A possible close supermassive black-hole binary in a quasar with optical periodicity

@article{Graham2015APC,
  title={A possible close supermassive black-hole binary in a quasar with optical periodicity},
  author={Matthew J. Graham and S. George Djorgovski and Daniel Stern and Eilat Glikman and Andrew J. Drake and Ashish A. Mahabal and Ciro Donalek and Stephen M. Larson and Eric J. Christensen},
  journal={Nature},
  year={2015},
  volume={518},
  pages={74-76}
}
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 model. The recent availability of large collections of astronomical time series of flux measurements (light curves) offers new data sets for a systematic exploration of quasar variability. Here we report the detection of a strong, smooth periodic signal in the… 
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