Publications

This is intended to be a reasonably complete list of my publications, though it may not always be completely up-to-date.

Other places to look are on my orcid profile and on NASA ADS.


GWTC-2.1: Deep Extended Catalog of Compact Binary Coalescences Observed by LIGO and Virgo During the First Half of the Third Observing Run

 
The second gravitational-wave transient catalog, GWTC-2, reported on 39 compact binary coalescences observed by the Advanced LIGO and Advanced Virgo detectors between 1 April 2019 15:00 UTC and 1 October 2019 15:00 UTC. Here, we present GWTC-2.1, which reports on a deeper list of candidate events observed over the same period. We analyze the final version of the strain data over this period, which is now publicly released. We employ three matched-filter search pipelines for candidate identification, and estimate the probability of astrophysical origin for each candidate event. While GWTC-2 used a false alarm rate threshold of 2 per year, we include in GWTC-2.1, 1201 candidates that pass a false alarm rate threshold of 2 per day. We calculate the source properties of a subset of 44 high-significance candidates that have a probability of astrophysical origin greater than 0.5, using the default priors. Of these candidates, 36 have been reported in GWTC-2. If the 8 additional high-significance candidates presented here are astrophysical, the mass range of candidate events that are unambiguously identified as binary black holes (both objects ≥3M⊙) is increased compared to GWTC-2, with total masses from ∼14M⊙ for GW190924_021846 to ∼184M⊙ for GW190426_190642. The primary components of two new candidate events (GW190403_051519 and GW190426_190642) fall in the mass gap predicted by pair-instability supernova theory. We also expand the population of binaries with significantly asymmetric mass ratios reported in GWTC-2 by an additional two events (q<0.61 and q<0.62 at 90% credibility for GW190403_051519 and GW190917_114630 respectively), and find that 2 of the 8 new events have effective inspiral spins χeff>0 (at 90% credibility), while no binary is consistent with χeff<0 at the same significance.
      @article{ 2021arXiv210801045T,
      
      title = { GWTC-2.1: Deep Extended Catalog of Compact Binary Coalescences Observed by LIGO and Virgo During the First Half of the Third Observing Run },
      year = { 2022 },
      }
    

GWTC-3: Compact Binary Coalescences Observed by LIGO and Virgo During the Second Part of the Third Observing Run

LIGO Scientific Collaboration, Virgo Collaboration, KAGRA Collaboration

The third major LIGO publication to make major use of the asimov framework for coordinating the analysis jobs. As with GWTC-2.1 I lead the coordination effort for the automation of the PE for all events presented in the paper.

| arxiv:2111.03606 |

 
The third Gravitational-wave Transient Catalog (GWTC-3) describes signals detected with Advanced LIGO and Advanced Virgo up to the end of their third observing run. Updating the previous GWTC-2.1, we present candidate gravitational waves from compact binary coalescences during the second half of the third observing run (O3b) between 1 November 2019, 15:00 UTC and 27 March 2020, 17:00 UTC. There are 35 compact binary coalescence candidates identified by at least one of our search algorithms with a probability of astrophysical origin pastro>0.5. Of these, 18 were previously reported as low-latency public alerts, and 17 are reported here for the first time. Based upon estimates for the component masses, our O3b candidates with pastro>0.5 are consistent with gravitational-wave signals from binary black holes or neutron star-black hole binaries, and we identify none from binary neutron stars. However, from the gravitational-wave data alone, we are not able to measure matter effects that distinguish whether the binary components are neutron stars or black holes. The range of inferred component masses is similar to that found with previous catalogs, but the O3b candidates include the first confident observations of neutron star-black hole binaries. Including the 35 candidates from O3b in addition to those from GWTC-2.1, GWTC-3 contains 90 candidates found by our analysis with pastro>0.5 across the first three observing runs. These observations of compact binary coalescences present an unprecedented view of the properties of black holes and neutron stars.
      @article{ 2021arXiv211103606T,
      author =  LIGO Scientific Collaboration and   Virgo Collaboration and   KAGRA Collaboration,
      title = { GWTC-3: Compact Binary Coalescences Observed by LIGO and Virgo During the Second Part of the Third Observing Run },
      year = { 2021 },
      }
    

