arXiv subjects

Neil J. Cornish

Publications and source records attributed to Neil J. Cornish.

At least 73 records · Page 4Linked to original sources

arXiv2022

Bayesian Characterisation of Circumbinary Exoplanets with LISA

The Laser Interferometer Space Antenna (LISA) will detect and characterize $\sim10^4$ Galactic Binaries consisting predominantly of two White Dwarfs (WD). An interesting prospect within this population is a third object--another WD star, a Circumbinary Exoplanet (CBP), or a Brown Dwarf (BD)--in orbit about the inner WD pair. We present the first fully Bayesian detection and posterior analysis of sub-stellar objects with LISA, focusing on the characterization of CBPs. We used an optimistic astrophysically motivated catalogue of these CBP third-body sources, including their orbital eccentricity around the inner binary for the first time. We examined Bayesian Evidence computations for detectability, as well as the effects on the posterior distributions for both the inner binary parameters and the third-body parameters. We find that the posterior behavior bifurcates based on whether the third-body period is above or below half the observation time. Additionally, we find that undetectable third-body sources can bias the inner binary parameters whether or not the correct template is used. We used the information retrieved from the study of the CBP population to make an initial conservative prediction for the number of detectable BD systems in the original catalogue. We end with commentary on the predicted qualitative effects on LISA global fitting and Galactic Binary population analysis. The procedure used in this work is generic and can be directly applied to other astrophysical effects expected within the Galactic Binary population.

astro-ph.EP↗

arXiv2022

Comparison of maximum likelihood mapping methods for gravitational-wave backgrounds

Detection of a stochastic background of gravitational waves is likely to occur in the next few years. Beyond searches for the isotropic component of SGWBs, there have been various mapping methods proposed to target anisotropic backgrounds. Some of these methods have been applied to data taken by the Laser Interferometer Gravitational-wave Observatories (LIGO) and Virgo. Specifically, these directional searches have focused on mapping the intensity of the signal on the sky via maximum likelihood solutions. We compare this intensity mapping approach to a previously proposed, but never employed, amplitude-phase mapping method to understand whether this latter approach may be employed in future searches. We build up our understanding of the differences between these two approaches by analysing simple toy models of time-stream data, and run mock-data mapping tests for the two methods. We find that the amplitude-phase method is only applicable to the case of a background which is phase-coherent on large scales or, at the very least, has an intrinsic coherence scale that is larger than that of the detector. Otherwise, the amplitude-phase mapping method leads to a loss of overall information, with respect to both phase and amplitude. Since we do not expect these phase-coherent properties to hold for any of the gravitational-wave background signals we hope to detect in the near future, we conclude that intensity mapping is the preferred method for such backgrounds.

gr-qc↗

arXiv2022

Searching For Gravitational Waves From Cosmological Phase Transitions With The NANOGrav 12.5-year dataset

We search for a first-order phase transition gravitational wave signal in 45 pulsars from the NANOGrav 12.5 year dataset. We find that the data can be modeled in terms of a strong first order phase transition taking place at temperatures below the electroweak scale. However, we do not observe any strong preference for a phase-transition interpretation of the signal over the standard astrophysical interpretation in terms of supermassive black holes mergers; but we expect to gain additional discriminating power with future datasets, improving the signal to noise ratio and extending the sensitivity window to lower frequencies. An interesting open question is how well gravitational wave observatories could separate such signals.

