22 results on '"Ashton, Gregory"'
Search Results
2. Timing variations in neutron stars : models, inference and their implications for gravitational waves
- Author
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Ashton, Gregory, Jones, David, and Andersson, Nils
- Subjects
523.8 - Abstract
Timing variations in pulsars, low frequency ubiquitous structure known as timing noise and sudden increases in the rotational frequency which we call glitches, provide a means to study neutron stars. Since the first observations, many models have been proposed, yet no definitive explanation has arisen. In this thesis, we aim to improve this situation by developing models of timing noise. We focus chiefly on precession models which explain periodic modulation seen in radio pulsar data. Developing models and testing them provides an opportunity to infer the elemental properties of neutron stars: evidence for long period precession has implications for the superfluid component predicted by models used to explain glitches. However, often more than one model can qualitatively explain the data, therefore we need a method to decide which model best fits the data. This is precisely the case for PSR B1828-11 which has been used as evidence for both precession and so-called magnetospheric switching. We address this confusion by applying the tools of probability theory to develop a Bayesian model comparison and find that the evidence is in favour of precession. In the second part of this thesis, we will discuss the implications of timing variations for the detection of continuous gravitational waves from neutron stars. To search for these signals, matched filtering methods are used which require a template, a guess for what the signal ‘looks like’. Timing variations, as seen in the electromagnetic signal, may also exist in the gravitational wave signal. If detected, these could provide an invaluable source of information about neutron stars. However, if not included in the template, they may mean that the gravitational wave signal is not detected in the first place. We investigate this issue for both timing noise and glitches, using electromagnetic observations to predict for what types of gravitational wave searches this may be an issue. We find that while timing noise is unlikely to be an issue for current gravitational wave searches, glitches may cause a significant problem in all-sky searches for gravitational waves from neutron stars.
- Published
- 2016
3. Cosmology with Gravitational Waves: A Review.
- Author
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Mastrogiovanni, Simone, Karathanasis, Christos, Gair, Jonathan, Ashton, Gregory, Rinaldi, Stefano, Huang, Hsiang‐Yu, and Dálya, Gergely
- Subjects
GRAVITATIONAL waves ,PHYSICAL cosmology ,BINARY black holes ,COMPACT objects (Astronomy) ,BINARY stars ,NEUTRON stars ,STELLAR mergers - Abstract
Standard sirens have been the central paradigm in gravitational‐wave cosmology so far. From the gravitational wave signature of compact star binaries, it is possible to measure the luminosity distance of the source directly, and if additional information on the source redshift is provided, a measurement of the cosmological expansion can be performed. This review article discusses several methodologies that have been proposed to use gravitational waves for cosmological studies. Methods that use only gravitational‐wave signals and methods that use gravitational waves in conjunction with additional observations such as electromagnetic counterparts and galaxy catalogs will be discussed. The review also discusses the most recent results on gravitational‐wave cosmology, starting from the binary neutron star merger GW170817 and its electromagnetic counterpart and finishing with the population of binary black holes, observed with the third Gravitational‐wave Transient Catalog GWTC–3. [ABSTRACT FROM AUTHOR]
- Published
- 2024
- Full Text
- View/download PDF
4. Rotational evolution of the Vela pulsar during the 2016 glitch
- Author
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Ashton, Gregory, Lasky, Paul D., Graber, Vanessa, and Palfreyman, Jim
- Published
- 2019
- Full Text
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5. Multimessenger parameter inference of gravitational-wave and electromagnetic observations of white dwarf binaries.
