Quasars have long been known to be variable sources at all wavelengths. Their optical variability is stochastic and can be due to a variety of physical mechanisms; it is also well-described statistically in terms of a damped random walk model1. The recent availability of large collections of astronomical time series of flux measurements (light curves2,3,4,5) offers new data sets for a systematic exploration of quasar variability. Here we report the detection of a strong, smooth periodic signal in the optical variability of the quasar PG 1302−102 with a mean observed period of 1,884 ± 88 days. It was identified in a search for periodic variability in a data set of light curves for 247,000 known, spectroscopically confirmed quasars with a temporal baseline of about 9 years. Although the interpretation of this phenomenon is still uncertain, the most plausible mechanisms involve a binary system of two supermassive black holes with a subparsec separation. Such systems are an expected consequence of galaxy mergers and can provide important constraints on models of galaxy formation and evolution.
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This work was supported in part by NSF grants AST-0909182, IIS-1118031 and AST-1313422. We thank J. S. Stuart, MIT Lincoln Laboratory, for assistance with the LINEAR data. We also thank the staff of the Keck and Palomar Observatories for their help with observations, and the CRTS team. Some of the data presented here were obtained at the W.M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and NASA. The observatory was made possible by the financial support of the W.M. Keck Foundation. The work of D.S. was performed at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA.
The authors declare no competing financial interests.
Extended data figures and tables
Shown are the CRTS light curves for 11 quasars reported48,49 to show periodicity in their radio emission. Each light curve has been normalized to zero mean and individual curves are offset by a constant of 1.5 mag from each other. The data are split across two panels for ease of viewing. Error bars shown are standard 1σ photometric errors. The CRTS light curve of PG 1302−102 (solid black stars) is also shown for comparison.
Shown are the CRTS light curves for 6 quasars reported51,52,53,54to have warped accretion disks. Each light curve has been normalized to zero mean and individual curves are offset by a constant of 0.5 mag from each other. Error bars shown are standard 1σ photometric errors. The CRTS light curve of PG1302−102 (solid black stars) is also shown for comparison.
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Graham, M., Djorgovski, S., Stern, D. et al. A possible close supermassive black-hole binary in a quasar with optical periodicity. Nature 518, 74–76 (2015). https://doi.org/10.1038/nature14143
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