arXiv subjects

Eric Thrane

Publications and source records attributed to Eric Thrane.

At least 73 records · Page 4Linked to original sources

arXiv2023

Measuring the properties of active galactic nuclei disks with gravitational waves

Active galactic nuclei (AGN) are promising environments for the assembly of merging binary black hole (BBH) systems. Interest in AGNs as nurseries for merging BBH is rising following the detection of gravitational waves from a BBH system from the purported pair-instability mass gap, most notably, GW190521. Active galactic nuclei have also been invoked to explain the formation of the high-mass-ratio system, GW190814. We draw on simulations of BBH systems in AGN to propose a phenomenological model for the distribution of black hole spins of merging binaries in AGN disks. The model incorporates distinct features that make the AGN channel potentially distinguishable from other channels, such as assembly in the field and in globular clusters. The model parameters can be mapped heuristically to the age and density of AGN disks. We estimate the extent to which different populations of mergers in AGNs can be distinguished. If most merging black holes are assembled in AGNs, future gravitational-wave observations may provide insights into the dynamics of AGN disks.

gr-qc↗

arXiv2023

Deep follow-up for gravitational-wave inference: a case study with GW151226

New analyses of gravitational wave events raise questions about the nature of some events. For example, LIGO--Virgo--KAGRA initially determined GW151226 to be a merger with a mass-ratio $q\approx0.5$ and effective inspiral spin $χ_{\text{eff}}\approx 0.2$. However, recent works offer an alternative picture: GW151226 is a lower-mass-ratio event $q \approx 0.3$ with slightly higher spin $χ_{\text{eff}}\approx 0.3$. This discrepancy has been challenging to resolve as a wide range of differences are employed for each analysis. This work introduces a ``deep follow-up'' framework to efficiently compute the posterior odds between two different peaks in parameter space. In doing so, we aim to help resolve disputes about the true nature of gravitational-wave events associated with conflicting astrophysical interpretations. Our proposal is not a replacement for standard inference techniques; instead, our method provides a diagnostic tool to understand discrepancies between conflicting results. We demonstrate this method by studying three {$q$-$χ_{\text{eff}}$} peaks proposed for GW151226. We find the $(q\sim0.5, χ_{\text{eff}}\sim0.2)$ interpretation is only slightly preferred over the $(q\sim0.3, χ_{\text{eff}}\sim0.3)$ hypothesis with a posterior odds of $\sim1.7\pm0.4$, suggesting that neither of the two peaks can be ruled out. We discuss strategies to produce more reliable parameter estimation studies in gravitational-wave astronomy.

astro-ph.HE↗

arXiv2023

Meet the parents: the progenitor binary for the supermassive black hole candidate in E1821+643

The remnants of binary black hole mergers can be given recoil kick velocities up to $5,000\text{ km s$^{-1}$}$ due to anisotropic emission of gravitational waves. E1821+643 is a recoiling supermassive black hole moving at $\sim 2,100\text{ km s$^{-1}$}$ along the line-of-sight relative to its host galaxy. This suggests a recoil kick of $\sim 2,240\text{ km s$^{-1}$}$. Such a kick is powerful enough to eject E1821+643 from its $M_\text{gal}\sim 2 \times 10^{12}\text{ M}_{\odot}$ host galaxy. In this work, we address the question: what are the likely properties of the progenitor binary that formed E1821+643? Using astrophysically motivated priors, we infer that E1821+643 was likely formed from a binary black hole system with masses of $m_1\sim 1.9^{+5.0}_{-3.8}\times 10^{9}\text{ M}_{\odot}$, $m_2\sim 8.1^{+3.9}_{-3.2} \times 10^{8}\text{ M}_{\odot}$ (90\% credible intervals). The black holes in this binary were likely to be spinning rapidly with dimensionless spin magnitudes of $χ_1 = 0.87^{+0.11}_{-0.26}$, $χ_2 = 0.77^{+0.19}_{-0.37}$. Such a high recoil velocity is impossible for spins aligned to the orbital angular momentum axis. This suggest that E1821+643 merged in hot gas, which is thought to provide an environment where spin alignment from accretion proceeds slowly relative to the merger timescale. We infer that E1821+643 is likely to be rapidly rotating with dimensionless spin $χ = 0.92\pm0.04$. A $2.6 \times 10^9 \text{ M}_{\odot}$ black hole, recoiling from a gas-rich environment at $v\sim 2,240\text{ km s$^{-1}$}$ is likely to persist as an active galactic nuclei for $\sim 860$ Myr, in which time it traverses $\sim 2$ Mpc.