- Author
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Johnson, Peyton T, Coughlin, Michael W, Hamilton, Ashlie, Bustamante-Rosell, María José, Ashton, Gregory, Corey, Samuel, Kupfer, Thomas, Littenberg, Tyson B, Reed, Draco, and Zimmerman, Aaron
- Subjects
WHITE dwarf stars ,LASER interferometers ,ELECTROMAGNETIC waves ,BAYESIAN field theory ,PARAMETER estimation ,GRAVITATIONAL waves ,ANTENNAS (Electronics) - Abstract
The upcoming Laser Interferometer Space Antenna (LISA) will detect a large gravitational-wave foreground of Galactic white dwarf binaries. These sources are exceptional for their probable detection at electromagnetic wavelengths, some long before LISA flies. Studies in both gravitational and electromagnetic waves will yield strong constraints on system parameters not achievable through measurements of one messenger alone. In this work, we present a Bayesian inference pipeline and simulation suite in which we study potential constraints on binaries in a variety of configurations. We show how using LISA detections and parameter estimation can significantly improve constraints on system parameters when used as a prior for the electromagnetic analyses. We also provide rules of thumb for how current measurements will benefit from LISA measurements in the future. [ABSTRACT FROM AUTHOR]
- Published
- 2023
- Full Text
- View/download PDF
6. Improving pulsar-timing solutions through dynamic pulse fitting.
- Author
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Nathan, Rowina S, Miles, Matthew T, Ashton, Gregory, Lasky, Paul D, Thrane, Eric, Reardon, Daniel J, Shannon, Ryan M, and Cameron, Andrew D
- Subjects
NEUTRON stars ,WHITE noise ,PULSARS ,SIGNAL-to-noise ratio ,GRAVITATIONAL waves ,TIME management ,NOISE control - Abstract
Precision pulsar timing is integral to the detection of the nanohertz stochastic gravitational-wave background as well as understanding the physics of neutron stars. Conventional pulsar timing often uses fixed time and frequency-averaged templates to determine the pulse times of arrival, which can lead to reduced accuracy when the pulse profile evolves over time. We illustrate a dynamic timing method that fits each observing epoch using basis functions. By fitting each epoch separately, we allow for the evolution of the pulse shape epoch to epoch. We apply our method to PSR J1103−5403 and find evidence that it undergoes mode changing, making it the fourth millisecond pulsar to exhibit such behaviour. Our method, which is able to identify and time a single mode, yields a timing solution with a root-mean-square error of |$1.343$| µs, a factor of 1.78 improvement over template fitting on both modes. In addition, the white-noise amplitude is reduced 4.3 times, suggesting that fitting the full data set causes the mode changing to be incorrectly classified as white noise. This reduction in white noise boosts the signal-to-noise ratio of a gravitational-wave background signal for this particular pulsar by 32 per cent. We discuss the possible applications for this method of timing to study pulsar magnetospheres and further improve the sensitivity of searches for nanohertz gravitational waves. [ABSTRACT FROM AUTHOR]
- Published
- 2023
- Full Text
- View/download PDF
7. Inferring the Astrophysical Population of Gravitational Wave Sources in the Presence of Noise Transients
- Author
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Heinzel, Jack, Talbot, Colm, Ashton, Gregory, and Vitale, Salvatore
- Subjects
High Energy Astrophysical Phenomena (astro-ph.HE) ,FOS: Physical sciences ,General Relativity and Quantum Cosmology (gr-qc) ,Astrophysics - High Energy Astrophysical Phenomena ,General Relativity and Quantum Cosmology - Abstract
The global network of interferometric gravitational wave (GW) observatories (LIGO, Virgo, KAGRA) has detected and characterized nearly 100 mergers of binary compact objects. However, many more real GWs are lurking sub-threshold, which need to be sifted from terrestrial-origin noise triggers (known as glitches). Because glitches are not due to astrophysical phenomena, inference on the glitch under the assumption it has an astrophysical source (e.g. binary black hole coalescence) results in source parameters that are inconsistent with what is known about the astrophysical population. In this work, we show how one can extract unbiased population constraints from a catalog of both real GW events and glitch contaminants by performing Bayesian inference on their source populations simultaneously. In this paper, we assume glitches come from a specific class with a well-characterized effective population (blip glitches). We also calculate posteriors on the probability of each event in the catalog belonging to the astrophysical or glitch class, and obtain posteriors on the number of astrophysical events in the catalog, finding it to be consistent with the actual number of events included., 13 pages, 10 figures
