Discovery of a Candidate Binary Supermassive Black Hole in a Periodic Quasar from Circumbinary Accretion Variability
Authors: Liao Wei-Ting , Chen Yu-Ching , Liu Xin , Holgado A Miguel , Guo Hengxiao , Gruendl Robert , Morganson Eric , Shen Yue , Davis Tamara , Kessler Richard , Martini Paul , McMahon Richard G , Allam Sahar , Annis James , Avila Santiago , Banerji Manda , Bechtol Keith , Bertin Emmanuel , Brooks David , Buckley-Geer Elizabeth , Rosell Aurelio Carnero , Kind Matias Carrasco , Carretero Jorge , Castander Francisco Javier , Cunha Carlos , D’Andrea Chris , da Costa Luiz , Davis Christopher , De Vicente Juan , Desai Shantanu , Diehl H Thomas , Doel Peter , Eifler Tim , Evrard August , Flaugher Brenna , Fosalba Pablo , Frieman Josh , Garcia-Bellido Juan , Gaztanaga Enrique , Glazebrook Karl , Gruen Daniel , Gschwend Julia , Gutierrez Gaston , Hartley Will , Hollowood Devon L , Honscheid Klaus , Hoyle Ben , James David , Krause Elisabeth , Kuehn Kyler , Lima Marcos , Maia Marcio , Marshall Jennifer , Menanteau Felipe , Miquel Ramon , Malagń Andrés Plazas , Roodman Aaron , Sanchez Eusebio , Scarpine Vic , Schubnell Michael , Serrano Santiago , Smith Mathew , Smith R Chris , Soares-Santos Marcelle , Sobreira Flavia , Suchyta Eric , Swanson Molly , Tarle Gregory , Vikram Vinu , Walker Alistair 
Published: October 2020

Binary supermassive black holes (BSBHs) are expected to be a generic byproduct from hierarchical galaxy formation. The final coalescence of BSBHs is thought to be the loudest gravitational wave (GW) siren, yet no confirmed BSBH is known in the GW-dominated regime. While periodic quasars have been proposed as BSBH candidates, the physical origin of the periodicity has been largely uncertain. Here we report discovery of a periodicity (P=1607±7 days) at 99.95% significance (with a global p-value of ∼10−3 accounting for the look elsewhere effect) in the optical light curves of a redshift 1.53 quasar, SDSS J025214.67−002813.7. Combining archival Sloan Digital Sky Survey data with new, sensitive imaging from the Dark Energy Survey, the total ∼20-yr time baseline spans ∼4.6 cycles of the observed 4.4-yr (restframe 1.7-yr) periodicity. The light curves are best fit by a bursty model predicted by hydrodynamic simulations of circumbinary accretion disks. The periodicity is likely caused by accretion rate modulation by a milli-parsec BSBH emitting GWs, dynamically coupled to the circumbinary accretion disk. A bursty hydrodynamic variability model is statistically preferred over a smooth, sinusoidal model expected from relativistic Doppler boost, a kinematic effect proposed for PG1302−102. Furthermore, the frequency dependence of the variability amplitudes disfavors Doppler boost, lending independent support to the circumbinary accretion variability hypothesis. Given our detection rate of one BSBH candidate from circumbinary accretion variability out of 625 quasars, it suggests that future large, sensitive synoptic surveys such as the Vera C. Rubin Observatory Legacy Survey of Space and Time may be able to detect hundreds to thousands of candidate BSBHs from circumbinary accretion with direct implications for Laser Interferometer Space Antenna.