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.

Direct evidence of non-disk optical continuum emission around an active black hole


Accretion onto black holes is key to their growth over cosmic time1, especially during the active galactic nuclei phase when the inflowing material forms a radiatively efficient accretion disk2. To probe the disk, indirect imaging methods such as reverberation mapping3,4,5,6 and microlensing7,8 are required. Recent findings suggest that the disk may be larger than theoretical predictions by a factor of a few4,6,9, thus casting doubt on our understanding of accretion in the general astrophysical context. Whether new physics is implied10,11,12 or poorly understood biases are in effect5,6,13,14 is a longstanding question. Here, we report new reverberation data based on a unique narrowband-imaging design15, and argue that time delays between adjacent optical bands are primarily associated with the reprocessing of light by a farther away under-appreciated non-disk component. This component is associated with high-density photoionized material that is uplifted from the outer accretion disk, probably by radiation-pressure force on dust, and thus may represent the long-sought origin of the broad-line region16. Our findings suggest that the optical phenomenology of some active galactic nuclei may be substantially affected by non-disk continuum emission with implications for measuring the fundamental properties of black holes and their active environs over cosmic time.

Access options

Rent or Buy article

Get time limited or full article access on ReadCube.


All prices are NET prices.

Fig. 1: Intermediate band light curves for Mrk 279.
Fig. 2: Continuum time delays.
Fig. 3: Spectral decomposition of the incident and delayed continuum components.
Fig. 4: A qualitative geometry for the central engines of AGN that is consistent with the reverberation mapping findings for Mrk 279.

Data availability

The data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request.


  1. 1.

    Yu, Q. & Tremaine, S. Observational constraints on growth of massive black holes. Mon. Not. R. Astron. Soc. 335, 965–976 (2002).

    ADS  Article  Google Scholar 

  2. 2.

    Shakura, N. I. & Sunyaev, R. A. Black holes in binary systems. Observational appearance. Astron. Astrophys. 24, 337–355 (1973).

    ADS  Google Scholar 

  3. 3.

    Collier, S. J. et al. Steps toward determination of the size and structure of the broad-line region in active galactic nuclei. XIV. Intensive optical spectrophotometric observations of NGC 7469. Astrophys. J. 500, 162–172 (1998).

    ADS  Article  Google Scholar 

  4. 4.

    Fausnaugh, M. M. et al. Space telescope and optical reverberation mapping project. III. Optical continuum emission and broadband time delays in NGC 5548. Astrophys. J. 821, 56 (2016).

    ADS  Article  Google Scholar 

  5. 5.

    Edelson, R. et al. Space telescope and optical reverberation mapping project. II. Swift and HST reverberation mapping of the accretion disk of NGC 5548. Astrophys. J. 806, 129 (2015).

    ADS  Article  Google Scholar 

  6. 6.

    Cackett, E. M. et al. Accretion disk reverberation with Hubble Space Telescope observations of NGC 4593: evidence for diffuse continuum lags. Astrophys. J. 857, 53 (2018).

    ADS  Article  Google Scholar 

  7. 7.

    Blackburne, J. A., Pooley, D., Rappaport, S. & Schechter, P. L. Sizes and temperature profiles of quasar accretion disks from chromatic microlensing. Astrophys. J. 729, 34 (2011).

    ADS  Article  Google Scholar 

  8. 8.

    Jiménez-Vicente, J. et al. The average size and temperature profile of quasar accretion disks. Astrophys. J. 783, 47 (2014).

    ADS  Article  Google Scholar 

  9. 9.

    Morgan, C. W., Kochanek, C. S., Morgan, N. D. & Falco, E. E. The quasar accretion disk size—black hole mass relation. Astrophys. J. 712, 1129–1136 (2010).

    ADS  Article  Google Scholar 

  10. 10.

    Dexter, J. & Agol, E. Quasar accretion disks are strongly inhomogeneous. Astrophys. J. Lett. 727, L24 (2011).

    ADS  Article  Google Scholar 

  11. 11.