astro-ph.CO↗

arXiv2021

Low Latency Detection of Massive Black Hole Binaries

The next decade is expected to see the launch of one or more space based gravitational wave detectors: the European lead Laser Interferometer Space Antenna (LISA); and one or more Chinese mission concepts, Taiji and TianQin. One of the primary scientific targets for these missions are the mergers of black holes with masses between $10^3 M_\odot$ and $10^8 M_\odot$. These systems may produce detectable electromagnetic signatures in additional to gravitational waves due to the presence of gas in mini-disks around each black hole, and a circumbinary disk surrounding the system. The electromagnetic emission may occur before, during and after the merger. In order to have the best chance of capturing all phases of the emission it is imperative that the gravitational wave signals can be detected in low latency, and used to produce reliable estimates for the sky location and distance to help guide the search for counterparts. The low latency detection also provides a starting point for the ``global fit'' of the myriad signals that are simultaneously present in the data. Here a low latency analysis pipeline is presented that is capable of analyzing months of data in just a few hours using a laptop from the last decade. The problem of performing a global fit is avoided by whitening out the bright foreground produced by nearby galactic binaries. The performance of the pipeline is illustrated using simulated data from the LISA Data Challenge.

gr-qc↗

arXiv2021

Spectral separation of the stochastic gravitational-wave background for LISA in the context of a modulated Galactic foreground

Within its observational band the Laser Interferometer Space Antenna, LISA, will simultaneously observe orbital modulated waveforms from Galactic white dwarf binaries, a binary black hole produced gravitational-wave background, and potentially a cosmologically created stochastic gravitational-wave background (SGWB). The overwhelming majority of stars end their lives as white dwarfs, making them very numerous in the Milky Way. We simulate Galactic white dwarf binary gravitational-wave emission based on distributions from various mock catalogs and determine a complex waveform from the Galactic foreground with $3.5 \times 10^{7}$ binaries. We describe the effects from the Galactic binary distribution population across mass, position within the Galaxy, core type, and orbital frequency distribution. We generate the modulated Galactic white dwarf signal detected by \textit{LISA} due to its orbital motion, and present a data analysis strategy to address it. The Fisher Information and Markov Chain Monte Carlo methods give an estimation of the \textit{LISA} noise and the parameters for the different signal classes. We estimate the detectable limits for the future LISA observation of the SGWB in the spectral domain with the 3 \textit{LISA} channels $A$, $E$, and $T$. We simultaneously estimate the Galactic foreground, the astrophysical and cosmological backgrounds. Assuming the expected astrophysical background and a Galactic foreground, a cosmological background energy density of around $Ω_{GW,Cosmo} \approx 8 \times 10^{-13}$ could be detected by LISA. LISA will either detect a cosmologically produced SGWB, or set a limit that will have important consequences.

gr-qc↗

arXiv2021

Spectral separation of the stochastic gravitational-wave background for LISA: observing both cosmological and astrophysical backgrounds

With the goal of attempting to observe a stochastic gravitational wave background (SGWB) with LISA, the spectral separability of the cosmological and astrophysical backgrounds is important to estimate. We attempt to determine the level with which a cosmological background can be observed given the predicted astrophysical background level. We predict detectable limits for the future LISA measurement of the SGWB. Adaptive Markov chain Monte-Carlo methods are used to produce estimates with the simulated data from the LISA Data challenge (LDC). We also calculate the Cramer-Rao lower bound on the variance of the SGWB parameter uncertainties based on the inverse Fisher Information using the Whittle Likelihood. The estimation of the parameters is done with the 3 LISA channels $A$, $E$, and $T$. We simultaneously estimate the noise using a LISA noise model. Assuming the expected astrophysical background, a cosmological background energy density of around $Ω_{GW,Cosmo} \approx 1 \times 10^{-12}$ to $1 \times 10^{-13}$ can be detected by LISA.

gr-qc↗

arXiv2021

The NANOGrav 12.5-year data set: Search for Non-Einsteinian Polarization Modes in theGravitational-Wave Background

