arXiv subjects

Eric Thrane

Publications and source records attributed to Eric Thrane.

At least 109 records · Page 6Linked to original sources

arXiv2020

Measuring the primordial gravitational-wave background in the presence of astrophysical foregrounds

Primordial gravitational waves are expected to create a stochastic background encoding information about the early Universe that may not be accessible by other means. However, the primordial background is obscured by an astrophysical foreground consisting of gravitational waves from compact binaries. We demonstrate a Bayesian method for estimating the primordial background in the presence of an astrophysical foreground. Since the background and foreground signal parameters are estimated simultaneously, there is no subtraction step, and therefore we avoid astrophysical contamination of the primordial measurement, sometimes referred to as "residuals". Additionally, since we include the non-Gaussianity of the astrophysical foreground in our model, this method represents the statistically optimal approach to the simultaneous detection of a multi-component stochastic background.

astro-ph.HE↗

arXiv2020

Searching for anisotropy in the distribution of binary black hole mergers

The standard model of cosmology is underpinned by the assumption of the statistical isotropy of the Universe. Observations of the cosmic microwave background, galaxy distributions, and supernovae, among other media, support the assumption of isotropy at scales $\gtrsim 100$\,Mpc. The recent detections of gravitational waves from merging stellar-mass binary black holes provide a new probe of anisotropy; complementary and independent of all other probes of the matter distribution in the Universe. We present an analysis using a spherical harmonic model to determine the level of anisotropy in the first LIGO/Virgo transient catalog. We find that the ten binary black hole mergers within the first transient catalog are consistent with an isotropic distribution. We carry out a study of simulated events to assess the prospects for future probes of anisotropy. Within a single year of operation, third-generation gravitational-wave observatories will probe anisotropies with an angular scale of $\sim36^\circ$ at the level of $\lesssim0.1\%$.

astro-ph.CO↗

arXiv2020

Identifying and mitigating noise sources in precision pulsar timing data sets

Pulsar timing array projects measure the pulse arrival times of millisecond pulsars for the primary purpose of detecting nanohertz-frequency gravitational waves. The measurements include contributions from a number of astrophysical and instrumental processes, which can either be deterministic or stochastic. It is necessary to develop robust statistical and physical models for these noise processes because incorrect models diminish sensitivity and may cause a spurious gravitational wave detection. Here we characterise noise processes for the 26 pulsars in the second data release of the Parkes Pulsar Timing Array using Bayesian inference. In addition to well-studied noise sources found previously in pulsar timing array data sets such as achromatic timing noise and dispersion measure variations, we identify new noise sources including time-correlated chromatic noise that we attribute to variations in pulse scattering. We also identify "exponential dip" events in four pulsars, which we attribute to magnetospheric effects as evidenced by pulse profile shape changes observed for three of the pulsars. This includes an event in PSR J1713$+$0747, which had previously been attributed to interstellar propagation. We present noise models to be used in searches for gravitational waves. We outline a robust methodology to evaluate the performance of noise models and identify unknown signals in the data. The detection of variations in pulse profiles highlights the need to develop efficient profile domain timing methods.

astro-ph.HE↗

arXiv2020

Constraining temperature distribution inside LIGO test masses from frequencies of their vibrational modes

Thermal distortion of test masses, as well as thermal drift of their vibrational mode frequencies, present a major challenge for operation of the Advanced LIGO and Advanced VIRGO interferometers, reducing optical efficiency, which limits sensitivity and potentially causing instabilities which reduce duty-cycle. In this paper, we demonstrate that test-mass vibrational mode frequency data can be used to overcome some of these difficulties. First, we derive a general expression for the change in a mode frequency as a function of temperature distribution inside the test mass. Then we show how the mode frequency dependence on temperature distribution can be used to identify the wavefunction of observed vibrational modes. We then show how monitoring the frequencies of multiple vibrational modes allows the temperature distribution inside the test mass to be strongly constrained. Finally, we demonstrate using simulations, the potential to improve the thermal model of the test mass, providing independent and improved estimates of important parameters such as the coating absorption coefficient and the location of point absorbers.

