WoU-MMA: Multi-messenger Titans: Probing The Dynamics & Environments Of Supermassive Binary Black Holes

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It is now known that nearly every galaxy contains at its center, a supermassive black hole, with a mass millions to billions of times greater than the mass of the Sun. If a supermassive black hole is in a dense environment, it can be detected by the light emitted by gas and dust falling into it. Such an object is called an active galactic nucleus (AGN). It is also known that some of these titanic objects exist in pairs, called supermassive binary black holes (SMBBHs), which are expected to eventually spiral into each other and collide, producing ripples in spacetime, or gravitational waves (GWs). These GWs cause tiny variations in the timing of rapidly spinning compact former stars called pulsars. By measuring these timing variations using multiple radio telescopes in what is called a pulsar timing array (PTA), astronomers expect to infer the existence of such GWs. A research team at Vanderbilt University will perform the first end-to-end coherent multi-messenger (MM) search for SMBBHs using the synthesis of precision pulsar timing array data with optical observations of the time-varying brightness and spectra of AGN. The methods resulting from the project will forecast the expected properties of the first MM binary detection in the 2020s, including the likely binary environment and host galaxy characteristics. The investigators will also establish a Vanderbilt node of Student Teams of Astrophysics ResarcherS (STARS), a highly successful national program established by the NANOGrav PTA team. The work will involve undergraduate students from Vanderbilt and Fisk (a nearby historically black university), who will be directly involved in pulsar and gravitational-wave research. This project will forge a path to the unambiguous identification of SMBBHs at sub-parsec separations, helping to resolve the perplexing enigma of this heretofore missing population. While dual AGN have been observed at separations from kiloparsecs down to seven parsecs, there are no unambiguously identified SMBBHs at sub-parsec separations. Some candidate sub-parsec pairs have been identified as quasi-periodic optical-flux or radial-velocity variations in AGN, but this is severely hampered by intrinsic disk and environmental noise processes that can mimic periodic behavior over short baselines. Only PTA data can directly infer the presence of SMBBHs through detection of their ~nanohertz-frequency GWs. By combining PTA GW searches with large photometric and spectroscopic surveys, this project will leverage the strength of multiple messengers to arbitrate existing candidates, forecast the expected properties and constraints on the first MM binary detection, and fuse the information provided by different messengers into a complete portrait of SMBBH dynamics at sub-parsec separations. This proposal represents a dramatic leap forward in understanding how EM signatures of SMBBHs track the GW signal. This project advances the goals of the NSF Windows on the Universe Big Idea.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.