title: A possible close supermassive black-hole binary in a quasar with optical periodicity.
abstract: 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 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.
authors.affiliation: California Institute of Technology, 1200 East California Boulevard, Pasadena, California 91125, USA.
authors.forename: Matthew J
authors.initials: MJ
authors.lastname: Graham
authors.affiliation: California Institute of Technology, 1200 East California Boulevard, Pasadena, California 91125, USA.
authors.forename: S G
authors.initials: SG
authors.lastname: Djorgovski
authors.affiliation: Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, California 91109, USA.
authors.forename: Daniel
authors.initials: D
authors.lastname: Stern
authors.affiliation: Department of Physics, Middlebury College, Middlebury, Vermont 05753, USA.
authors.forename: Eilat
authors.initials: E
authors.lastname: Glikman
authors.affiliation: California Institute of Technology, 1200 East California Boulevard, Pasadena, California 91125, USA.
authors.forename: Andrew J
authors.initials: AJ
authors.lastname: Drake
authors.affiliation: California Institute of Technology, 1200 East California Boulevard, Pasadena, California 91125, USA.
authors.forename: Ashish A
authors.initials: AA
authors.lastname: Mahabal
authors.affiliation: California Institute of Technology, 1200 East California Boulevard, Pasadena, California 91125, USA.
authors.forename: Ciro
authors.initials: C
authors.lastname: Donalek
authors.affiliation: Lunar and Planetary Laboratory, Department of Planetary Sciences, University of Arizona, 1629 East University Boulevard, Tucson, Arizona 85721, USA.
authors.forename: Steve
authors.initials: S
authors.lastname: Larson
authors.affiliation: Lunar and Planetary Laboratory, Department of Planetary Sciences, University of Arizona, 1629 East University Boulevard, Tucson, Arizona 85721, USA.
authors.forename: Eric
authors.initials: E
authors.lastname: Christensen