GWTC-2: Compact Binary Coalescences Observed by LIGO and Virgo During the First Half of the Third Observing Run

LIGO Scientific Collaboration, Virgo Collaboration

Physical Review X, 11

| arxiv:2010.14527 | doi:10.1103/PhysRevX.11.021053 |

 
We report on gravitational-wave discoveries from compact binary coalescences detected by Advanced LIGO and Advanced Virgo in the first half of the third observing run (O3a) between 1 April 2019 15∶00 UTC and 1 October 2019 15∶00 UTC. By imposing a false-alarm-rate threshold of two per year in each of the four search pipelines that constitute our search, we present 39 candidate gravitational-wave events. At this threshold, we expect a contamination fraction of less than 10%. Of these, 26 candidate events were reported previously in near-real time through gamma-ray coordinates network notices and circulars; 13 are reported here for the first time. The catalog contains events whose sources are black hole binary mergers up to a redshift of approximately 0.8, as well as events whose components cannot be unambiguously identified as black holes or neutron stars. For the latter group, we are unable to determine the nature based on estimates of the component masses and spins from gravitational-wave data alone. The range of candidate event masses which are unambiguously identified as binary black holes (both objects ≥3  M⊙) is increased compared to GWTC-1, with total masses from approximately 14  M⊙ for GW190924_021846 to approximately 150  M⊙ for GW190521. For the first time, this catalog includes binary systems with significantly asymmetric mass ratios, which had not been observed in data taken before April 2019. We also find that 11 of the 39 events detected since April 2019 have positive effective inspiral spins under our default prior (at 90% credibility), while none exhibit negative effective inspiral spin. Given the increased sensitivity of Advanced LIGO and Advanced Virgo, the detection of 39 candidate events in approximately 26 weeks of data (approximately 1.5 per week) is consistent with GWTC-1.
      @article{ 2020arXiv201014527A,
      author =  LIGO Scientific Collaboration and   Virgo Collaboration,
      title = { GWTC-2: Compact Binary Coalescences Observed by LIGO and Virgo During the First Half of the Third Observing Run },
      journal = { Physical Review X },
      year = { 2021 },
      month = { jun },
      volume = { 11 },
      number = { 2 },
      doi = { 10.1103/PhysRevX.11.021053 },
      }
    

Comparing Short Gamma-Ray Burst Jet Structure Models

Fergus Hayes, Ik Siong Heng, John Veitch, Daniel Williams

Astrophysical Journal, 891, 124

gamma-ray burst: general gravitational waves

| arxiv:1911.04190 | doi:10.3847/1538-4357/ab72fc |

 
A structured gamma-ray burst jet could explain the dimness of the prompt emission observed from GRB170817A but the exact form of this structure is still ambiguous. However, with the promise of future joint gravitational wave and gamma-ray burst observations, we shall be able to examine populations of binary neutron star mergers rather than a case-by-case basis. We present an analysis that considers gravitational wave triggered binary neutron star events both with and without short gamma-ray burst counterparts assuming that events without a counterpart were observed off-axis. This allows for Bayes factors to be calculated to compare different jet structure models. We perform model comparison between a Gaussian and power-law apparent jet structure on simulated data to demonstrate that the correct model can be distinguished with a log Bayes factor of >5 after less than 100 events. Constraints on the apparent structure jet model parameters are also made. After 25(100) events the angular width of the core of a power-law jet structure can be constrained within a 90% credible interval of width ∼9.1(4.4)∘, and the outer beaming angle to be within ∼19.9(8.5)∘. Similarly we show the width of a Gaussian jet structure to be constrained to ∼2.8(1.6)∘.
      @article{ 2020ApJ...891..124H,
      author = { Hayes, F } and { Heng, IS } and { Veitch, J } and { Williams, D },
      title = { Comparing Short Gamma-Ray Burst Jet Structure Models },
      journal = { Astrophysical Journal },
      year = { 2020 },
      month = { mar },
      volume = { 891 },
      number = { 2 },
      pages = { 124 },
      doi = { 10.3847/1538-4357/ab72fc },
      }
    