astro-ph.GA↗

arXiv2023

Model exploration in gravitational-wave astronomy with the maximum population likelihood

Hierarchical Bayesian inference is an essential tool for studying the population properties of compact binaries with gravitational waves. The basic premise is to infer the unknown prior distribution of binary black hole and/or neutron star parameters such component masses, spin vectors, and redshift. These distributions shed light on the fate of massive stars, how and where binaries are assembled, and the evolution of the Universe over cosmic time. Hierarchical analyses model the binary black hole population using a prior distribution conditioned on hyper-parameters, which are inferred from the data. However, a misspecified model can lead to faulty astrophysical inferences. In this paper we answer the question: given some data, which prior distribution--from the set of all possible prior distributions--produces the largest possible population likelihood? This distribution (which is not a true prior) is $\pistroke$ (pronounced "pi stroke"), and the associated \textit{maximum population likelihood} is $\Lstroke$ (pronounced "L stroke"). The structure of $\pistroke$ is a linear superposition of delta functions, a result which follows from Carath{é}odory's theorem. We show how $\pistroke$ and $\Lstroke$ can be used for model exploration/criticism. We apply this $\Lstroke$ formalism to study the population of binary black hole mergers observed in LIGO--Virgo--KAGRA's third Gravitational-Wave Transient Catalog. Based on our results, we discuss possible improvements for gravitational-wave population models.

astro-ph.IM↗

arXiv2022

Do unequal-mass binary black hole systems have larger $χ_\text{eff}$? Probing correlations with copulas in gravitational-wave astronomy

The formation history of binary black hole systems is imprinted on the distribution of their masses, spins, and eccentricity. While much has been learned studying these parameters in turn, recent studies have explored the joint distribution of binary black hole parameters in two or more dimensions. Most notably, it has recently been argued that binary black hole mass ratio and effective inspiral spin $χ_\text{eff}$ are anti-correlated. We point out a previously overlooked subtlety in such two-dimensional population studies: in order to conduct a controlled test for correlation, one ought to fix the two marginal distributions -- lest the purported correlation be driven by improved fit in just one dimension. We address this subtlety using a tool from applied statistics: the copula density function. We use the previous work correlating mass ratio and $χ_\text{eff}$ as a case study to demonstrate the power of copulas in gravitational-wave astronomy while scrutinising their astrophysical inferences. Our findings, however, affirm their conclusions that binary black holes with unequal component masses exhibit larger $χ_\text{eff}$ (98.7% credibility). We conclude by discussing potential astrophysical implications of these findings as well as prospects for future studies using copulas.

astro-ph.HE↗

arXiv2022

Gravitational-wave inference for eccentric binaries: the argument of periapsis

Gravitational waves from binary black hole mergers have allowed us to directly observe stellar-mass black hole binaries for the first time, and therefore explore their formation channels. One of the ways to infer how a binary system is assembled is by measuring the system's orbital eccentricity. Current methods of parameter estimation do not include all physical effects of eccentric systems such as spin-induced precession, higher-order modes, and the initial argument of periapsis: an angle describing the orientation of the orbital ellipse. We explore how varying the argument of periapsis changes gravitational waveforms and study its effect on the inference of astrophysical parameters. We use the eccentric spin-aligned waveforms TEOBResumS and SEOBNRE to measure the change in the waveforms as the argument of periapsis is changed. We find that the argument of periapsis could already be impacting analyses performed with TEOBResumS. However, it is likely to be well-resolvable in the foreseeable future only for the loudest events observed by LIGO--Virgo--KAGRA. The systematic error in previous, low-eccentricity analyses that have not considered the argument of periapsis is likely to be small.