We search NANOGrav's 12.5-year data set for evidence of a gravitational wave background (GWB) with all the spatial correlations allowed by general metric theories of gravity. We find no substantial evidence in favor of the existence of such correlations in our data. We find that scalar-transverse (ST) correlations yield signal-to-noise ratios and Bayes factors that are higher than quadrupolar (tensor transverse, TT) correlations. Specifically, we find ST correlations with a signal-to-noise ratio of 2.8 that are preferred over TT correlations (Hellings and Downs correlations) with Bayesian odds of about 20:1. However, the significance of ST correlations is reduced dramatically when we include modeling of the Solar System ephemeris systematics and/or remove pulsar J0030$+$0451 entirely from consideration. Even taking the nominal signal-to-noise ratios at face value, analyses of simulated data sets show that such values are not extremely unlikely to be observed in cases where only the usual TT modes are present in the GWB. In the absence of a detection of any polarization mode of gravity, we place upper limits on their amplitudes for a spectral index of $γ= 5$ and a reference frequency of $f_\text{yr} = 1 \text{yr}^{-1}$. Among the upper limits for eight general families of metric theories of gravity, we find the values of $A^{95\%}_{TT} = (9.7 \pm 0.4)\times 10^{-16}$ and $A^{95\%}_{ST} = (1.4 \pm 0.03)\times 10^{-15}$ for the family of metric spacetime theories that contain both TT and ST modes.

gr-qc↗

arXiv2021

Heterodyned Likelihood for Rapid Gravitational Wave Parameter Inference

Inferring the source properties of a gravitational wave signal has traditionally been very computationally intensive and time consuming. In recent years, several techniques have been developed that can significantly reduce the computational cost while delivering rapid and accurate parameter inference. One of the most powerful of these techniques is the heterodyned likelihood, which uses a reference waveform to base-band the likelihood calculation. Here an efficient implementation of the heterodyned likelihood is presented that can be used for a wide range of signal types and for both ground based and space based interferometers. The computational savings relative to direct calculation of the likelihood vary between two and four orders of magnitude depending on the system. The savings are greatest for low mass systems such as neutron star binaries. The heterodyning procedure can incorporate marginalization over calibration uncertainties and the noise power spectrum.

gr-qc↗

arXiv2021

Spectral separation of the stochastic gravitational-wave background for LISA: galactic, cosmological and astrophysical backgrounds

In its observation band, the Laser Interferometer Space Antenna (LISA) will simultaneously observe stochastic gravitational-wave background (SGWB) signals of different origins; orbitally modulated waveforms from galactic white dwarf binaries, a binary black hole produced background, and possibly a cosmologically produced SGWB. We simulate the emission of gravitational waves from galactic white dwarf binaries based on the Lamberts \cite{Lamberts} distributions and determine a complex waveform from the galactic foreground. We generate the modulated galactic signal detected by LISA due to its orbital motion, and present a data analysis strategy to address it. The Fisher Information and Markov Chain Monte Carlo methods give an estimate of the LISA noise and parameters for the different signal sources. We simultaneously estimate the galactic foreground, the astrophysical and cosmological backgrounds, and estimate detection limits for the future LISA observation of the SGWB in the spectral domain with the 3 LISA channels $ A $, $ E $ and $ T $. In the context of the expected astrophysical background and a galactic foreground, a cosmological background energy density of about $ Ω_{GW,Cosmo} \approx 8 \times 10^{-13} $ could be detected by LISA with our spectral separation strategy.

gr-qc↗

arXiv2021

Massive Black Hole Binaries and Where to Find Them with Dual Detector Networks

A single space-based gravitational wave detector will push the boundaries of astronomy and fundamental physics. Having a network of two or more detectors would significantly improve source localization. Here we consider how dual networks of space-based detectors would improve parameter estimation of massive black hole binaries. We consider two scenarios: a network comprised of the Laser Interferometer Space Antenna (LISA) and an additional LISA-like heliocentric detector (e.g. Taiji); and a network comprised of LISA with an an additional geocentric detector (e.g. TianQin). We use Markov chain Monte Carlo techniques and Fisher matrix estimates to explore the impact of a two detector network on sky localization and distance determination. The impact on other source parameters is also studied. With the addition of a Taiji or TianQin, we find orders of magnitude improvements in sky localization for the more massive MBHBs, while also seeing improvements for lower mass systems, and for other source parameters.