- Published
- 2023
8. Gaussian processes for glitch-robust gravitational-wave astronomy.
- Author
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Ashton, Gregory
- Subjects
- *
GAUSSIAN processes , *BINARY black holes , *GRAVITATIONAL waves , *ASTRONOMY , *STELLAR evolution , *GENERAL relativity (Physics) , *BLACK holes - Abstract
Interferometric gravitational-wave observatories have opened a new era in astronomy. The rich data produced by an international network enable detailed analysis of the curved space-time around black holes. With nearly 100 signals observed so far and thousands expected in the next decade, their population properties enable insights into stellar evolution and the expansion of our Universe. However, the detectors are afflicted by transient noise artefacts known as 'glitches' which contaminate the signals and bias inferences. Of the 90 signals detected to date, 18 were contaminated by glitches. This feasibility study explores a new approach to transient gravitational-wave data analysis using Gaussian processes, which model the underlying physics of the glitch-generating mechanism rather than the explicit realization of the glitch itself. We demonstrate that if the Gaussian process kernel function can adequately model the glitch morphology, we can recover the parameters of simulated signals. Moreover, we find that the Gaussian processes kernels used in this work are well suited to modelling long-duration glitches which are most challenging for existing glitch-mitigation approaches. Finally, we show how the time-domain nature of our approach enables a new class of time-domain tests of General Relativity, performing a re-analysis of the inspiral-merger-ringdown test on the first observed binary black hole merger. Our investigation demonstrates the feasibility of the Gaussian processes as an alternative to the traditional framework but does not yet establish them as a replacement. Therefore, we conclude with an outlook on the steps needed to realize the full potential of the Gaussian process approach. [ABSTRACT FROM AUTHOR]
- Published
- 2023
- Full Text
- View/download PDF
9. follow-up on intermediate-mass black hole candidates in the second LIGO–Virgo observing run with the Bayes Coherence Ratio.
- Author
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Vajpeyi, Avi, Smith, Rory, Thrane, Eric, Ashton, Gregory, Alford, Thomas, Garza, Sierra, Isi, Maximiliano, Kanner, Jonah, Massinger, T J, and Xiao, Liting
- Subjects
BLACK holes ,BINARY black holes ,SUPERMASSIVE black holes ,GLOBULAR clusters ,ACTIVE galactic nuclei ,GRAVITATIONAL waves - Abstract
The detection of an intermediate-mass black hole population (10
2 –106 M⊙ ) will provide clues to their formation environments (e.g. discs of active galactic nuclei, globular clusters) and illuminate a potential pathway to produce supermassive black holes. Ground-based gravitational-wave detectors are sensitive to mergers that can form intermediate-mass black holes weighing up to ∼450 M⊙ . However, ground-based detector data contain numerous incoherent short duration noise transients that can mimic the gravitational-wave signals from merging intermediate-mass black holes, limiting the sensitivity of searches. Here, we follow-up on binary black hole merger candidates using a ranking statistic that measures the coherence or incoherence of triggers in multiple-detector data. We use this statistic to rank candidate events, initially identified by all-sky search pipelines, with lab-frame total masses ≳ 55 M⊙ using data from LIGO's second observing run. Our analysis does not yield evidence for new intermediate-mass black holes. However, we find support for eight stellar-mass binary black holes not reported in the first LIGO–Virgo gravitational wave transient catalogue GWTC-1, seven of which have been previously reported by other catalogues. [ABSTRACT FROM AUTHOR]- Published
- 2022
- Full Text
- View/download PDF
10. CDF-S XT1: The off-axis afterglow of a neutron star merger at $z=2.23$
- Author
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Sarin, Nikhil, Ashton, Gregory, Lasky, Paul D., Ackley, Kendall, Mong, Yik-Lun, and Galloway, Duncan K.
