Periodic Reporting for period 1 - GWsFromEMRIs (Gravitational waves from extreme mass-ratio inspirals)
Berichtszeitraum: 2018-02-01 bis 2020-01-31
The goal of the GWFromEMRIs project was the computation of gravitational waveforms emitted by extreme mass-ratio-inspirals (EMRIs), one of LISA most-awaited sources. In an EMRI, a stellar-mass objects slowly fall towards a supermassive black hole, tracing tens of thousands of orbits around its companion. The long time spent in the LISA frequency band (months to years) will allow to explore the strong-field regime in unprecedented detail and to test the core laws of General Relativity. These systems can be successfully described within the gravitational-self-force formalism, which builds upon black-hole perturbation theory techniques to compute the orbital motion of the secondary mass.
To this purpose, the project envisaged the development of new numerical tools to compute the self-force in Kerr spacetime. While this has been part of my research interests, the action has morphed into a more comprehensive and ambitious project, where I complemented numerical work on the computation of the gravitational self-force with an intense modelling effort within the LIGO collaboration. In particular, I was one of the core developers of a new family of waveform approximants for binary-black-hole coalescences (Pratten et al. Phys. Rev. D 102, 064001, 2020, García-Quirós, Colleoni et al. Phys. Rev. D 102, 064002, 2020) spanning a large region in parameter space, extending from extreme-mass-ratio to comparable-mass binaries. The main application of these models is in data analysis for ground-based interferometry, but they could be useful for preliminary parameter estimation studies for LISA.
The main motivation behind this shift of focus was the start of LIGO’s observing run (O3). O3 called for renewed efforts to deliver a new generation of black-hole binary waveform models that could meet the requirements of increasingly challenging data-analysis campaigns. In preparation for new and possibly exceptional detections, much work was devoted to the construction of templates going beyond the quadrupole-mode approximation, which had been the basis for previous parameter estimation studies. This development was pushed also by a fresh injection of numerical relativity data (Class. Quantum Grav. 36, 195006, 2019), on which modern waveform models rely to construct a faithful representation of the signal. Although the models developed during the project are not specifically targeting EMRIs, they are flexible enough to incorporate self-force information once this becomes available.
To this purpose, the project envisaged the development of new numerical tools to compute the self-force in Kerr spacetime. While this has been part of my research interests, the action has morphed into a more comprehensive and ambitious project, where I complemented numerical work on the computation of the gravitational self-force with an intense modelling effort within the LIGO collaboration. In particular, I was one of the core developers of a new family of waveform approximants for binary-black-hole coalescences (Pratten et al. Phys. Rev. D 102, 064001, 2020, García-Quirós, Colleoni et al. Phys. Rev. D 102, 064002, 2020) spanning a large region in parameter space, extending from extreme-mass-ratio to comparable-mass binaries. The main application of these models is in data analysis for ground-based interferometry, but they could be useful for preliminary parameter estimation studies for LISA.
The main motivation behind this shift of focus was the start of LIGO’s observing run (O3). O3 called for renewed efforts to deliver a new generation of black-hole binary waveform models that could meet the requirements of increasingly challenging data-analysis campaigns. In preparation for new and possibly exceptional detections, much work was devoted to the construction of templates going beyond the quadrupole-mode approximation, which had been the basis for previous parameter estimation studies. This development was pushed also by a fresh injection of numerical relativity data (Class. Quantum Grav. 36, 195006, 2019), on which modern waveform models rely to construct a faithful representation of the signal. Although the models developed during the project are not specifically targeting EMRIs, they are flexible enough to incorporate self-force information once this becomes available.
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.
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.
The main goal of the project was the development of gravitational waveform models to be employed in LISA parameter estimation. While EMRIs represented the initial target of the action, the project has ended up being more comprehensive, embracing also the modelling of comparable-ass binaries and projecting my research within a large international collaboration. With the launch of the LISA mission steadily approaching, it will be key to complement the lessons learned from ground-based interferometry with knowledge of the challenges posed by new types of experiments and sources.