astro-ph.IM↗

arXiv2020

GW190521: orbital eccentricity and signatures of dynamical formation in a binary black hole merger signal

Pair instability supernovae are thought to restrict the formation of black holes in the mass range ~50 - 135 solar masses. However, black holes with masses within this "high mass gap" are expected to form as the remnants of binary black hole mergers. These remnants can merge again dynamically in densely populated environments such as globular clusters. The hypothesis that the binary black hole merger GW190521 formed dynamically is supported by its high mass. Orbital eccentricity can also be a signature of dynamical formation, since a binary that merges quickly after becoming bound may not circularize before merger. In this work, we measure the orbital eccentricity of GW190521. We find that the data prefer a signal with eccentricity $e \geq 0.1$ at 10 Hz to a non-precessing, quasi-circular signal, with a log Bayes factor $\ln{\cal B}=5.0$. When compared to precessing, quasi-circular analyses, the data prefer a non-precessing, $e \geq 0.1$ signal, with log Bayes factors $\ln{\cal B}\approx2$. Using injection studies, we find that a non-spinning, moderately eccentric ($e = 0.13$) GW190521-like binary can be mistaken for a quasi-circular, precessing binary. Conversely, a quasi-circular binary with spin-induced precession may be mistaken for an eccentric binary. We therefore cannot confidently determine whether GW190521 was precessing or eccentric. Nevertheless, since both of these properties support the dynamical formation hypothesis, our findings support the hypothesis that GW190521 formed dynamically.

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arXiv2020

Black hole genealogy: Identifying hierarchical mergers with gravitational waves

In dense stellar environments, the merger products of binary black hole mergers may undergo additional mergers. These hierarchical mergers are predicted to have higher masses than the first generation of black holes made from stars. The components of hierarchical mergers are expected to have significant characteristic spins $χ\sim 0.7$. However, since the population properties of first-generation black holes are uncertain, it is difficult to know if any given merger is first-generation or hierarchical. We use observations of gravitational waves to reconstruct the binary black hole mass and spin spectrum of a population containing hierarchical merger events. We employ a phenomenological model that captures the properties of merging binary black holes from simulations of dense stellar environments. Inspired by recent work on the isolated formation of low-spin black holes, we include a zero-spin subpopulation. We analyze binary black holes from LIGO and Virgo's first two observing runs, and find that this catalog is consistent with having no hierarchical mergers. We find that the most massive system in this catalog, GW170729, is mostly likely a first-generation merger, having a $4\%$ probability of being a hierarchical merger assuming a $5 \times 10^5 M_{\odot}$ globular cluster mass. Using our model, we find that $99\%$ of first-generation black holes in coalescing binaries have masses below 44 $M_{\odot}$, and the fraction of binaries with near-zero spin is $0.051^{+0.156}_{-0.048}$ ($90\%$ credible interval). Upcoming observations will determine if hierarchical mergers are a common source of gravitational waves.

astro-ph.HE↗

arXiv2020

Machine Learning for Transient Recognition in Difference Imaging With Minimum Sampling Effort

The amount of observational data produced by time-domain astronomy is exponentially in-creasing. Human inspection alone is not an effective way to identify genuine transients fromthe data. An automatic real-bogus classifier is needed and machine learning techniques are commonly used to achieve this goal. Building a training set with a sufficiently large number of verified transients is challenging, due to the requirement of human verification. We presentan approach for creating a training set by using all detections in the science images to be thesample of real detections and all detections in the difference images, which are generated by the process of difference imaging to detect transients, to be the samples of bogus detections. This strategy effectively minimizes the labour involved in the data labelling for supervised machine learning methods. We demonstrate the utility of the training set by using it to train several classifiers utilizing as the feature representation the normalized pixel values in 21-by-21pixel stamps centered at the detection position, observed with the Gravitational-wave Optical Transient Observer (GOTO) prototype. The real-bogus classifier trained with this strategy can provide up to 95% prediction accuracy on the real detections at a false alarm rate of 1%.

astro-ph.IM↗

arXiv2020

A scalable random forest regressor for combining neutron-star equation of state measurements: A case study with GW170817 and GW190425