    Gardner, E. & Done, C. The origin of the UV/optical lags in NGC 5548. Mon. Not. R. Astron. Soc. 470, 3591–3605 (2017).

    ADS  Article  Google Scholar 

  12. 12.

    Hall, P. B., Sarrouh, G. T. & Horne, K. Non-blackbody disks can help explain inferred AGN accretion disk sizes. Astrophys. J. 854, 93 (2018).

    ADS  Article  Google Scholar 

  13. 13.

    Korista, K T. & Goad, M. R. The variable diffuse continuum emission of broad-line clouds. Astrophys. J. 553, 695–708 (2001).

    ADS  Article  Google Scholar 

  14. 14.

    McHardy, I. et al. X-ray/UV/optical variability of NGC 4593 with swift: reprocessing of X-rays by an extended reprocessor. Mon. Not. R. Astron. Soc. 480, 2881 (2018).

    ADS  Article  Google Scholar 

  15. 15.

    Pozo Nuñez, F., Chelouche, D., Kaspi, S. & Niv, S. Automatized photometric monitoring of active galactic nuclei with the 46 cm telescope of the wise observatory. Publ. Astron. Soc. Pac. 129, 094101 (2017).

    ADS  Article  Google Scholar 

  16. 16.

    Czerny, B. & Hryniewicz, K. The origin of the broad line region in active galactic nuclei. Astron. Astrophys. 525, L8 (2011).

    ADS  Article  Google Scholar 

  17. 17.

    Chelouche, D. The case for standard irradiated accretion disks in active galactic nuclei. Astrophys. J. 772, 9 (2013).

    ADS  Article  Google Scholar 

  18. 18.

    Gaskell, C. M. The case for cases B and C: intrinsic hydrogen line ratios of the broad-line region of active galactic nuclei, reddenings, and accretion disc sizes. Mon. Not. R. Astron. Soc. 467, 226–238 (2017).

    ADS  Google Scholar 

  19. 19.

    Pancoast, A. et al. Modelling reverberation mapping data—II. Dynamical modelling of the lick AGN monitoring project 2008 data set. Mon. Not. R. Astron. Soc. 445, 3073–3091 (2014).

    ADS  Article  Google Scholar 

  20. 20.

    Santos-Lleó, M. et al. Monitoring of the optical and 2.5–11.7 μm spectrum and mid-IR imaging of the Seyfert 1 galaxy Mrk 279 with ISO. Astron. Astrophys. 369, 57–64 (2001).

    ADS  Article  Google Scholar 

  21. 21.

    Phinney, E. S. Dusty disks and the infrared emission from AGN. NATO ASI C 290, 457 (1989).

    Google Scholar 

  22. 22.

    Sirko, E. & Goodman, J. Spectral energy distributions of marginally self-gravitating quasi-stellar object discs. Mon. Not. R. Astron. Soc. 341, 501–508 (2003).

    ADS  Article  Google Scholar 

  23. 23.

    Bentz, M. & Katz, S. The AGN black hole mass database. Publ. Astron. Soc. Pac. 127, 67 (2015).

    ADS  Article  Google Scholar 

  24. 24.

    Laor, A. & Davis, S. W. Cold accretion discs and lineless quasars. Mon. Not. R. Astron. Soc. 417, 681–688 (2011).

    ADS  Article  Google Scholar 

  25. 25.

    Baskin, A., Laor, A. & Stern, J. Radiation pressure confinement—II. Application to the broad-line region in active galactic nuclei. Mon. Not. R. Astron. Soc. 438, 604–619 (2014).

    ADS  Article  Google Scholar 

  26. 26.

    Ferland, G. J. et al. The 2017 release CLOUDY. Rev. Mex. Astron. Astr. 53, 385–438 (2017).

    ADS  Google Scholar 

  27. 27.

    Czerny, B. et al. Failed radiatively accelerated dusty outflow model of the broad line region in active galactic nuclei. I. Analytical solution. Astrophys. J. 846, 154 (2017).

    ADS  Article  Google Scholar 

  28. 28.