gr-qc↗

arXiv2022

The population properties of spinning black holes using Gravitational-wave Transient Catalog 3

Binary black holes formed via different pathways are predicted to have distinct spin properties. Measuring these properties with gravitational waves provides an opportunity to unveil the origins of binary black holes. Recent work draws conflicting conclusions regarding the spin distribution observed by LIGO--Virgo--KAGRA (LVK). Some analyses suggest that a fraction of the observed black-hole spin vectors are significantly misaligned (by $>90^\circ$) relative to the orbital angular momentum. This has been interpreted to mean that some binaries in the LVK dataset are assembled dynamically in dense stellar environments. Other analyses find support for a sub-population of binaries with negligible spin and no evidence for significantly misaligned spin -- a result consistent with the field formation scenario. In this work, we study the spin properties of binary black holes in the third LVK gravitational-wave transient catalog. We find that there is insufficient data to resolve the existence of a sub-population of binaries with negligible black-hole spin (the presence of this sub-population is supported by a modest Bayes factor of 1.7). We find modest support for the existence of mergers with extreme spin tilt angles $> 90^\circ$ (the presence of extreme-tilt binaries is favored by a Bayes factor of 10.1). Only one thing is clear: at least some of the LVK binaries formed in the field. At most $89\%$ of binaries are assembled dynamically (99\% credibility), though, the true branching fraction could be much lower, even negligible.

astro-ph.HE↗

arXiv2022

Four eccentric mergers increase the evidence that LIGO--Virgo--KAGRA's binary black holes form dynamically

The growing population of compact binary mergers detected with gravitational waves contains multiple events that are challenging to explain through isolated binary evolution. Such events have higher masses than are expected in isolated binaries, component spin-tilt angles that are misaligned, and/or non-negligible orbital eccentricities. We investigate the orbital eccentricities of 62 binary black hole candidates from the third gravitational-wave transient catalogue of the LIGO-Virgo-KAGRA Collaboration with an aligned-spin, moderate-eccentricity waveform model. Within this framework, we find that at least four of these events show significant support for eccentricity $e_{10} \geq 0.1$ at a gravitational-wave frequency of $10$~Hz ($> 60\%$ credibility, under a log-uniform eccentricity prior that spans the range $10^{-4} < e_{10} < 0.2$). Two of these events are new additions to the population: GW191109 and GW200208\_22. If the four eccentric candidates are truly eccentric, our results suggest that densely-populated star clusters may produce 100\% of the observed mergers. However, it remains likely that other formation environments with higher yields of eccentric mergers -- for example, active galactic nuclei -- also contribute. We estimate that we will be able to confidently distinguish which formation channel dominates the eccentric merger rate after $\gtrsim 80$ detections of events with $e_{10} \geq 0.05$ at LIGO--Virgo sensitivity, with only $\sim 5$ detectably-eccentric events required to distinguish formation channels with third-generation gravitational-wave detectors.

astro-ph.HE↗

arXiv2022

A Rosetta Stone for eccentric gravitational waveform models

Orbital eccentricity is a key signature of dynamical binary black hole formation. The gravitational waves from a coalescing binary contain information about its orbital eccentricity, which may be measured if the binary retains sufficient eccentricity near merger. Dedicated waveforms are required to measure eccentricity. Several models have been put forward, and show good agreement with numerical relativity at the level of a few percent or better. However, there are multiple ways to define eccentricity for inspiralling systems, and different models internally use different definitions of eccentricity, making it difficult to directly compare eccentricity measurements. In this work, we systematically compare two eccentric waveform models, $\texttt{SEOBNRE}$ and $\texttt{TEOBResumS}$, by developing a framework to translate between different definitions of eccentricity. This mapping is constructed by minimizing the relative mismatch between the two models over eccentricity and reference frequency, before evolving the eccentricity of one model to the same reference frequency as the other model. We show that for a given value of eccentricity passed to $\texttt{SEOBNRE}$, one must input a $20$-$50\%$ smaller value of eccentricity to $\texttt{TEOBResumS}$ in order to obtain a waveform with the same empirical eccentricity. We verify this mapping by repeating our analysis for eccentric numerical relativity simulations, demonstrating that $\texttt{TEOBResumS}$ reports a correspondingly smaller value of eccentricity than $\texttt{SEOBNRE}$.

