Skip to main content

Thank you for visiting You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

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


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.

This is a preview of subscription content

Access options

Rent or Buy article

Get time limited or full article access on ReadCube.


All prices are NET prices.

Figure 1: The parameter space of SMBH binary pairs.
Figure 2: The composite light curve for PG 1302−102 over a period of 7,338 days (20 years).
Figure 3: The composite spectrum for PG 1302−102.
Figure 4: The profiles of the Balmer and Paschen series lines of PG 1302−102.


  1. 1

    Kelly, B. C., Bechtold, J. & Siemiginowska, A. Are the variations in quasar optical flux driven by thermal fluctuations? Astrophys. J. 698, 895–910 (2009)

    ADS  Article  Google Scholar 

  2. 2

    Pojmanski, G. The All Sky Automated Survey. Catalog of Variable Stars. I. 0 h – 6 h Quarter of the Southern Hemisphere. Acta Astronom. 52, 397–427 (2002)

    ADS  Google Scholar 

  3. 3

    Udalski, A. et al. The optical gravitational lensing experiment. Discovery of the first candidate microlensing event in the direction of the Galactic Bulge. Acta Astronom. 43, 289–294 (1993)

    ADS  Google Scholar 

  4. 4

    Rau, A. et al. Exploring the optical transient sky with the Palomar Transient Factory. Publ. Astron. Soc. Pacif. 121, 1334–1351 (2009)

    ADS  Article  Google Scholar 

  5. 5

    Sesar, B. et al. Exploring the variable sky with LINEAR. I. Photometric recalibration with the Sloan Digital Sky Survey. Astron. J. 142, 190–202 (2011)

    ADS  Article  Google Scholar 

  6. 6

    Valtonen, M. J. et al. A massive binary black-hole system in OJ287 and a test of general relativity. Nature 452, 851–853 (2008)

    ADS  CAS  PubMed  Article  Google Scholar 

  7. 7

    Valtonen, M. J., Lehto, H. J., Takalo, L. O. & Sillanpää, A. Testing the 1995 binary black hole model of OJ287. Astrophys. J. 729, 33–38 (2011)

    ADS  Article  Google Scholar 

  8. 8

    Ju, W., Greene, J. E., Rafikov, R. R., Bickerton, S. J. & Badenes, C. Search for supermassive black hole binaries in the Sloan Digital Sky Survey spectroscopic sample. Astrophys. J. 777, 44–59 (2013)

    ADS  Article  CAS  Google Scholar 

  9. 9

    Shen, Y., Liu, X., Loeb, A. & Tremaine, S. Constraining sub-parsec binary supermassive black holes in quasars with multi-epoch spectroscopy. I. The general quasar population. Astrophys. J. 775, 49–71 (2013)

    ADS  Article  CAS  Google Scholar 

  10. 10

    Tsalmantza, P., Decarli, R., Dotti, M. & Hogg, D. W. A systematic search for massive black hole binaries in the Sloan Digital Sky Survey spectroscopic sample. Astrophys. J. 738, 20–28 (2011)

    ADS  Article  CAS  Google Scholar 

  11. 11

    Drake, A. J. et al. First results from the Catalina Real-Time Transient Survey. Astrophys. J. 696, 870–884 (2009)

    ADS  Article  Google Scholar 

  12. 12

    Djorgovski, S. G. et al. in The First Year of MAXI: Monitoring Variable X-ray Sources (eds Mihara, T. & Kawai, N. ) 263–268 (JAXA Special Publication, 2010); available at

  13. 13

    Mahabal, A. A. et al. Discovery, classification, and scientific exploration of transient events from the Catalina Real-time Transient Survey. Bull. Astron. Soc. India 39, 387–408 (2011)

    ADS  Google Scholar 

  14. 14

    Hinshaw, G. et al. Nine-year Wilkinson Microwave Anisotropy Probe (WMAP) observations: cosmological parameter results. Astrophys. J. 208 (suppl.). 19–43 (2013)

    Article  Google Scholar 

  15. 15

    Garcia, A., Sodré, L., Jablonski, F. J. & Terlevich, R. J. Optical monitoring of quasars — I. Variability. Mon. Not. R. Astron. Soc. 309, 803–816 (1999)

    ADS  Article  Google Scholar 

  16. 16

    Eggers, D., Shaffer, D. B. & Weistrop, D. Optical variability of radio-luminous PG quasars. Astron. J. 119, 460–468 (2000)

    ADS  Article  Google Scholar 

  17. 17

    Bahcall, J. N., Kirhakos, S. & Schneider, D. P. Hubble Space Telescope images of nearby luminous quasars. II. Results for eight quasars and tests of the detection sensitivity. Astrophys. J. 450, 486–500 (1995)

