Letter

Blazar spectral variability as explained by a twisted inhomogeneous jet

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Abstract

Blazars are active galactic nuclei, which are powerful sources of radiation whose central engine is located in the core of the host galaxy. Blazar emission is dominated by non-thermal radiation from a jet that moves relativistically towards us, and therefore undergoes Doppler beaming1. This beaming causes flux enhancement and contraction of the variability timescales, so that most blazars appear as luminous sources characterized by noticeable and fast changes in brightness at all frequencies. The mechanism that produces this unpredictable variability is under debate, but proposed mechanisms include injection, acceleration and cooling of particles2, with possible intervention of shock waves3,4 or turbulence5. Changes in the viewing angle of the observed emitting knots or jet regions have also been suggested as an explanation of flaring events6,7,8,9,10 and can also explain specific properties of blazar emission, such as intra-day variability11, quasi-periodicity12,13 and the delay of radio flux variations relative to optical changes14. Such a geometric interpretation, however, is not universally accepted because alternative explanations based on changes in physical conditions—such as the size and speed of the emitting zone, the magnetic field, the number of emitting particles and their energy distribution—can explain snapshots of the spectral behaviour of blazars in many cases15,16. Here we report the results of optical-to-radio-wavelength monitoring of the blazar CTA 102 and show that the observed long-term trends of the flux and spectral variability are best explained by an inhomogeneous, curved jet that undergoes changes in orientation over time. We propose that magnetohydrodynamic instabilities17 or rotation of the twisted jet6 cause different jet regions to change their orientation and hence their relative Doppler factors. In particular, the extreme optical outburst of 2016–2017 (brightness increase of six magnitudes) occurred when the corresponding emitting region had a small viewing angle. The agreement between observations and theoretical predictions can be seen as further validation of the relativistic beaming theory.

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Acknowledgements

The Acknowledgements are listed in the Supplementary Information.

Author information

Affiliations

  1. INAF, Osservatorio Astrofisico di Torino, I-10025 Pino Torinese, Italy

    • C. M. Raiteri
    • , M. Villata
    •  & M. I. Carnerero
  2. Instituto de Astrofisica de Canarias (IAC), La Laguna, E-38200 Tenerife, Spain

    • J. A. Acosta-Pulido
    • , W. Boschin
    • , C. Lázaro
    • , F. Pinna
    • , C. Protasio
    • , F. J. Redondo-Lorenzo
    •  & G. Rodriguez-Coira
  3. Departamento de Astrofisica, Universidad de La Laguna, La Laguna, E-38205 Tenerife, Spain

    • J. A. Acosta-Pulido
    • , W. Boschin
    • , N. Castro-Segura
    • , C. Lázaro
    • , F. Pinna
    • , C. Protasio
    • , F. J. Redondo-Lorenzo
    •  & G. Rodriguez-Coira
  4. Instituto de Astrofísica de Andalucía (CSIC), E-18080 Granada, Spain

    • I. Agudo
    • , C. Casadio
    • , A. Fuentes
    • , J. L. Gómez
    •  & S. N. Molina
  5. Pulkovo Observatory, 196140 St Petersburg, Russia

    • A. A. Arkharov
    • , N. V. Efimova
    • , V. M. Larionov
    •  & A. A. Mokrushina
  6. Institute of Astronomy and NAO, Bulgarian Academy of Sciences, 1784 Sofia, Bulgaria

    • R. Bachev
    • , B. Mihov
    • , E. Semkov
    • , L. Slavcheva-Mihova
    •  & A. Strigachev
  7. Crimean Astrophysical Observatory RAS, Nauchny 298409, Russia

    • G. V. Baida
    • , G. A. Borman
    • , M. S. Butuzova
    • , S. V. Nazarov
    •  & D. N. Okhmat
  8. Instituto de Astronomía, Universidad Nacional Autónoma de México, Mexico

    • E. Benítez
    • , J. Echevarría
    • , C. Espinosa
    • , D. Hiriart
    •  & R. Michel
  9. INAF, TNG Fundación Galileo Galilei, E-38712 La Palma, Spain

    • W. Boschin
    •  & D. Carosati
  10. Department of Astronomy, Faculty of Physics, University of Sofia, BG-1164 Sofia, Bulgaria