- Subjects
High Energy Astrophysical Phenomena (astro-ph.HE) ,Astrophysics::High Energy Astrophysical Phenomena ,FOS: Physical sciences ,High Energy Physics::Experiment ,Astrophysics::Cosmology and Extragalactic Astrophysics ,Astrophysics - High Energy Astrophysical Phenomena ,Astrophysics::Galaxy Astrophysics - Abstract
CDF-S XT1 is a fast-rising non-thermal X-ray transient detected by \textit{Chandra} in the Deep-Field South Survey. Although various hypotheses have been suggested, the origin of this transient remains unclear. Here, we show that the observations of CDF-S XT1 are well explained as the X-ray afterglow produced by a relativistic structured jet viewed off-axis. We measure properties of the jet, showing that they are similar to those of GRB170817A, albeit at cosmological distances. We measure the observers viewing angle to be $\theta_{\textrm{obs}} = 10^{\circ}\pm3^{\circ}$ and the core of the ultra-relativistic jet to be $\theta_{\textrm{core}} = 4.4^{\circ}\pm0.9^{\circ}$, where the uncertainties are the $68\%$ credible interval. The inferred properties and host galaxy combined with Hubble, radio, and optical non detections favour the hypothesis that CDF-S XT1 is the off-axis afterglow of a binary neutron star merger. We find that other previously suggested hypotheses are unable to explain all properties of CDF-S XT1. At a redshift of $z=2.23$, this is potentially the most distant observed neutron star merger to date and the first orphan afterglow of a short gamma-ray burst. We discuss the implications of a binary neutron star merger at such a high redshift for the star-formation rate in the early Universe, the nucleosynthesis of heavy elements, and the prospect of identifying other off-axis afterglows., Comment: Submitted to ApJL, 8 Pages, 3 figures
- Published
- 2021
11. Parameterised population models of transient non-Gaussian noise in the LIGO gravitational-wave detectors.
- Author
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Ashton, Gregory, Thiele, Sarah, Lecoeuche, Yannick, McIver, Jess, and Nuttall, Laura K
- Subjects
- *
GRAVITATIONAL waves , *NOISE , *DETECTORS - Abstract
The two interferometric LIGO gravitational-wave observatories provide the most sensitive data to date to study the gravitational-wave universe. As part of a global network, they have completed their third observing run in which they observed many tens of signals from merging compact binary systems. It has long been known that a limiting factor in identifying transient gravitational-wave signals is the presence of transient non-Gaussian noise, which reduce the ability of astrophysical searches to detect signals confidently. Significant efforts are taken to identify and mitigate this noise at the source, but its presence persists, leading to the need for software solutions. Taking a set of transient noise artefacts categorised by the GravitySpy software during the O3a observing era, we produce parameterised population models of the noise projected into the space of astrophysical model parameters of merging binary systems. We compare the inferred population properties of transient noise artefacts with observed astrophysical systems from the GWTC2.1 catalogue. We find that while the population of astrophysical systems tend to have near equal masses and moderate spins, transient noise artefacts are typically characterised by extreme mass ratios and large spins. This work provides a new method to calculate the consistency of an observed candidate with a given class of noise artefacts. This approach could be used in assessing the consistency of candidates found by astrophysical searches (i.e. determining if they are consistent with a known glitch class). Furthermore, the approach could be incorporated into astrophysical searches directly, potentially improving the reach of the detectors, though only a detailed study would verify this. [ABSTRACT FROM AUTHOR]
- Published
- 2022
- Full Text
- View/download PDF
12. Faster search for long gravitational-wave transients: GPU implementation of the transient F-statistic
- Author
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Keitel, David and Ashton, Gregory
- Subjects
GPUs ,Physics and Astronomy (miscellaneous) ,glitches ,Astrophysics::High Energy Astrophysical Phenomena ,FOS: Physical sciences ,General Relativity and Quantum Cosmology (gr-qc) ,General Relativity and Quantum Cosmology ,neutron stars ,gravitational waves ,Astrophysics - Instrumentation and Methods for Astrophysics ,Instrumentation and Methods for Astrophysics (astro-ph.IM) ,QC ,QB - Abstract
The F-statistic is an established method to search for continuous gravitational waves from spinning neutron stars. Prix et al. (2011) introduced a variant for transient quasi-monochromatic signals. Possible astrophysical scenarios for such transients include glitching pulsars, newborn neutron stars and accreting systems. Here we present a new implementation of the transient F-statistic, using pyCUDA to leverage the power of modern graphics processing units (GPUs). The obtained speedup allows efficient searches over much wider parameter spaces, especially when using more realistic transient signal models including time-varying (e.g. exponentially decaying) amplitudes. Hence, it can enable comprehensive coverage of glitches in known nearby pulsars, improve the follow-up of outliers from continuous-wave searches, and might be an important ingredient for future blind all-sky searches for unknown neutron stars., 13 pages, 3 figures; v2: updated reference to 1710.02327 and its erratum
