1) Preparation of a time-domain code to compute the metric perturbation sourced by a small body around a Kerr black hole
This was the primary objective of the original research plan and, although results have not been used yet in the production of waveforms, it will be possibly to do so in the future. The numerical infrastructure builds on previous work in Schwarzschild spacetime (Phys. Rev. D 95, 104033 (2018)), where the Teukolsky equation is numerically solved for on a double-null grid. A generalisation to Kerr spacetime, improved accuracy and computational efficiency are among the achievements of the project so far.
2) The PhenomX waveform suite
I was one of the main developers of the PhenomX suite, a family of state-of-the-art phenomenological waveform models for binary black-hole coalescences. The PhenomX family represents a substantial upgrade with respect to its predecessors, and all the multipoles benefit from a calibration set of numerical relativity and black-hole perturbation theory waveforms that is 25 times larger than that of PhenomD.
The first building block was a new model for the quadrupolar wave emission of aligned-spin binaries, IMRPhenomXAS (Pratten et al. Phys. Rev. D 102, 064001, 2020). I was then one of the main developers of another model targeting aligned-spin binaries, IMRPhenomXHM, which incorporates the effects of higher multipoles (García-Quirós, Colleoni et al. Phys. Rev. D 102, 064002, 2020) and leverages adapted grids to speed up template evaluation. I worked on the mathematical formulation of the model and on its calibration to numerical waveforms and, subsequently, on its implementation in the LIGO Algorithm library lalsuite. I was then involved in the development of IMRPhenomXPHM (Pratten et al.,arXiv:2004.06503) an extension of the previous models to precessing binaries. IMRPhenomXPHM relies crucially on the previous models, as it approximates the signal in the detector’s frame through time-dependent transformations of aligned-spin waveforms, which are identified with the signal measured in the co-precessing frame. The user base of PhenomX is rapidly extending and it has the potential of becoming one of the reference waveform family in future parameter estimation studies.
3) Follow-up of exceptional events
To prepare the ground for systematic use of the PhenomX suite in data-analysis, I have also carried out follow-up studies on real data, focussing on GW190412, the first gravitational-wave observation of a binary black-hole system with asymmetric masses. We relied on the new models to estimate the source properties with Bayesian inference techniques, examined sampler convergence and assessed potential parameter biasses. Results will be presented in a forthcoming paper.
Many of the above results have been presented in journal publications, as well as in presentations to the LIGO collaboration. I also had the chance to contribute to several outreach projects covering the physics of black holes and gravitational waves. In particular, I took part to to the local edition of Pint of Science, as well to an episode of “Balears fa Ciència”, a popular science programme broadcast by the Balearic television channel.