    Bentz, M. C. et al. The low-luminosity end of the radius–luminosity relationship for active galactic nuclei. Astrophys. J. 767, 149 (2013).

    ADS  Article  Google Scholar 

  29. 29.

    Davis, S. W. & Laor, A. The radiative efficiency of accretion flows in individual active galactic nuclei. Astrophys. J. 728, 98 (2011).

    ADS  Article  Google Scholar 

  30. 30.

    Netzer, H. Revisiting the unified model of active galactic nuclei. Annu. Rev. Astron. Astrophys. 53, 365–408 (2015).

    ADS  Article  Google Scholar 

  31. 31.

    Bertin, E. & Arnouts, S. SExtractor: software for source extraction. Astron. Astrophys. Suppl. 117, 393–404 (1996).

    ADS  Article  Google Scholar 

  32. 32.

    Bertin, E. et al. The TERAPIX pipeline. In Astronomical Data Analysis Software and Systems XI (eds Bohlender, D. A., Durand, D. & Handley, T. H.) 228 (Conference Series Volume 281, Astronomical Society of the Pacific, 2002).

  33. 33.

    Chelouche, D., Pozo-Nuñez, F. & Zucker, S. Methods of reverberation mapping. I. Time-lag determination by measures of randomness. Astrophys. J. 844, 146 (2017).

    ADS  Article  Google Scholar 

  34. 34.

    Peterson, B. M. et al. On uncertainties in cross-correlation lags and the reality of wavelength-dependent continuum lags in active galactic nuclei. Publ. Astron. Soc. Pac. 110, 660–670 (1998).

    ADS  Article  Google Scholar 

  35. 35.

    Schlafly, E. F. & Finkbeiner, D. P. Measuring reddening with Sloan Digital Sky Survey stellar spectra and recalibrating SFD. Astrophys. J. 737, 103 (2011).

    ADS  Article  Google Scholar 

  36. 36.

    Choloniewski, J. The shape and variability of the nonthermal component of the optical spectra of active galaxies. Acta Astronom. 31, 293 (1981).

    ADS  Google Scholar 

  37. 37.

    Winkler, H. et al. Variability studies of Seyfert galaxies. I—broad-band optical photometry. Mon. Not. R. Astron. Soc. 257, 659–676 (1992).

    ADS  Article  Google Scholar 

  38. 38.

    Pozo Nuñez, F. et al. Photometric reverberation mapping of 3C 120. Astron. Astrophys. 545, A84 (2012).

    Article  Google Scholar 

  39. 39.

    Sakata, Y. et al. Long-term optical continuum color variability of nearby active galactic nuclei. Astrophys. J. 711, 461–483 (2010).

    ADS  Article  Google Scholar 

  40. 40.

    Kinney, A. et al. Template ultraviolet to near-infrared spectra of star-forming galaxies and their application to K-corrections. Astrophys. J. 467, 38 (1996).

    ADS  Article  Google Scholar 

  41. 41.

    Pogge, R. & Martini, P. Hubble Space Telescope imaging of the circumnuclear environments of the CfA Seyfert galaxies: nuclear spirals and fueling. Astrophys. J. 569, 624–640 (2002).

    ADS  Article  Google Scholar 

  42. 42.

    Peterson, B. M. Reverberation mapping of active galactic nuclei. Publ. Astron. Soc. Pac. 105, 247–268 (1993).

    ADS  Article  Google Scholar 

  43. 43.

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

  44. 44.

    Rybicki, G. B. & Press, W. H. Interpolation, realization, and reconstruction of noisy, irregularly sampled data. Astrophys. J. 398, 169–176 (1992).

    ADS  Article  Google Scholar 

  45. 45.

    Zu, Y., Kochanek, C. S. & Peterson, B. M. An alternative approach to measuring reverberation lags in active galactic nuclei. Astrophys. J. 735, 80 (2011).

    ADS  Article  Google Scholar 

  46. 46.

    Li, Y.-R., Wang, J.-M. & Bai, J.-M. A non-parametric approach to constrain the transfer function in reverberation mapping. Astrophys. J. 831, 206 (2016).