- Published
- 2018
13. Standard-siren Cosmology Using Gravitational Waves from Binary Black Holes.
- Author
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You, Zhi-Qiang, Zhu, Xing-Jiang, Ashton, Gregory, Thrane, Eric, and Zhu, Zong-Hong
- Subjects
PHYSICAL cosmology ,GRAVITATIONAL waves ,BINARY black holes ,HUBBLE constant ,BLACK holes ,ASTRONOMY - Abstract
Gravitational-wave astronomy provides a unique new way to study the expansion history of the universe. In this work, we investigate the impact future gravitational-wave observatories will have on cosmology. Third-generation observatories like the Einstein Telescope and Cosmic Explorer will be sensitive to essentially all of the binary black hole coalescence events in the universe. Recent work by Farr et al. points out that features in the stellar-mass black hole population break the mass–redshift degeneracy, facilitating precise determination of the Hubble parameter without electromagnetic counterparts or host galaxy catalogs. Using a hierarchical Bayesian inference model, we show that with one year of observations by the Einstein Telescope, the Hubble constant will be measured to ≲1%. We also show that this method can be used to perform Bayesian model selection between cosmological models. As an illustrative example, we find that a decisive statement can be made comparing the ΛCDM and RHCT cosmological models using two weeks of data from the Einstein Telescope. [ABSTRACT FROM AUTHOR]
- Published
- 2021
- Full Text
- View/download PDF
14. Interpreting the X-ray afterglows of gamma-ray bursts with radiative losses and millisecond magnetars.
- Author
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Sarin, Nikhil, Lasky, Paul D, and Ashton, Gregory
- Subjects
GAMMA ray bursts ,MAGNETARS ,X-rays - Abstract
The spin-down energy of millisecond magnetars has been invoked to explain X-ray afterglow observations of a significant fraction of short and long gamma-ray bursts. Here, we extend models previously introduced in the literature, incorporating radiative losses with the spin-down of a magnetar central engine through an arbitrary braking index. Combining this with a model for the tail of the prompt emission, we show that our model can better explain the data than millisecond-magnetar models without radiative losses or those that invoke spin-down solely through vacuum dipole radiation. We find that our model predicts a subset of X-ray flares seen in some gamma-ray bursts. We can further explain the diversity of X-ray plateaus by altering the radiative efficiency and measure the braking index of newly born millisecond magnetars. We measure the braking index of GRB061121 as |$n=4.85^{+0.11}_{-0.15}$| suggesting the millisecond-magnetar born in this gamma-ray burst spins down predominantly through gravitational-wave emission. [ABSTRACT FROM AUTHOR]
- Published
- 2020
- Full Text
- View/download PDF
15. Massively parallel Bayesian inference for transient gravitational-wave astronomy.
- Author
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Smith, Rory J E, Ashton, Gregory, Vajpeyi, Avi, and Talbot, Colm
- Subjects
- *
GRAVITATIONAL waves , *ASTRONOMY , *COMPUTER workstation clusters , *PARALLEL processing , *SCIENTIFIC community - Abstract
Understanding the properties of transient gravitational waves (GWs) and their sources is of broad interest in physics and astronomy. Bayesian inference is the standard framework for astrophysical measurement in transient GW astronomy. Usually, stochastic sampling algorithms are used to estimate posterior probability distributions over the parameter spaces of models describing experimental data. The most physically accurate models typically come with a large computational overhead which can render data analsis extremely time consuming, or possibly even prohibitive. In some cases highly specialized optimizations can mitigate these issues, though they can be difficult to implement, as well as to generalize to arbitrary models of the data. Here, we investigate an accurate, flexible, and scalable method for astrophysical inference: parallelized nested sampling. The reduction in the wall-time of inference scales almost linearly with the number of parallel processes running on a high-performance computing cluster. By utilizing a pool of several hundreds or thousands of CPUs in a high-performance cluster, the large wall times of many astrophysical inferences can be alleviated while simultaneously ensuring that any GW signal model can be used 'out of the box', i.e. without additional optimization or approximation. Our method will be useful to both the LIGO-Virgo-KAGRA collaborations and the wider scientific community performing astrophysical analyses on GWs. An implementation is available in the open source gravitational-wave inference library pBilby (parallel bilby). [ABSTRACT FROM AUTHOR]