Gravitational-wave observations of binary neutron star coalescences constrain the neutron-star equation of state by enabling measurement of the tidal deformation of each neutron star. This deformation is determined by the tidal deformability parameter $Λ$, which was constrained using the first binary neutron star gravitational-wave observation, GW170817. Now, with the measurement of the second binary neutron star, GW190425, we can combine different gravitational-wave measurements to obtain tighter constraints on the neutron-star equation of state. In this paper, we combine data from GW170817 and GW190425 to place constraints on the neutron-star equation of state. To facilitate this calculation, we derive interpolated marginalized likelihoods for each event using a machine learning algorithm. These likelihoods, which we make publicly available, allow for results from multiple gravitational-wave signals to be easily combined. Using these new data products, we find that the radius of a fiducial 1.4 $M_\odot$ neutron star is constrained to $11.6^{+1.6}_{-0.9}$ km at 90% confidence and the pressure at twice the nuclear saturation density is constrained to $3.1^{+3.1}_{-1.3}\times10^{34}$ dyne/cm$^2$ at 90% confidence. This result is dominated by GW170817 and is consistent with findings from other works.

astro-ph.HE↗

arXiv2020

Gravitational-wave astronomy with a physical calibration model

We carry out astrophysical inference for compact binary merger events in LIGO-Virgo's first gravitational-wave transient catalog (GWTC-1) using a physically motivated calibration model. We demonstrate that importance sampling can be used to reduce the cost of what would otherwise be a computationally challenging analysis. We show that including the physical estimate for the calibration error distribution has negligible impact on the inference of parameters for the events in GWTC-1. Studying a simulated signal with matched filter signal-to-noise ratio $\text{SNR}=200$, we project that a calibration error estimate typical of GWTC-1 is likely to be negligible for the current generation of gravitational-wave detectors. We argue that other sources of systematic error---from waveforms, prior distributions, and noise modelling---are likely to be more important. Finally, using the events in GWTC-1 as standard sirens, we infer an astrophysically-informed improvement on the estimate of the calibration error in the LIGO interferometers.

astro-ph.IM↗

arXiv2020

The astrophysical odds of GW151216

The gravitational-wave candidate GW151216 is a proposed binary black hole event from the first observing run of the Advanced LIGO detectors. Not identified as a bona fide signal by the LIGO--Virgo collaboration, there is disagreement as to its authenticity, which is quantified by $p_\text{astro}$, the probability that the event is astrophysical in origin. Previous estimates of $p_\text{astro}$ from different groups range from 0.18 to 0.71, making it unclear whether this event should be included in population analyses, which typically require $p_\text{astro}>0.5$. Whether GW151216 is an astrophysical signal or not has implications for the population properties of stellar-mass black holes and hence the evolution of massive stars. Using the astrophysical odds, a Bayesian method which uses the signal coherence between detectors and a parameterised model of non-astrophysical detector noise, we find that $p_\text{astro}=0.03$, suggesting that GW151216 is unlikely to be a genuine signal. We also analyse GW150914 (the first gravitational-wave detection) and GW151012 (initially considered to be an ambiguous detection) and find $p_\text{astro}$ values of 1 and 0.997 respectively. We argue that the astrophysical odds presented here improve upon traditional methods for distinguishing signals from noise.

astro-ph.HE↗

arXiv2020

Toward the unambiguous identification of supermassive binary black holes through Bayesian inference

Supermassive binary black holes at sub-parsec orbital separations have yet to be discovered, with the possible exception of blazar OJ~287. In parallel to the global hunt for nanohertz gravitational waves from supermassive binaries using pulsar timing arrays, there has been a growing sample of candidates reported from electromagnetic surveys, particularly searches for periodic variations in optical light curves of quasars. However, the periodicity search is prone to false positives from quasar red noise and quasi-periodic oscillations from the accretion disc of a single supermassive black hole---especially when the data span fewer than a few signal cycles. We present a Bayesian method for the detection of quasar (quasi-)periodicity in the presence of red noise. We apply this method to the binary candidate PG1302$-$102, and show that a) there is very strong support (Bayes factor $>10^6$) for quasi-periodicity, and b) the data slightly favour a quasi-periodic oscillation over a sinusoidal signal, which we interpret as modest evidence against the binary black hole hypothesis. We also find that the prevalent damped random walk red-noise model is disfavored with more than 99.9\% credibility. Finally, we outline future work that may enable the unambiguous identification of supermassive binary black holes.

astro-ph.HE↗

arXiv2020

Is there a spectral turnover in the spin noise of millisecond pulsars?