gr-qc↗

arXiv2022

Consistency of the Parkes Pulsar Timing Array Signal with a Nanohertz Gravitational-wave Background

Pulsar timing array experiments have recently reported strong evidence for a common-spectrum stochastic process with a strain spectral index consistent with that expected of a nanohertz-frequency gravitational-wave background, but with negligible yet non-zero evidence for spatial correlations required for a definitive detection. However, it was pointed out by the Parkes Pulsar Timing Array (PPTA) collaboration that the same models used in recent analyses resulted in strong evidence for a common-spectrum process in simulations where none is present. In this work, we introduce a methodology to distinguish pulsar power spectra with the same amplitude from noise power spectra of similar but distinct amplitudes. The former is the signature of a spatially uncorrelated pulsar term of a nanohertz gravitational-wave background, whereas the latter could represent ensemble pulsar noise properties. We test the methodology on simulated data sets. We find that the reported common process in PPTA pulsars is indeed consistent with the spectral feature of a pulsar term. We recommend this methodology as one of the validity tests that the real astrophysical and cosmological backgrounds should pass, as well as for inferences about the spatially uncorrelated component of the background.

gr-qc↗

arXiv2022

Signatures of mass ratio reversal in gravitational waves from merging binary black holes

The spins of merging binary black holes offer insights into their formation history. Recently it has been argued that in isolated binary evolution of two massive stars the firstborn black hole is slowly rotating, whilst the progenitor of the second-born black hole can be tidally spun up if the binary is tight enough. Naively, one might therefore expect that only the less massive black hole in merging binaries exhibits non-negligible spin. However, if the mass ratio of the binary is "reversed" (typically during the first mass transfer episode), it is possible for the tidally spun up second-born to become the more massive black hole. We study the properties of such mass-ratio reversed (MRR) binary black hole mergers using a large set of 560 population synthesis models. We find that the more massive black hole is formed second in $\gtrsim 70\%$ of binary black holes observable by LIGO, Virgo, and KAGRA for most model variations we consider, with typical total masses $\gtrsim 20$ M$_{\odot}$ and mass ratios $q = m_2 / m_1 \sim 0.7$ (where $m_1 > m_2$). The formation history of these systems typically involves only stable mass transfer episodes. The second-born black hole has non-negligible spin ($χ> 0.05$) in up to $25\%$ of binary black holes, with among those the more (less) massive black hole spinning in $0\%$--$80\%$ ($20\%$--$100\%$) of cases, varying greatly in our models. We discuss our models in the context of several observed gravitational-wave events and the observed mass ratio - effective spin correlation.

astro-ph.HE↗

arXiv2022

GWCloud: a searchable repository for the creation and curation of gravitational-wave inference results

There are at present ${\cal O}(100)$ gravitational-wave candidates from compact binary mergers reported in the astronomical literature. As detector sensitivities are improved, the catalog will swell in size: first to ${\cal O}(1000)$ events in the A+ era and then to ${\cal O}(10^6)$ events in the era of third-generation observatories like Cosmic Explorer and the Einstein Telescope. Each event is analyzed using Bayesian inference to determine properties of the source including component masses, spins, tidal parameters, and the distance to the source. These inference products are the fodder for some of the most exciting gravitational-wave science, enabling us to measure the expansion of the Universe with standard sirens, to characterise the neutron star equation of state, and to unveil how and where gravitational-wave sources are assembled. In order to maximize the science from the coming deluge of detections, we introduce GWCloud, a searchable repository for the creation and curation of gravitational-wave inference products. It is designed with five pillars in mind: uniformity of results, reproducibility of results, stability of results, access to the astronomical community, and efficient use of computing resources. We describe how to use GWCloud with examples, which readers can replicate using the companion code to this paper. We describe our long-term vision for GWCloud.

