations in the waveform model, including the content of spherical harmonic modes. We clarify some issues raised in a recent paper [Nitz&Capano], associated with possible support for a high-mass ratio source, but confirm their finding of a multi-modal posterior distribution, albeit with important differences in the statistical significance of the peaks. In particular, we find that the support for both masses being outside the PISN mass-gap, and the support for an intermediate mass ratio binary are drastically reduced with respect to what Nitz&Capano found. We also provide updated probabilities for associating GW190521 to the potential electromagnetic counterpart from ZTF

Theoretical groundwork supporting the precessing-spin two-body dynamics of the effective-one-body waveform models SEOBNRv5

Waveform models are essential for gravitational-wave (GW) detection andparameter estimation of coalescing compact-object binaries. More accuratemodels are required for the increasing sensitivity of current and future GWdetectors. The effective-one-body (EOB) formalism combines the post-Newtonian(PN) and small mass-ratio approximations with numerical-relativity results, andproduces highly accurate inspiral-merger-ringdown waveforms. In this paper, wederive the analytical precessing-spin two-body dynamics for the\texttt{SEOBNRv5} waveform model, which has been developed for the upcomingLIGO-Virgo-KAGRA observing run. We obtain an EOB Hamiltonian that reduces tothe exact Kerr Hamiltonian in the test-mass limit. It includes the full 4PNprecessing-spin information, and is valid for generic compact objects (i.e.,for black holes or neutron stars). We also build an efficient and accurate EOBHamiltonian that includes partial precessional effects, notably orbit-averagedin-plane spin effects for circular orbits, and derive 4PN-expandedprecessing-spin equations of motion, consistent with such an EOB Hamiltonian.The results were used to build the computationally-efficient precessing-spinmultipolar \texttt{SEOBNRv5PHM} waveform model.<br

pySEOBNR: a software package for the next generation of effective-one-body multipolar waveform models

We present pySEOBNR, a Python package for gravitational-wave (GW) modelingdeveloped within the effective-one-body (EOB) formalism. The package containsan extensive framework to generate state-of-the-art inspiral-merger-ringdownwaveform models for compact-object binaries composed of black holes and neutronstars. We document and demonstrate how to use the built-in quasi-circularprecessing-spin model SEOBNRv5PHM, whose aligned-spin limit (SEOBNRv5HM) hasbeen calibrated to numerical-relativity simulations and the nonspinning sectorto gravitational self-force data using pySEOBNR. Furthermore, pySEOBNR containsthe infrastructure necessary to construct, calibrate, test, and profile newwaveform models in the EOB approach. The efficiency and flexibility of pySEOBNRwill be crucial to overcome the data-analysis challenges posed by upcoming andnext-generation GW detectors on the ground and in space, which will afford thepossibility to observe all compact-object binaries in our Universe.<br

Next generation of accurate and efficient multipolar precessing-spin effective-one-body waveforms for binary black holes

Spin precession is one of the key physical effects that could unveil theorigin of the compact binaries detected by ground- and space-basedgravitational-wave (GW) detectors, and shed light on their possible formationchannels. Efficiently and accurately modeling the GW signals emitted by thesesystems is crucial to extract their properties. Here, we present SEOBNRv5PHM, amultipolar precessing-spin waveform model within the effective-one-body (EOB)formalism for the full signal (i.e. inspiral, merger and ringdown) of binaryblack holes (BBHs). In the non-precessing limit, the model reduces toSEOBNRv5HM, which is calibrated to $442$ numerical-relativity (NR) simulations,13 waveforms from BH perturbation theory, and non-spinning energy flux fromsecond-order gravitational self-force theory. We remark that SEOBNRv5PHM is notcalibrated to precessing-spin NR waveforms from the Simulating eXtremeSpacetimes Collaboration. We validate SEOBNRv5PHM by computing theunfaithfulness against 1543 precessing-spin NR waveforms, and find that for99.8% (84.4%) of the cases, the maximum value, in the total mass range 20-300$M_\odot$, is below 3% (1%). These numbers reduce to 95.3% (60.8%) when usingthe previous version of the SEOBNR family, SEOBNRv4PHM, and to 78.2% (38.3%)when using the state-of-the-art frequency-domain multipolar precessing-spinphenomenological IMRPhenomXPHM model. Due to much better computationalefficiency of SEOBNRv5PHM compared to SEOBNRv4PHM, we are also able to performextensive Bayesian parameter estimation on synthetic signals and GW eventsobserved by LIGO-Virgo detectors. We show that SEOBNRv5PHM can be used as astandard tool for inference analyses to extract astrophysical and cosmologicalinformation of large catalogues of BBHs.<br