Neutrino Emission from Supermassive Binary Black Hole Mergers

The first high-probability association of an extragalactic neutrino to the blazar TXS 0506+056 in 2017 identified such active galaxies as potential high-energy neutrino emitters, as expected for a long time. A detailed follow-up investigation revealed a second neutrino flare from the same blazar nearly 3 years prior, indicating a possible periodicity. A supermassive binary black hole system close to its merger explains such a periodic neutrino emission as a result of jet precession and jet interactions with surrounding molecular clouds. We present an analytical model for predicting the arrival times of neutrino flares and gravitational waves for such systems and apply it to the blazar TXS 0506+056 under the assumption of quasi-periodic neutrino emission. We conclude that the blazar can indeed be an ongoing binary merger with the next neutrino emission possibly occurring before the end of 2021 and deliver binary properties for a successful detection of its gravitational waves by LISA. As supermassive binary black hole mergers occur more frequently due to merging of their host galaxies, we further investigate a connection between their radiated gravitational wave energy and the diffuse astrophysical neutrino flux that is measured by IceCube. We estimate the contributions of these mergers and additionally of binary stellar mass black hole mergers in starburst galaxies to the diffuse neutrino flux.