gr-qc↗

arXiv2022

Orbital dynamics and extreme scattering event properties from long-term scintillation observations of PSR J1603-7202

We model long-term variations in the scintillation of binary pulsar PSR J1603$-$7202, observed by the 64 m Parkes radio telescope (Murriyang) between 2004 and 2016. We find that the time variation in the scintillation arc curvature is well-modelled by scattering from an anisotropic thin screen of plasma between the Earth and the pulsar. Using our scintillation model, we measure the inclination angle and longitude of ascending node of the orbit, yielding a significant improvement over the constraints from pulsar timing. From our measurement of the inclination angle, we place a lower bound on the mass of J1603$-$7202's companion of $\gtrsim 0.5\,\text{M}_\odot$ assuming a pulsar mass of $\gtrsim1.2\,\text{M}_\odot$. We find that the scintillation arcs are most pronounced when the electron column density along the line of sight is increased, and that arcs are present during a known extreme scattering event. We measure the distance to the interstellar plasma and its velocity, and we discuss some structures seen in individual scintillation arcs within the context of our model.

astro-ph.HE↗

arXiv2022

Building better spin models for merging binary black holes: Evidence for non-spinning and rapidly spinning nearly aligned sub-populations

Recent work paints a conflicting portrait of the distribution of black hole spins in merging binaries measured with gravitational waves. Some analyses find that a significant fraction of merging binaries contain at least one black hole with a spin tilt $>90^\circ$ with respect to the orbital angular momentum vector, which has been interpreted as a signature for dynamical assembly. Other analyses find the data are consistent with a bimodal population in which some binaries contain black holes with negligible spin while the rest contain black holes with spin vectors preferentially aligned with the orbital angular momentum vector. In this work, we scrutinize models for the distribution of black hole spins to pinpoint possible failure modes in which the model yields a faulty conclusion. We reanalyze data from the second LIGO--Virgo gravitational-wave transient catalog (GWTC-2) using a revised spin model, which allows for a sub-population of black holes with negligible spins. In agreement with recent results by Roulet et al., we show that the GWTC-2 detections are consistent with two distinct sub-populations. We estimate that $29-75\%$ (90\% credible interval) of merging binaries contain black holes with negligible spin $χ\approx 0$. The remaining binaries are part of a second sub-population in which the spin vectors are preferentially (but not exactly) aligned to the orbital angular momentum. The black holes in this second sub-population are characterized by spins of $χ\sim0.45$. We suggest that the inferred spin distribution is consistent with the hypothesis that all merging binaries form via the field formation scenario.

gr-qc↗

arXiv2022

Linking the rates of neutron star binaries and short gamma-ray bursts

Short gamma-ray bursts are believed to be produced by both binary neutron star (BNS) and neutron star-black hole (NSBH) mergers. We use current estimates for the BNS and NSBH merger rates to calculate the fraction of observable short gamma-ray bursts produced through each channel. This allows us to constrain merger rates of BNS to $\mathcal{R}_{\rm{BNS}}=384^{+431}_{-213}{\rm{Gpc}^{-3} \rm{yr}^{-1}}$ ($90\%$ credible interval), a $16\%$ decrease in the rate uncertainties from the second LIGO--Virgo Gravitational-Wave Transient Catalog, GWTC-2. Assuming a top-hat emission profile with a large Lorentz factor, we constrain the average opening angle of gamma-ray burst jets produced in BNS mergers to $\approx 15^\circ$. We also measure the fraction of BNS and NSBH mergers that produce an observable short gamma-ray burst to be $0.02^{+0.02}_{-0.01}$ and $0.01 \pm 0.01$, respectively and find that $\gtrsim 40\%$ of BNS mergers launch jets (90\% confidence). We forecast constraints for future gravitational-wave detections given different modelling assumptions, including the possibility that BNS and NSBH jets are different. With $24$ BNS and $55$ NSBH observations, expected within six months of the LIGO-Virgo-KAGRA network operating at design sensitivity, it will be possible to constrain the fraction of BNS and NSBH mergers that launch jets with $10\%$ precision. Within a year of observations, we can determine whether the jets launched in NSBH mergers have a different structure than those launched in BNS mergers and rule out whether $\gtrsim 80\%$ of binary neutron star mergers launch jets. We discuss the implications of future constraints on understanding the physics of short gamma-ray bursts and binary evolution.

