Such a map would provide a vital new way to explore and understand astronomy and physics, just as X-rays and radio waves did in earlier eras, the researchers say. The new protocol demonstrated by the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) offers a detection protocol to populate the map.

"Our finding provides the scientific community with the first concrete benchmarks for developing and testing detection protocols for individual, continuous gravitational wave sources," said Chiara Mingarelli, assistant professor of physics in Yale's Faculty of Arts and Sciences (FAS), member of NANOGrav, and corresponding author of a new study in the Astrophysical Journal Letters.

According to the researchers, even a small number of confirmed black hole binaries will enable them to anchor a map of the gravitational wave background. In the months ahead, NANOGrav will continue identifying and locating binaries.

Previous theoretical work led by Mingarelli and collaborators suggested that black hole mergers are five times more likely to be found in galaxies with a quasar, a brightly lit "beacon" in space fueled by gases falling into a black hole. Informed by this research, the new study details an end-to-end, targeted search framework from continuous gravitational waves from individual black hole merger candidates.

In 2023, NANOGrav reported on the discovery of the first direct evidence of a background of gravitational waves. The discovery suggested that gravitational waves, caused by slowly merging pairs of supermassive black holes, could be detected from Earth within a background field of low-frequency energy.

NANOGrav centered its detection methods around pulsars, which are the collapsed cores of massive stars that have exploded. Pulsars, which rotate rapidly, emit precisely timed radio signals.

The international collaborators then pivoted to the search for individual waves.

For the new study, a research team led by Mingarelli tested a novel methodology that combines measurements of the gravitational wave background with variable measurements of quasars. She and her colleagues conducted targeted searches for supermassive black hole binaries in 114 active galactic nuclei - areas in the center of galaxies where a black hole is drawing in matter.

That's how they found SDSS J1536+0411 (aka "Rohan") and SDSS J0729+4008 (aka "Gondor") - named, in part, after locales from J.R.R. Tolkien's "The Lord of the Rings" novels.

"The names come from both people and pop culture," Mingarelli said. "Rohan was first, for Rohan Shivakumar, the Yale student who first analyzed it, and Gondor was next, because, well - the beacons were lit!"

In "The Lord of the Rings," heroes joined forces after beacons were lit in Gondor and Rohan.

Mingarelli said the discovery offers intriguing possibilities across a range of astrophysics research - from gravitational wave theory and data analysis to galaxy mergers, and black hole astrophysics.

"Our work has laid out a roadmap for a systemic supermassive black hole binary detection framework," she said. "We carried out a systematic, targeted search, developed a rigorous protocol - and two targets rose to the top as examples motivating follow-up."

A number of Yale researchers are co-authors of the study. They include Priyamvada Natarajan, the Joseph S. and Sophia S. Fruton Professor and Chair of Astronomy and professor of physics; Paolo Coppi, professor of astronomy; Forrest Hutchison, Bjorn Larsen, and Qinyuan Zheng, who are students in the Yale Graduate School of Arts and Sciences; and Yale College students Rohan Shivakumar, Ellis Eisenberg, and Yu-Ting Chang.

NANOGrav receives support from the National Science Foundation, the Gordon and Betty Moore Foundation, a National Sciences and Engineering Research Council of Canada (NSERC) Discovery Grant, and the Canadian Institute for Advanced Research.

Research Report:The NANOGrav 15 yr Dataset: Targeted Searches for Supermassive Black Hole Binaries

Related Links
NANOGrav
Understanding Time and Space