Sharon Mary Tomson

Galaxy observations suggest that mergers of supermassive black hole binaries (SMBHBs) are rare events on cosmological scales, with characteristic rates of order one per decade across the observable Universe. In this work, we present a framework to search for merging SMBHBs in pulsar timing array (PTA) data using a physically complete waveform model that includes the late inspiral, merger, ringdown, and gravitational-wave memory. This approach enables a unified treatment of continuous premerger emission and the nonoscillatory memory signal associated with coalescence. Using simulated PTA datasets, we demonstrate a proof of principle for parameter estimation of merging SMBHBs, focusing on representative systems with chirp masses of 10 8 and 10 10 M ⊙ at distances of 3 Mpc and 100 Mpc respectively. These signals are distinguishable for a PTA consisting of 25 pulsars observed over 13 years with 100 ns timing precision. For these sufficiently strong signals, we recover the simulated binaries with log Bayes factors exceeding 10, and show that chirp mass and luminosity distance can be jointly constrained with uncertainties governed by the characteristic mass-distance degeneracy of the signal. The sky position uncertainties are of the order of a few degrees, which could potentially enable electromagnetic follow-up and multimessenger observations of SMBHB mergers. We also show that the measurement uncertainties on the parameters of simulated merging binaries depend weakly on the presence of the gravitational wave background with Hellings-Downs correlations in our simulated data. We further demonstrate that commonly used memory burst approximations lead to biased strain amplitudes and inferred source parameters when compared to the full SMBHB waveform, even when optimally tuned. These results establish a pathway toward searching for SMBHB mergers and late-stage inspirals with PTAs using physically complete waveform models, complementing existing searches for continuous gravitational waves.

We perform searches for gravitational-wave memory in the data of two major Pulsar Timing Array (PTA) experiments located in Europe and Australia. Supermassive black hole binaries (SMBHBs) are the primary sources of gravitational waves in PTA experiments. We develop and carry out the first search for late inspirals and mergers of these sources based on full numerical relativity waveforms with null (nonlinear) gravitational-wave memory. Additionally, we search for generic bursts of null gravitational-wave memory, exploring the possibilities of reducing the computational costs of these searches through kernel density estimation and normalizing flow approximations of the posteriors. We rule out mergers of SMBHBs with a chirp mass of 10 ¹⁰ M _⊙ up to 700 Mpc over 18 yr of observation at 95% credibility. We rule out the observation of generic displacement memory bursts with strain amplitudes >10 ⁻¹⁴ in brief periods of observation time but across the sky, or over the whole observation time but for certain preferred sky positions, at 95% credibility.

We perform searches for gravitational wave memory in the data of two major Pulsar Timing Array (PTA) experiments located in Europe and Australia. Supermassive black hole binaries (SMBHBs) are the primary sources of gravitational waves in PTA experiments. We develop and carry out the first search for late inspirals and mergers of these sources based on full numerical relativity waveforms with null (nonlinear) gravitational wave memory. Additionally, we search for generic bursts of null gravitational wave memory, exploring possibilities of reducing the computational cost of these searches through kernel density and normalizing flow approximation of the posteriors. We rule out the mergers of SMBHBs with a chirp mass of 10^10 Solar Mass up to 700 Mpc over 18 years of observation at 95% credibility. We rule out the observation of generic displacement memory bursts with strain amplitudes > 10^-14 in brief periods of the observation time but across the sky, or over the whole observation time but for certain preferred sky positions, at 95%$credibility.

Constraining the origin of the nanohertz gravitational-wave background necessitates precise noise modelling to avoid parameter estimation biases. In this work, we find the inferred properties of the putative gravitational wave background in the second data release of the European Pulsar Timing Array to be in better agreement with theoretical expectations under the improved noise model. In particular, our improved noise models show consistency of the background’s strain spectral index with the value of −2/3, favoring the population of supermassive black hole binaries as the origin of the background. Our results further suggest that the observed gravitational wave emission is the dominant source of the binary energy loss, with no evidence of environmental effects or eccentric orbits. At the reference gravitational wave frequency of yr ⁻¹ , we also find a lower power-law strain amplitude of the background than in previous data analyses. This mitigates some of the tensions of the strain amplitude with the expected number density and mass scale of binaries discussed in the literature. Our analysis demonstrates the importance of accurate modelling of radio pulsar pulse profile variations, hierarchical properties of noise across pulsars, as well as noise model averaging, when inferring properties of the gravitational wave background.

We show that the gravitational waveform emitted by a binary on an eccentric orbit can be naturally decomposed into a series of harmonics. The frequencies of these harmonics depend upon the radial frequency, f r , determined by the time to return to apoapsis, and the azimuthal frequency, f ϕ , determined by the time to complete one orbit relative to a fixed axis. These frequencies differ due to periapsis advance. Restricting to the (2, 2) multipole, we find that the frequencies can be expressed as f = 2 f ϕ + k f r . We introduce a straightforward method of generating these harmonics and show that the majority of the signal power is contained in the k = − 1 , 0 , 1 harmonics for moderate eccentricities. We demonstrate that by filtering these three leading harmonics, we are able to obtain a good estimate of the orbital eccentricity from their relative amplitudes.