    ADS  Article  Google Scholar 

  47. 47.

    Chelouche, D. & Zucker, S. Quasar cartography: from black hole to broad-line region scales. Astrophys. J. 769, 124 (2013).

    ADS  Article  Google Scholar 

  48. 48.

    Pijpers, F. P. & Wanders, I. Reverberation mapping of active galactic nuclei: the SOLA method for time-series inversion. Mon. Not. R. Astron. Soc. 271, 183–196 (1994).

    ADS  Article  Google Scholar 

  49. 49.

    Welsh, W. F. On the reliability of cross-correlation function lag determinations in active galactic nuclei. Publ. Astron. Soc. Pac. 111, 1347–1366 (1999).

    ADS  Article  Google Scholar 

  50. 50.

    Scott, J. E. et al. Variable intrinsic absorption in Mrk 279. Astrophys. J. 694, 438–448 (2009).

    ADS  Article  Google Scholar 

  51. 51.

    Stevans, M. L., Shull, J. M., Danforth, C. W. & Tilton, E. M. HST-COS observations of AGNs. II. Extended survey of ultraviolet composite spectra from 159 active galactic nuclei. Astrophys. J. 794, 75 (2014).

    ADS  Article  Google Scholar 

  52. 52.

    Mehdipour, M. et al. Multi-wavelength campaign on NGC 7469. III. Spectral energy distribution and the AGN wind photoionisation modelling, plus detection of diffuse X-rays from the starburst with Chandra HETGS. Astron. Astrophys. 615, A72 (2018).

    Article  Google Scholar 

  53. 53.

    Warner, C., Hamann, F. & Dietrich, M. A relation between supermassive black hole mass and quasar metallicity?. Astrophys. J. 596, 72–84 (2003).

    ADS  Article  Google Scholar 

  54. 54.

    Dietrich, M. et al. Continuum and emission-line strength relations for a large active galactic nuclei sample. Astrophys. J. 581, 912–912 (2002).

    ADS  Article  Google Scholar 

  55. 55.

    Maoz, D. et al. High-rate spectroscopic active galactic nucleus monitoring at the Wise Observatory. I—Markarian 279. Astrophys. J. 351, 75–82 (1990).

    ADS  Article  Google Scholar 

  56. 56.

    Stern, J. & Laor, A. Type 1 AGN at low z. I. Emission properties. J. Phys. Conf. Ser. 423, 600–631 (2012).

    Google Scholar 

  57. 57.

    Baldwin, J., Ferland, G., Korista, K. & Verner, D. Locally optimally emitting clouds and the origin of quasar emission lines. Astrophys. J. Lett. 455, L119 (1995).

    ADS  Article  Google Scholar 

Download references


We thank D. Maoz for continuous support of the project at the Wise Observatory, and S. Niv for enabling the robotic use of the C18 telescope. This work was partly supported by grants 950/15 from the Israeli Science Foundation and 3555/14-1 from the Deutsche Forschungsgemeinschaft. Computations on the Hive computer cluster at the University of Haifa are partly supported by Israeli Science Foundation grant 2155/15.

Author information




D.C. conceived the project, post-processed the light curves, carried out time-series analyses and photoionization calculations, and wrote the paper. F.P.N. acquired the data, reduced them, carried out flux variation gradient calculations and time-series analyses, and contributed to writing the paper. S.K. assisted with the data reduction and time-series analyses, and provided technical support of telescope operations.

Corresponding authors

Correspondence to Doron Chelouche or Francisco Pozo Nuñez.

Ethics declarations

Competing interests

The authors declare no competing interests.

Additional information

Publisher’s note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Supplementary information

Supplementary Information

Supplementary Figures 1–10, Supplementary Tables 1–2, Supplementary References 1–3

Rights and permissions

Reprints and Permissions

About this article

Verify currency and authenticity via CrossMark

Cite this article

Chelouche, D., Pozo Nuñez, F. & Kaspi, S. Direct evidence of non-disk optical continuum emission around an active black hole. Nat Astron 3, 251–257 (2019).

Download citation

Further reading


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