- Published
- 2020
- Full Text
- View/download PDF
16. The astrophysical odds of GW151216.
- Author
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Ashton, Gregory and Thrane, Eric
- Subjects
- *
BINARY black holes , *SUPERGIANT stars , *BLACK holes , *STELLAR evolution - Abstract
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 astro, the probability that the event is astrophysical in origin. Previous estimates of p 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 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 that uses the signal coherence between detectors and a parametrized model of non-astrophysical detector noise, we find that p 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 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. [ABSTRACT FROM AUTHOR]
- Published
- 2020
- Full Text
- View/download PDF
17. Characterizing Astrophysical Binary Neutron Stars with Gravitational Waves.
- Author
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Zhu, Xing-Jiang and Ashton, Gregory
- Published
- 2020
- Full Text
- View/download PDF
18. Comments on: 'Echoes from the abyss: Evidence for Planck-scale structure at black hole horizons'
- Author
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Ashton, Gregory, Birnholtz, Ofek, Cabero, Miriam, Collin Capano, Dent, Thomas, Krishnan, Badri, Meadors, Grant David, Nielsen, Alex B., Nitz, Alex, and Westerweck, Julian
- Subjects
General Relativity and Quantum Cosmology ,Astrophysics::High Energy Astrophysical Phenomena ,FOS: Physical sciences ,General Relativity and Quantum Cosmology (gr-qc) ,Astrophysics - Instrumentation and Methods for Astrophysics ,Instrumentation and Methods for Astrophysics (astro-ph.IM) - Abstract
Recently, Abedi, Dykaar and Afshordi claimed evidence for a repeating damped echo signal following the binary black hole merger gravitational-wave events recorded in the first observational period of the Advanced LIGO interferometers. We discuss the methods of data analysis and significance estimation leading to this claim, and identify several important shortcomings. We conclude that their analysis does not provide significant observational evidence for the existence of Planck-scale structure at black hole horizons, and suggest renewed analysis correcting for these shortcomings.
- Published
- 2016
19. Multiwaveform inference of gravitational waves.
- Author
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Ashton, Gregory and Khan, Sebastian
- Subjects
- *
GRAVITATIONAL waves , *INDEPENDENT sets , *UNCERTAINTY - Abstract
Bayesian inference of gravitational wave signals is subject to systematic error due to modeling uncertainty in waveform signal models coined approximants. A growing collection of approximants are available which use different approaches and make different assumptions to ease the process of model development. We provide a method to marginalize over the uncertainty in a set of waveform approximants by constructing a mixture-model multiwaveform likelihood. This method fits into existing workflows by determining the mixture parameters from the per-waveform evidence, enabling the production of marginalized combined sample sets from independent runs. [ABSTRACT FROM AUTHOR]
- Published
- 2020
- Full Text
- View/download PDF
20. Gravitational wave detection without boot straps: A Bayesian approach.
- Author
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Ashton, Gregory, Thrane, Eric, and Smith, Rory J. E.
- Subjects
- *
GRAVITATIONAL waves , *TIME perception , *FALSE alarms , *STRAPS , *BOOTS , *NOISE - Abstract
In order to separate astrophysical gravitational-wave signals from instrumental noise, which often contains transient non-Gaussian artifacts, astronomers have traditionally relied on bootstrap methods such as time slides. Bootstrap methods sample with replacement, comparing single-observatory data to construct a background distribution, which is used to assign a false-alarm probability to candidate signals. While bootstrap methods have played an important role establishing the first gravitational-wave detections, there are limitations. First, as the number of detections increases, it makes increasingly less sense to treat single-observatory data as bootstrap-estimated noise, when we know that the data are filled with astrophysical signals, some resolved, some unresolved. Second, it has been known for a decade that background estimation from time slides eventually breaks down due to saturation effects, yielding incorrect estimates of significance. Third, the false alarm probability cannot be used to weight candidate significance, for example when performing population inference on a set of candidates. Given recent debate about marginally resolved gravitational-wave detection claims, the question of significance has practical consequences. We propose a Bayesian framework for calculating the odds that a signal is of astrophysical origin versus instrumental noise without bootstrap noise estimation. We show how the astrophysical odds can safely accommodate glitches. We argue that it is statistically optimal. We demonstrate the method with simulated noise and provide examples to build intuition about this new approach to significance. [ABSTRACT FROM AUTHOR]