gr-qc↗

arXiv2021

Rapid and Robust Parameter Inference for Binary Mergers

The detection rate for compact binary mergers has grown as the sensitivity of the global network of ground based gravitational wave detectors has improved, now reaching the stage where robust automation of the analyses is essential. Automated low-latency algorithms have been developed that send out alerts when candidate signals are detected. The alerts include sky maps to facilitate electromagnetic follow up observations, along with probabilities that the system might contain a neutron star, and hence be more likely to generate an electromagnetic counterpart. Data quality issues, such as loud noise transients (glitches), can adversely affect the low-latency algorithms, causing false alarms and throwing off parameter estimation. Here a new analysis method is presented that is robust against glitches, and capable of producing fully Bayesian parameter inference, including sky maps and mass estimates, in a matter of minutes. Key elements of the method are wavelet-based de-noising, penalized maximization of the likelihood during the initial search, rapid sky localization using pre-computed inner products, and heterodyned likelihoods for full Bayesian inference.

gr-qc↗

arXiv2021

Bayesian search for gravitational wave bursts in pulsar timing array data

The nanohertz frequency band explored by pulsar timing arrays provides a unique discovery space for gravitational wave signals. In addition to signals from anticipated sources, such as those from supermassive black hole binaries, some previously unimagined sources may emit transient gravitational waves (a.k.a. bursts) with unknown morphology. Unmodeled transients are not currently searched for in this frequency band, and they require different techniques from those currently employed. Possible sources of such gravitational wave bursts in the nanohertz regime are parabolic encounters of supermassive black holes, cosmic string cusps and kinks, or other, as-yet-unknown phenomena. In this paper we present BayesHopperBurst, a Bayesian search algorithm capable of identifying generic gravitational wave bursts by modeling both coherent and incoherent transients as a sum of Morlet-Gabor wavelets. A trans-dimensional Reversible Jump Markov Chain Monte Carlo sampler is used to select the number of wavelets best describing the data. We test BayesHopperBurst on various simulated datasets including different combinations of signals and noise transients. Its capability to run on real data is demonstrated by analyzing data of the pulsar B1855+09 from the NANOGrav 9-year dataset. Based on a simulated dataset resembling the NANOGrav 12.5-year data release, we predict that at our most sensitive time-frequency location we will be able to probe gravitational wave bursts with a root-sum-squared amplitude higher than $\sim 5 \times 10^{-11}$ Hz$^{-1/2}$, which corresponds to $\sim 40 M_{\odot} c^2$ emitted in GWs at a fiducial distance of 100 Mpc.

gr-qc↗

arXiv2021

Astrophysics Milestones For Pulsar Timing Array Gravitational Wave Detection

The NANOGrav Collaboration reported strong Bayesian evidence for a common-spectrum stochastic process in its 12.5-yr pulsar timing array dataset, with median characteristic strain amplitude at periods of a year of $A_{\rm yr} = 1.92^{+0.75}_{-0.55} \times 10^{-15}$. However, evidence for the quadrupolar Hellings \& Downs interpulsar correlations, which are characteristic of gravitational wave signals, was not yet significant. We emulate and extend the NANOGrav dataset, injecting a wide range of stochastic gravitational wave background (GWB) signals that encompass a variety of amplitudes and spectral shapes, and quantify three key milestones: (I) Given the amplitude measured in the 12.5 yr analysis and assuming this signal is a GWB, we expect to accumulate robust evidence of an interpulsar-correlated GWB signal with 15--17 yrs of data, i.e., an additional 2--5 yrs from the 12.5 yr dataset; (II) At the initial detection, we expect a fractional uncertainty of $40\%$ on the power-law strain spectrum slope, which is sufficient to distinguish a GWB of supermassive black-hole binary origin from some models predicting more exotic origins;(III) Similarly, the measured GWB amplitude will have an uncertainty of $44\%$ upon initial detection, allowing us to arbitrate between some population models of supermassive black-hole binaries. In addition, power-law models are distinguishable from those having low-frequency spectral turnovers once 20~yrs of data are reached. Even though our study is based on the NANOGrav data, we also derive relations that allow for a generalization to other pulsar-timing array datasets. Most notably, by combining the data of individual arrays into the International Pulsar Timing Array, all of these milestones can be reached significantly earlier.