    ADS  Article  Google Scholar 

  18. 18

    McLure, R. J. et al. A comparative HST imaging study of the host galaxies of radio-quiet quasars, radio-loud quasars and radio galaxies — I. Mon. Not. R. Astron. Soc. 308, 377–404 (1999)

    ADS  CAS  Article  Google Scholar 

  19. 19

    Hutchings, J. B., Morris, S. C., Gower, A. C. & Lister, M. L. Correlated optical and radio structure in the QSO 1302–102. Publ. Astron. Soc. Pacif. 106, 642–645 (1994)

    ADS  Article  Google Scholar 

  20. 20

    Benítez, E. et al. The close environment of OJ 287: underlying nebulosity and a possible optical jet? Astrophys. J. 464, L47–L50 (1996)

    ADS  Article  Google Scholar 

  21. 21

    Jackson, N. et al. Monitoring of active galactic nuclei. I — The quasars 1302-102 and 1217+023. Astron. Astrophys. 262, 17–25 (1992)

    ADS  CAS  Google Scholar 

  22. 22

    Shen, Y. & Loeb, A. Identifying supermassive black hole binaries with broad emission line diagnosis. Astrophys. J. 725, 249–260 (2010)

    ADS  Article  Google Scholar 

  23. 23

    Gaskell, C. M. Close supermassive binary black holes. Nature 463, E1 (2010)

    ADS  CAS  PubMed  Article  PubMed Central  Google Scholar 

  24. 24

    Lu, J.-F. & Zhou, B.-Y. Observational evidence of jet precession in galactic nuclei caused by accretion disks. Astrophys. J. 635, L17–L20 (2005)

    ADS  CAS  Article  Google Scholar 

  25. 25

    Shen, Y. et al. A catalog of quasar properties from Sloan Digital Sky Survey data release 7. Astrophys. J. 194 (suppl.). 45–65 (2011)

    ADS  Article  CAS  Google Scholar 

  26. 26

    Farris, B. D., Duffell, P., MacFadyen, A. I. & Haiman, Z. Binary black hole accretion from a circumbinary disk: gas dynamics inside the central cavity. Astrophys. J. 783, 134–145 (2014)

    ADS  Article  Google Scholar 

  27. 27

    Rafikov, R. R. Structure and evolution of circumbinary disks around supermassive black hole binaries. Astrophys. J. 774, 144 (2013)

    ADS  Article  Google Scholar 

  28. 28

    Tremaine, S. & Davis, S. W. Dynamics of warped accretion discs. Mon. Not. R. Astron. Soc. 441, 1408–1434 (2014)

    ADS  Article  Google Scholar 

  29. 29

    Bogdanović, T., Smith, B. D., Sigurdsson, S. & Eracleous, M. Modeling of emission signatures of massive black hole binaries. I. Methods. Astrophys. J. 174 (suppl.). 455–480 (2008)

    ADS  Article  Google Scholar 

  30. 30

    Skrutskie, M. F. et al. The Two Micron All Sky Survey. 2MASS. Astron. J. 131, 1163–1183 (2006)

    ADS  Article  Google Scholar 

  31. 31

    Larson, S. et al. The CSS and SSS NEO surveys. Bull. Am. Astron. Soc. 35, 982 (2003)

    ADS  Google Scholar 

  32. 32

    Landolt, A. U. UBVRI photometric standard stars around the sky at −50° declination. Astron. J. 133, 2502–2523 (2007)

    ADS  Article  Google Scholar 

  33. 33

    Landolt, A. U. UBVRI photometric standard stars around the celestial equator: updates and additions. Astron. J. 137, 4186–4269 (2009)

    ADS  Article  Google Scholar 

  34. 34

    Foster, G. Wavelets for period analysis of unevenly sampled time series. Astron. J. 112, 1709–1729 (1996)

    ADS  Article  Google Scholar 

  35. 35

    Alexander, T. Improved AGN light curve analysis with the z-transformed discrete correlation function. Preprint at (2013)

  36. 36

    McQuillan, A., Aigrain, S. & Mazeh, T. Measuring the rotation period distribution of field M dwarfs with Kepler. Mon. Not. R. Astron. Soc. 432, 1203–1216 (2013)

    ADS  Article  Google Scholar 

  37. 37

    MacLeod, C. L. et al. Modeling the time variability of SDSS Stripe 82 quasars as a damped random walk. Astrophys. J. 721, 1014–1033 (2010)

    ADS  CAS  Article  Google Scholar 

  38. 38

    Shen, Y. et al. A catalog of quasar properties from Sloan Digital Sky Survey data release 7. Astrophys. J. 194 (Supp.). 45–65 (2011)