A Precessing Numerical Relativity Waveform Surrogate Model for Binary Black Holes: A Gaussian Process Regression Approach

Daniel Williams, Ik Siong Heng, Jonathan Gair, James Clark, Bhavesh Khamesra

Physical Review D, 101, 6

general relativity and quantum cosmology physics - data analysis statsistics and probability gravitational waves gravitational waves: modelling

| arxiv:1903.09204 | doi:10.1103/PhysRevD.101.063011 |

 
Gravitational wave astrophysics relies heavily on the use of matched filtering both to detect signals in noisy data from detectors, and to perform parameter estimation on those signals. Matched filtering relies upon prior knowledge of the signals expected to be produced by a range of astrophysical systems, such as binary black holes. These waveform signals can be computed using numerical relativity techniques, where the Einstein field equations are solved numerically, and the signal is extracted from the simulation. Numerical relativity simulations are, however, computationally expensive, leading to the need for a surrogate model which can predict waveform signals in regions of the physical parameter space which have not been probed directly by simulation. We present a method for producing such a surrogate using Gaussian process regression which is trained directly on waveforms generated by numerical relativity. This model returns not just a single interpolated value for the waveform at a new point, but a full posterior probability distribution on the predicted value. This model is therefore an ideal component in a Bayesian analysis framework, through which the uncertainty in the interpolation can be taken into account when performing parameter estimation of signals.
      @article{ 2019arXiv190309204W,
      author = { Williams, D } and { Heng, IS } and { Gair, J } and { Clark, JA } and { Khamesra, B },
      title = { A Precessing Numerical Relativity Waveform Surrogate Model for Binary Black Holes: A Gaussian Process Regression Approach },
      journal = { Physical Review D },
      year = { 2020 },
      volume = { 101 },
      doi = { 10.1103/PhysRevD.101.063011 },
      }
    

Constraints On Short, Hard Gamma-Ray Burst Beaming Angles From Gravitational Wave Observations

Daniel Williams, James Clark, Andrew Williamson, Ik Siong Heng

Astrophysical Journal, 858, 2

gamma-ray burst: general gravitational waves

| arxiv:1712.02585 | doi:10.3847/1538-4357/aab847 | data:10.5281/zenodo.1066019 |

 
The first detection of a binary neutron star merger, GW170817, and an associated short gamma-ray burst confirmed that neutron star mergers are responsible for at least some of these bursts. The prompt gamma ray emission from these events is thought to be highly relativistically beamed. We present a method for inferring limits on the extent of this beaming by comparing the number of short gamma-ray bursts observed electromagnetically to the number of neutron star binary mergers detected in gravitational waves. We demonstrate that an observing run comparable to the expected Advanced LIGO 2016--2017 run would be capable of placing limits on the beaming angle of approximately θ∈(2.88∘,14.15∘), given one binary neutron star detection. We anticipate that after a year of observations with Advanced LIGO at design sensitivity in 2020 these constraints would improve to θ∈(8.10∘,14.95∘).
      @article{ 2017arXiv171202585W,
      author = { Daniel Williams } and { James Clark } and { Andrew Williamson } and { Ik Siong Heng },
      title = { Constraints On Short, Hard Gamma-Ray Burst Beaming Angles From Gravitational Wave Observations },
      journal = { Astrophysical Journal },
      year = { 2018 },
      volume = { 858 },
      doi = { 10.3847/1538-4357/aab847 },
      }
    