astro-ph.HE↗

arXiv2021

The BayesWave analysis pipeline in the era of gravitational wave observations

We describe updates and improvements to the BayesWave gravitational wave transient analysis pipeline, and provide examples of how the algorithm is used to analyze data from ground-based gravitational wave detectors. BayesWave models gravitational wave signals in a morphology-independent manner through a sum of frame functions, such as Morlet-Gabor wavelets or chirplets. BayesWave models the instrument noise using a combination of a parametrized Gaussian noise component and non-stationary and non-Gaussian noise transients. Both the signal model and noise model employ trans-dimensional sampling, with the complexity of the model adapting to the requirements of the data. The flexibility of the algorithm makes it suitable for a variety of analyses, including reconstructing generic unmodeled signals; cross checks against modeled analyses for compact binaries; as well as separating coherent signals from incoherent instrumental noise transients (glitches). The BayesWave model has been extended to account for gravitational wave signals with generic polarization content and the simultaneous presence of signals and glitches in the data. We describe updates in the BayesWave prior distributions, sampling proposals, and burn-in stage that provide significantly improved sampling efficiency. We present standard review checks indicating the robustness and convergence of the BayesWave trans-dimensional sampler.

gr-qc↗

arXiv2021

The NANOGrav 12.5-year Data Set: Search For An Isotropic Stochastic Gravitational-Wave Background

We search for an isotropic stochastic gravitational-wave background (GWB) in the $12.5$-year pulsar timing data set collected by the North American Nanohertz Observatory for Gravitational Waves. Our analysis finds strong evidence of a stochastic process, modeled as a power-law, with common amplitude and spectral slope across pulsars. The Bayesian posterior of the amplitude for an $f^{-2/3}$ power-law spectrum, expressed as the characteristic GW strain, has median $1.92 \times 10^{-15}$ and $5\%$--$95\%$ quantiles of $1.37$--$2.67 \times 10^{-15}$ at a reference frequency of $f_\mathrm{yr} = 1 ~\mathrm{yr}^{-1}$. The Bayes factor in favor of the common-spectrum process versus independent red-noise processes in each pulsar exceeds $10,000$. However, we find no statistically significant evidence that this process has quadrupolar spatial correlations, which we would consider necessary to claim a GWB detection consistent with general relativity. We find that the process has neither monopolar nor dipolar correlations, which may arise from, for example, reference clock or solar system ephemeris systematics, respectively. The amplitude posterior has significant support above previously reported upper limits; we explain this in terms of the Bayesian priors assumed for intrinsic pulsar red noise. We examine potential implications for the supermassive black hole binary population under the hypothesis that the signal is indeed astrophysical in nature.

astro-ph.HE↗

arXiv2021

The NANOGrav 11yr Data Set: Limits on Supermassive Black Hole Binaries in Galaxies within 500Mpc