    ADS  Article  CAS  Google Scholar 

  39. 39

    Haiman, Z., Kocsis, B. & Menou, K. The population of viscosity- and gravitational wave-driven supermassive black hole binaries among luminous active galactic nuclei. Astrophys. J. 700, 1952–1969 (2009)

    ADS  Article  Google Scholar 

  40. 40

    Volonteri, M., Miller, J. M. & Dotti, M. Sub-parsec supermassive binary quasars: expectations at z<1. Astrophys. J. 703, L86–L89 (2009)

    ADS  Article  Google Scholar 

  41. 41

    Oke, J. B. et al. The Keck Low-Resolution Imaging Spectrometer. Publ. Astron. Soc. Pacif. 107, 375–385 (1995)

    ADS  Google Scholar 

  42. 42

    Cushing, M. C., Vacca, W. D. & Rayner, J. T. Spextool: a spectral extraction package for SpeX, a 0.8-5.5 micron cross-dispersed spectrograph. Publ. Astron. Soc. Pacif. 116, 362–376 (2004)

    ADS  Article  Google Scholar 

  43. 43

    Vacca, W. D., Cushing, M. C. & Rayner, J. T. A method of correcting near-infrared spectra for telluric absorption. Publ. Astron. Soc. Pacif. 115, 389–409 (2003)

    ADS  Article  Google Scholar 

  44. 44

    Shen, Y. The mass of quasars. Bull. Astron. Soc. India 41, 61–115 (2013)

    ADS  CAS  Google Scholar 

  45. 45

    Kim, D., Im, M. & Kim, M. New estimators of black hole mass in active galactic nuclei with hydrogen Paschen lines. Astrophys. J. 724, 386–399 (2010)

    ADS  CAS  Article  Google Scholar 

  46. 46

    Greene, J. E. & Ho, L. C. Active galactic nuclei with candidate intermediate-mass black holes. Astrophys. J. 610, 722–736 (2004)

    ADS  CAS  Article  Google Scholar 

  47. 47

    Glikman, E. et al. The FIRST-2MASS red quasar survey. Astrophys. J. 667, 673–703 (2007)

    ADS  CAS  Article  Google Scholar 

  48. 48

    Raiteri, C. M. et al. Optical and radio variability of the BL Lacertae object AO 0235+16: a possible 5-6 year periodicity. Astron. Astrophys. 377, 396–412 (2001)

    ADS  Article  Google Scholar 

  49. 49

    Fan, J. H. et al. Optical periodicity analysis for radio selected BL Lacertae objects (RBLs). Astron. Astrophys. 381, 1–5 (2002)

    ADS  Article  Google Scholar 

  50. 50

    Kudryavtseva, N. A. et al. A possible jet precession in the periodic quasar B0605-085. Astron. Astrophys. 526, A51–A64 (2011)

    Article  Google Scholar 

  51. 51

    Greenhill, L. J. et al. A warped accretion disk and wide-angle outflow in the inner parsec of the Circinus Galaxy. Astrophys. J. 590, 162–173 (2003)

    ADS  CAS  Article  Google Scholar 

  52. 52

    Herrnstein, J. R., Moran, J. M., Greenhill, L. J. & Trotter, A. S. The geometry of and mass accretion rate through the maser accretion disk in NGC 4258. Astrophys. J. 629, 719–738 (2005)

    ADS  CAS  Article  Google Scholar 

  53. 53

    Kondratko, P. T., Greenhill, L. J. & Moran, J. M. The parsec-scale accretion disk in NGC 3393. Astrophys. J. 678, 87–95 (2008)

    ADS  CAS  Article  Google Scholar 

  54. 54

    Kuo, C. Y. et al. The Megamaser Cosmology Project. III. Accurate masses of seven supermassive black holes in active galaxies with circumnuclear megamaser disks. Astrophys. J. 727, 20–34 (2011)

    ADS  Article  Google Scholar 

Download references


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.

Author information




M.J.G. performed the analysis and wrote the paper. S.G.D. is the PI of the CRTS survey and obtained the Keck spectrum. E.G. obtained and reduced the near-infrared data and provided the Balmer and Paschen line fits. D.S. reduced the Keck data. A.J.D. is the co-PI of the CRTS survey and provided the CRTS data. S.L. and E.C. are the PIs of the CSS survey. All authors contributed to the text.

Corresponding author

Correspondence to Matthew J. Graham.

Ethics declarations

Competing interests

The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 The optical light curves of quasars showing radio periodicity.

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.

Extended Data Figure 2 The optical light curves of quasars with warped accretion disks.

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.

PowerPoint slides

Rights and permissions

Reprints and Permissions

About this article

Verify currency and authenticity via CrossMark

Cite this article

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).

Download citation

Further reading


By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.


Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing