WoU-MMA: Targeted Search for Binary Mergers with Multiple Harmonics in Gravitational Wave Data

Black holes and neutron stars are some of the final endpoints of stars that we see in the universe. These massive and dense compact objects are hard to detect by their electromagnetic emission. Since 2016, interferometric detectors such as LIGO, VIRGO, and KAGRA have helped detect compact objects through the gravitational waves that they emit as they merge with each other.

As the detectors' sensitivity improved, the field has moved from the era of notable single detections to one in which we can systematically survey the population of compact objects in the universe. The most interesting and informative detections are those that show complexity (such as overtones and modulations) in the signal, as these effects ultimately inform us about the environment these binaries were assembled in; existing searches do not account for these effects as they complicate the search algorithms.

This research program will boost the sensitivity of searches to such signals, and hence extend the reach in the most interesting parts of parameter space - the highest mass mergers (extending all the way to several hundred solar masses), and binaries with very high mass ratios and misaligned spins. Detecting and characterizing these special systems can answer a range of fundamental astrophysical questions, such as the link between stellar-mass black holes and the more massive ones at the centers of galaxies, the endpoint of stellar evolution for the most massive stars (which collapse to the most massive black holes), and the means by which black hole binaries are assembled (from the configurations and sizes of the component spins).

In a broader sense, the research program will also facilitate NSF’s long-term vision of building an outside community around the LIGO strain data, introduce fresh perspectives to the analyses, and maximize the potential of pre-existing open data: this will be realized by means of a workshop for the field, and collaborations with local researchers and established university programs to disseminate ideas among high school students.

The research is motivated by the presence of subtle effects such as higher harmonics and general-relativistic precession in the loudest individual signals, as well as tentative trends in the astrophysical population, that have already been detected. These events were found by search pipelines that made simplifying assumptions about the signals, i.e., included only a subset of the relevant physical effects; the most important assumption being that the sources are quasi-circular binaries with component spins that are aligned with the orbital angular momentum, and that the waveform is composed of a single harmonic.

The aim is to develop a new search that incorporates the effects of higher harmonics, and possibly even precession, and hence is sensitive to a wider range of signals. This will require revisiting all search algorithms in order to achieve the increased sensitivity without paying too large a computational price. The main tasks are to (a) organize templates to account for the diversity in the space of signals, while still retaining the ability to maximize over several geometric parameters, which will control the template bank’s size, and (b) devise methods of optimally combining the individual harmonics, as well as data from multiple detectors.

This step is crucial to do correctly to avoid losing sensitivity to existing signals due to enlarging the search space. The methods build on existing full-physics waveform models that are routinely used for parameter inference, and insights from previous searches that used simplified waveform models.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.