- Published
- 2019
- Full Text
- View/download PDF
21. Proposal of a micromagnetic standard problem for ferromagnetic resonance simulations.
- Author
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Baker, Alexander, Beg, Marijan, Ashton, Gregory, Albert, Maximilian, Chernyshenko, Dmitri, Wang, Weiwei, Zhang, Shilei, Bisotti, Marc-Antonio, Franchin, Matteo, Hu, Chun Lian, Stamps, Robert, Hesjedal, Thorsten, and Fangohr, Hans
- Subjects
- *
MICROMAGNETICS , *FERROMAGNETIC resonance , *MAGNETIZATION , *EXCITATION spectrum , *FOURIER transforms , *MOLECULAR dynamics - Abstract
Nowadays, micromagnetic simulations are a common tool for studying a wide range of different magnetic phenomena, including the ferromagnetic resonance. A technique for evaluating reliability and validity of different micromagnetic simulation tools is the simulation of proposed standard problems. We propose a new standard problem by providing a detailed specification and analysis of a sufficiently simple problem. By analyzing the magnetization dynamics in a thin permalloy square sample, triggered by a well defined excitation, we obtain the ferromagnetic resonance spectrum and identify the resonance modes via Fourier transform. Simulations are performed using both finite difference and finite element numerical methods, with OOMMF and Nmag simulators, respectively. We report the effects of initial conditions and simulation parameters on the character of the observed resonance modes for this standard problem. We provide detailed instructions and code to assist in using the results for evaluation of new simulator tools, and to help with numerical calculation of ferromagnetic resonance spectra and modes in general. [ABSTRACT FROM AUTHOR]
- Published
- 2017
- Full Text
- View/download PDF
22. Gravitational waves or deconfined quarks: What causes the premature collapse of neutron stars born in short gamma-ray bursts?
- Author
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Sarin, Nikhil, Lasky, Paul D., and Ashton, Gregory
- Subjects
- *
NEUTRON stars , *GRAVITATIONAL collapse , *GRAVITATIONAL waves , *BINARY stars , *QUARKS , *BLACK holes , *GAMMA ray bursts - Abstract
We infer the collapse times of long-lived neutron stars into black holes using the x-ray afterglows of 18 short gamma-ray bursts. We then apply hierarchical inference to infer properties of the neutron star equation of state and dominant spin-down mechanism. We measure the maximum non-rotating neutron star mass MTOV=2.31-0.21+0.36M⊙ and constrain the fraction of remnants spinning down predominantly through gravitational-wave emission to η=0.69-0.39+0.21 with 68% uncertainties. In principle, this method can determine the difference between hadronic and quark equation of states. In practice, however, the data is not yet informative with indications that these neutron stars do not have hadronic equation of states at the 1σ level. These inferences all depend on the underlying progenitor mass distribution for short gamma-ray bursts produced by binary neutron star mergers. The recently announced gravitational-wave detection of GW190425 suggests this underlying distribution is different from the locally measured population of double neutron stars. We show that MTOV and η constraints depend on the fraction of binary mergers that form through a distribution consistent with the locally measured population and a distribution that can explain GW190425. The more binaries that form from the latter distribution, the larger MTOV needs to be to satisfy the x-ray observations. Our measurements above are marginalized over this unknown fraction. If instead, we assume GW190425 is not a binary neutron star merger, i.e., the underlying mass distribution of double neutron stars is the same as observed locally, we measure MTOV=2.26-0.17+0.31M⊙. [ABSTRACT FROM AUTHOR]
- Published
- 2020
- Full Text
- View/download PDF
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