Supermassive black hole binaries (SMBHBs) should form frequently in galactic nuclei as a result of galaxy mergers. At sub-parsec separations, binaries become strong sources of low-frequency gravitational waves (GWs), targeted by Pulsar Timing Arrays (PTAs). We used recent upper limits on continuous GWs from the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) 11yr dataset to place constraints on putative SMBHBs in nearby massive galaxies. We compiled a comprehensive catalog of ~44,000 galaxies in the local universe (up to redshift ~0.05) and populated them with hypothetical binaries, assuming that the total mass of the binary is equal to the SMBH mass derived from global scaling relations. Assuming circular equal-mass binaries emitting at NANOGrav's most sensitive frequency of 8nHz, we found that 216 galaxies are within NANOGrav's sensitivity volume. We ranked the potential SMBHBs based on GW detectability by calculating the total signal-to-noise ratio (S/N) such binaries would induce within the NANOGrav array. We placed constraints on the chirp mass and mass ratio of the 216 hypothetical binaries. For 19 galaxies, only very unequal-mass binaries are allowed, with the mass of the secondary less than 10 percent that of the primary, roughly comparable to constraints on a SMBHB in the Milky Way. Additionally, we were able to exclude binaries delivered by major mergers (mass ratio of at least 1/4) for several of these galaxies. We also derived the first limit on the density of binaries delivered by major mergers purely based on GW data.

astro-ph.GA↗

arXiv2020

The NANOGrav 12.5 yr Data Set: Observations and Narrowband Timing of 47 Millisecond Pulsars

We present time-of-arrival (TOA) measurements and timing models of 47 millisecond pulsars (MSPs) observed from 2004 to 2017 at the Arecibo Observatory and the Green Bank Telescope by the North American Nanohertz Observatory for Gravitational Waves (NANOGrav). The observing cadence was three to four weeks for most pulsars over most of this time span, with weekly observations of six sources. These data were collected for use in low-frequency gravitational wave searches and for other astrophysical purposes. We detail our observational methods and present a set of TOA measurements, based on "narrowband" analysis, in which many TOAs are calculated within narrow radio-frequency bands for data collected simultaneously across a wide bandwidth. A separate set of "wideband" TOAs will be presented in a companion paper. We detail a number of methodological changes compared to our previous work which yield a cleaner and more uniformly processed data set. Our timing models include several new astrometric and binary pulsar measurements, including previously unpublished values for the parallaxes of PSRs J1832-0836 and J2322+2057, the secular derivatives of the projected semi-major orbital axes of PSRs J0613-0200 and J2229+2643, and the first detection of the Shapiro delay in PSR J2145-0750. We report detectable levels of red noise in the time series for 14 pulsars. As a check on timing model reliability, we investigate the stability of astrometric parameters across data sets of different lengths. We report flux density measurements for all pulsars observed. Searches for stochastic and continuous gravitational waves using these data will be subjects of forthcoming publications.

astro-ph.HE↗

arXiv2020

The NANOGrav 12.5-year Data Set: Wideband Timing of 47 Millisecond Pulsars

We present a new analysis of the profile data from the 47 millisecond pulsars comprising the 12.5-year data set of the North American Nanohertz Observatory for Gravitational Waves (NANOGrav), which is presented in a parallel paper (Alam et al. 2021a; NG12.5). Our reprocessing is performed using "wideband" timing methods, which use frequency-dependent template profiles, simultaneous time-of-arrival (TOA) and dispersion measure (DM) measurements from broadband observations, and novel analysis techniques. In particular, the wideband DM measurements are used to constrain the DM portion of the timing model. We compare the ensemble timing results to NG12.5 by examining the timing residuals, timing models, and noise model components. There is a remarkable level of agreement across all metrics considered. Our best-timed pulsars produce encouragingly similar results to those from NG12.5. In certain cases, such as high-DM pulsars with profile broadening, or sources that are weak and scintillating, wideband timing techniques prove to be beneficial, leading to more precise timing model parameters by 10-15%. The high-precision, multi-band measurements of several pulsars indicate frequency-dependent DMs. Compared to the narrowband analysis in NG12.5, the TOA volume is reduced by a factor of 33, which may ultimately facilitate computational speed-ups for complex pulsar timing array analyses. This first wideband pulsar timing data set is a stepping stone, and its consistent results with NG12.5 assure us that such data sets are appropriate for gravitational wave analyses.

astro-ph.HE↗