astro-ph.HE↗

arXiv2022

When models fail: an introduction to posterior predictive checks and model misspecification in gravitational-wave astronomy

Bayesian inference is a powerful tool in gravitational-wave astronomy. It enables us to deduce the properties of merging compact-object binaries and to determine how these mergers are distributed as a population according to mass, spin, and redshift. As key results are increasingly derived using Bayesian inference, there is increasing scrutiny on Bayesian methods. In this review, we discuss the phenomenon of \textit{model misspecification}, in which results obtained with Bayesian inference are misleading because of deficiencies in the assumed model(s). Such deficiencies can impede our inferences of the true parameters describing physical systems. They can also reduce our ability to distinguish the "best fitting" model: it can be misleading to say that Model~A is preferred over Model~B if both models are manifestly poor descriptions of reality. Broadly speaking, there are two ways in which models fail: models that fail to adequately describe the data (either the signal or the noise) have misspecified likelihoods. Population models -- designed, for example, to describe the distribution of black hole masses -- may fail to adequately describe the true population due to a misspecified prior. We recommend tests and checks that are useful for spotting misspecified models using examples inspired by gravitational-wave astronomy. We include companion python notebooks to illustrate essential concepts.

astro-ph.IM↗

arXiv2022

Bayesian WIMP detection with the Cherenkov Telescope Array

Over the past decades Bayesian methods have become increasingly popular in astronomy and physics as stochastic samplers have enabled efficient investigation of high-dimensional likelihood surfaces. In this work we develop a hierarchical Bayesian inference framework to detect the presence of dark matter annihilation events in data from the Cherenkov Telescope Array (CTA). Cosmic rays are weighted based on their measured sky position $\hatΩ_m$ and energy $E_m$ in order to derive a posterior distribution for the dark matter's velocity averaged cross section $\langleσv\rangle$. The dark matter signal model and the astrophysical background model are cast as prior distributions for $(\hatΩ_m, E_m)$. The shape of these prior distributions can be fixed based on first-principle models; or one may adopt flexible priors to include theoretical uncertainty, for example, in the dark matter annihilation spectrum or the astrophysical distribution of sky location. We demonstrate the utility of this formalism using simulated data with a contribution from a scalar singlet dark-matter model. The sensitivity according to our method is comparable to previous estimates of the CTA sensitivity.

astro-ph.HE↗

arXiv2022

Fast, flexible, and accurate evaluation of gravitational-wave Malmquist bias with machine learning

Many astronomical surveys are limited by the brightness of the sources, and gravitational-wave searches are no exception. The detectability of gravitational waves from merging binaries is affected by the mass and spin of the constituent compact objects. To perform unbiased inference on the distribution of compact binaries, it is necessary to account for this selection effect, which is known as Malmquist bias. Since systematic error from selection effects grows with the number of events, it will be increasingly important over the coming years to accurately estimate the observational selection function for gravitational-wave astronomy. We employ density estimation methods to accurately and efficiently compute the compact binary coalescence selection function. We introduce a simple pre-processing method, which significantly reduces the complexity of the required machine learning models. We demonstrate that our method has smaller statistical errors at comparable computational cost than the method currently most widely used allowing us to probe narrower distributions of spin magnitudes. The currently used method leaves $10-50\%$ of the interesting black hole spin models inaccessible; our new method can probe $>99\%$ of the models and has a lower uncertainty for $>80\%$ of the models.

gr-qc↗