A Bayesian method for detecting stellar flares

Pitkin, Williams, Fletcher, Grant

Monthly Notices of the Royal Astronomical Society, 445, 3, 2268-2284

methods: data analysis methods: statistical stars: flare

| arxiv:1406.1712 | doi:10.1093/mnras/stu1889 | ascl:1407.015 |

 
We present a Bayesian-odds-ratio-based algorithm for detecting stellar flares in light curve data. We assume flares are described by a model in which there is a rapid rise with a half-Gaussian profile, followed by an exponential decay. Our signal model also contains a polynomial background model. This is required to fit underlying light curve variations that are expected in the data, which could otherwise partially mimic a flare. We characterise the false alarm probability and efficiency of this method and compare it with a simpler thresholding method based on that used in Walkowicz et al (2011). We find our method has a significant increase in detection efficiency for low signal-to-noise ratio (S/N) flares. For a conservative false alarm probability our method can detect 95% of flares with S/N less than ~20, as compared to S/N of ~25 for the simpler method. As an example we have applied our method to a selection of stars in Kepler Quarter 1 data. The method finds 687 flaring stars with a total of 1873 flares after vetos have been applied. For these flares we have characterised their durations and and signal-to-noise ratios.
      @article{ 2014MNRAS.445.2268P,
      author = { Pitkin, MD } and { Williams, D } and { Fletcher, L } and { Grant, S },
      title = { A Bayesian method for detecting stellar flares },
      journal = { Monthly Notices of the Royal Astronomical Society },
      year = { 2014 },
      volume = { 445 },
      pages = { 2268-2284 },
      doi = { 10.1093/mnras/stu1889 },
      }
    

Mimicking Mergers: Mistaking Black Hole Captures as Mergers

Weichangfeng Guo, Daniel Williams, Ik Siong Heng, Hunter Gabbard, Yeong-Bok Bae, Gungwon Kang, Zong-Hong Zhu

The first publication for one of my students!

| arxiv:2203.06969 |

 
As the number of gravitational wave observations has increased in recent years, the variety of sources has broadened. Here we investigate whether it is possible for the current generation of detectors to distinguish between very short-lived gravitational wave signals from mergers between high-mass black holes, and the signal produced by a close encounter between two black holes which results in gravitational capture, and ultimately a merger. We compare the posterior probability distributions produced by analysing simulated signals from both types of progenitor events, both under ideal and realistic scenarios. We show that while, under ideal conditions it is possible to distinguish both progenitors, under more realistic conditions they are indistinguishable. This has important implications for the interpretation of such short signals, and we therefore advocate that these signals be the focus of additional investigation even when satisfactory results have been achieved from standard analyses.
      @article{ ,
      author = { Guo } and { Williams, D } and { Heng, IS } and { Gabbard } and { Bae } and { Kang } and { Zhu },
      title = { Mimicking Mergers: Mistaking Black Hole Captures as Mergers },
      }
    

Beyond GWTC-3: Analysing and verifying new gravitational-wave events from the 4-OGC Catalogue

 
      @article{ ,
      author = { Williams },
      title = { Beyond GWTC-3: Analysing and verifying new gravitational-wave events from the 4-OGC Catalogue },
      }
    

Rapid Generation of Kilonova Light Curves Using Conditional Variational Autoencoder

Surojit Saha, Michael J Williams, Laurence Datrier, Fergus Hayes, Matt Nicholl, Albert K H Kong, Martin Hendry, Ik Siong Heng, Gavin P Lamb, En-Tzu Lin, Daniel Williams

| arxiv:2310.17450 |

 
      @article{ ,
      author = { Saha } and { Williams } and { Datrier } and { Hayes } and { Nicholl } and { Kong } and { Hendry } and { Heng } and { Lamb } and { Lin } and { Williams },
      title = { Rapid Generation of Kilonova Light Curves Using Conditional Variational Autoencoder },
      }