Pulsar timing arrays provide a unique means to detect nanohertz gravitational waves through long-term measurements of pulse arrival times from an ensemble of millisecond pulsars. After years of observations, some timing array pulsars have been shown to be dominated by low-frequency red noise, including spin noise that might be associated with pulsar rotational irregularities. The power spectral density of pulsar timing red noise is usually modeled with a power law or a power law with a turnover frequency below which the noise power spectrum plateaus. If there is a turnover in the spin noise of millisecond pulsars, residing within the observation band of current and/or future pulsar timing measurements, it may be easier than projected to resolve the gravitational-wave background from supermassive binary black holes. Additionally, the spectral turnover can provide valuable insights on neutron star physics. In the recent study by Melatos and Link, the authors provided a derivation of the model for power spectral density of spin noise from superfluid turbulence in the core of a neutron star, from first principles. The model features a spectral turnover, which depends on the dynamical response time of the superfluid and the steady-state angular velocity lag between the crust and the core of the star. In this work, we search for a spectral turnover in spin noise using the first data release of the International Pulsar Timing Array. Through Bayesian model selection, we find no evidence of a spectral turnover. Our analysis also shows that data from pulsars J1939+2134, J1024$-$0719 and J1713+0747 prefers the power-law model to the superfluid turbulence model.

astro-ph.HE↗

arXiv2020

On the origin of GW190425

The LIGO/Virgo collaborations recently announced the detection of a likely binary neutron star merger, GW190425. The total mass of GW190425 is significantly larger than the masses of Galactic double neutron stars known through radio astronomy. This suggests that GW190425 formed differently from Galactic double neutron stars. We hypothesize that GW190425 formed via unstable "case BB" mass transfer. According to this hypothesis, the progenitor of GW190425 was a binary consisting of a neutron star and a ${\sim} 4-5 {M_\odot}$ helium star, which underwent a common-envelope process. Following the supernovae of the helium star core, a tight, eccentric, double neutron star was formed, which merged in ${\lesssim}$ 10 Myr. The helium star progenitor may explain the unusually large mass of GW190425, while the short time to merger may explain why we do not see similar systems in radio. In order to test this hypothesis, we measure the eccentricity of GW190425 using publicly available LIGO/Virgo data. We constrain the eccentricity at 10 Hz to be $e \leq 0.007$ with $90\%$ confidence. This result provides no evidence for or against the unstable mass transfer scenario because the binary is likely to have circularized to $e\lesssim10^{-4}$ by the time it entered the LIGO/Virgo band. Future detectors operating in lower frequency bands will enable us to discern the formation channel of mergers similar to GW190425 using eccentricity measurements.

astro-ph.HE↗

arXiv2020

Memory Effect or Cosmic String? Classifying Gravitational-Wave Bursts with Bayesian Inference

A variety of gravitational-wave transient sources can be modeled in the Fourier domain using a power law. This simple power-law model provides a reasonable approximation for gravitational-wave bursts from cosmic string cusps, cosmic string kinks, and the memory effect. Each of these sources is described using a different spectral index. In this work, we simulate interferometer strain data with injections of power-law and memory bursts to demonstrate parameter estimation, signal detection, and model selection. We show how Bayesian inference can be used to measure the power-law spectral index, thereby distinguishing between different astrophysical scenarios.

gr-qc↗

arXiv2020

Gravitational-wave astronomy with an uncertain noise power spectral density

In order to extract information about the properties of compact binaries, we must estimate the noise power spectral density of gravitational-wave data, which depends on the properties of the gravitational-wave detector. In practice, it is not possible to know this perfectly, only to estimate it from the data. Multiple estimation methods are commonly used and each has a corresponding statistical uncertainty. However, this uncertainty is widely ignored when measuring the physical parameters describing compact binary coalescences, and the appropriate likelihoods which account for the uncertainty are not well known. In order to perform increasingly precise astrophysical inference and model selection, it will be essential to account for this uncertainty. In this work, we derive the correct likelihood for one of the most widely used estimation methods in gravitational-wave transient analysis, the median average. We demonstrate that simulated Gaussian noise follows the predicted distributions. We then examine real gravitational-wave data at and around the time of GW151012, a relatively low-significance binary black hole merger event. We show that the data are well described by stationary-Gaussian noise and explore the impact of different noise power spectral density estimation methods on the astrophysical inferences we draw about GW151012.

astro-ph.IM↗

arXiv2020

Measuring the neutron star equation of state with gravitational waves: the first forty binary neutron star mergers

Gravitational waves from binary neutron star coalescences contain rich information about matter at supranuclear densities encoded by the neutron star equation of state. We can measure the equation of state by analyzing the tidal interactions between neutron stars, which is quantified by the tidal deformability. Multiple merger events are required to probe the equation of state over a range of neutron star masses. The more events included in the analysis, the stronger the constraints on the equation of state. In this paper, we build on previous work to explore the constraints that LIGO and Virgo are likely to place on the neutron star equation of state by combining the first forty binary neutron star detections, a milestone we project to be reached during the first year of accumulated design-sensitivity data. We carry out Bayesian inference on a realistic mock dataset of binaries to obtain posterior distributions for neutron star tidal parameters. In order to combine posterior samples from multiple observations, we employ a random forest regressor, which allows us to efficiently interpolate the likelihood distribution. Assuming a merger rate of 1540 Gpc$^{-3}$ yr$^{-1}$ and a LIGO-Virgo detector network operating for one year at the sensitivity of the third-observation run, plus an additional eight months of design sensitivity, we find that the radius of a 1.4 $M_\odot$ neutron star can be constrained to $\sim 10$% at 90% confidence. At the same time, the pressure at twice the nuclear saturation density can be constrained to $\sim 45$ % at 90% confidence. Finally, we add an appendix following publication of the paper in the journal, showing the posterior distribution of the maximum neutron star mass allowed by the equation of state. We find that the maximum mass can be constrained to $\sim 0.3$ $M_\odot$ at 90% confidence.

gr-qc↗

arXiv2020

Inferring the population properties of binary black holes from unresolved gravitational waves

The vast majority of compact binary mergers in the Universe produce gravitational waves that are too weak to yield unambiguous detections; they are unresolved. We present a method to infer the population properties of compact binaries -- such as their merger rates, mass spectrum, and spin distribution -- using both resolved and unresolved gravitational waves. By eliminating entirely the distinction between resolved and unresolved signals, we eliminate bias from selection effects. To demonstrate this method, we carry out a Monte Carlo study using an astrophysically motivated population of binary black holes. We show that some population properties of compact binaries are well constrained by unresolved signals after about one week of observation with Advanced LIGO at design sensitivity.

astro-ph.HE↗

arXiv2020

Constraining short gamma-ray burst jet properties with gravitational waves and gamma rays

Gamma-ray burst (GRB) prompt emission is highly beamed, and understanding the jet geometry and beaming configuration can provide information on the poorly understood central engine and circum-burst environment. Prior to the advent of gravitational-wave astronomy, astronomers relied on observations of jet breaks in the multi-wavelength afterglow to determine the GRB opening angle, since the observer's viewing angle relative to the system cannot be determined from the electromagnetic data alone. Gravitational-wave observations, however, provide an independent measurement of the viewing angle. We describe a Bayesian method for determining the geometry of short GRBs using coincident electromagnetic and gravitational-wave observations. We demonstrate how an ensemble of multi-messenger detections can be used to measure the distributions of the jet energy, opening angle, Lorentz factor, and angular profile of short GRBs; we find that for a population of 100 such observations, we can constrain the mean of the opening angle distribution to within 10 degrees regardless of the angular emission profile. Conversely, the constraint on the energy distribution depends on the shape of the profile, which can be distinguished.

astro-ph.HE↗