    • V. Bozhilov
    • , M. Minev
    •  & E. Ovcharov
  11. Osservatorio Astronomico della Regione Autonoma Valle d’Aosta, I-11020 Nus, Italy

    • P. Calcidese
  12. EPT Observatories, Tijarafe, E-38780 La Palma, Spain

    • D. Carosati
  13. Max-Planck-Institut für Radioastronomie, D–53121 Bonn, Germany

    • C. Casadio
  14. School of Physics and Astronomy, University of Southampton, Southampton SO17 1BJ, UK

    • N. Castro-Segura
  15. Graduate Institute of Astronomy, National Central University, Jhongli City, Taoyuan County 32001, Taiwan

    • W.-P. Chen
    • , C. S. Lin
    •  & M. R. Samal
  16. Astronomical Observatory, 11060 Belgrade, Serbia

    • G. Damljanovic
    •  & O. Vince
  17. Dipartimento di Fisica e Astronomia, Università di Bologna, I-40129 Bologna, Italy

    • F. D’Ammando
  18. INAF, Istituto di Radioastronomia, I-40129 Bologna, Italy

    • F. D’Ammando
  19. INAF, Osservatorio Astronomico di Roma, I-00040 Monte Porzio Catone, Italy

    • A. Di Paola
    •  & A. Giunta
  20. Ulugh Beg Astronomical Institute, Maidanak Observatory, Tashkent 100052, Uzbekistan

    • Sh. A. Ehgamberdiev
    •  & D. O. Mirzaqulov
  21. Astronomical Institute, St Petersburg State University, 198504 St Petersburg, Russia

    • T. S. Grishina
    • , S. G. Jorstad
    • , E. N. Kopatskaya
    • , V. M. Larionov
    • , E. G. Larionova
    • , L. V. Larionova
    • , A. A. Mokrushina
    • , D. A. Morozova
    • , S. S. Savchenko
    • , Yu. V. Troitskaya
    • , I. S. Troitsky
    •  & A. A. Vasilyev
  22. Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts 02138, USA

    • M. A. Gurwell
  23. Astrophysics Research Institute, Liverpool John Moores University, Liverpool L3 5RF, UK

    • H. Jermak
    •  & I. A. Steele
  24. School of Cosmic Physics, Dublin Institute For Advanced Studies, Dublin, Ireland

    • B. Jordan
  25. Institute for Astrophysical Research, Boston University, Boston, Massachusetts 02215, USA

    • S. G. Jorstad
    • , M. Joshi
    • , M. P. Malmrose
    •  & A. P. Marscher
  26. NNLOT, Al-Farabi Kazakh National University, Almaty, Kazakhstan

    • K. Kuratov
  27. Fesenkov Astrophysical Institute, Almaty, Kazakhstan

    • K. Kuratov
  28. Abastumani Observatory, Mt Kanobili, 0301 Abastumani, Georgia

    • O. M. Kurtanidze
    • , S. O. Kurtanidze
    •  & M. G. Nikolashvili
  29. Engelhardt Astronomical Observatory, Kazan Federal University, Tatarstan, Russia

    • O. M. Kurtanidze
  30. Landessternwarte, Zentrum für Astronomie der Universität Heidelberg, 69117 Heidelberg, Germany

    • O. M. Kurtanidze
  31. Center for Astrophysics, Guangzhou University, Guangzhou 510006, China

    • O. M. Kurtanidze
  32. Aalto University Metsähovi Radio Observatory, FI-02540 Kylmälä, Finland

    • A. Lähteenmäki
    • , J. Tammi
    •  & M. Tornikoski
  33. Aalto University Department of Electronics and Nanoengineering, FI-00076 Aalto, Finland

    • A. Lähteenmäki
  34. Tartu Observatory, 61602 Tõravere, Estonia

    • A. Lähteenmäki
  35. Astronomical Institute, Osaka Kyoiku University, Osaka 582-8582, Japan

    • K. Matsumoto
    •  & K. Sadakane
  36. School of Physics, University College Dublin, Dublin 4, Ireland

    • B. McBreen
  37. Department of Physics and Astronomy, Brigham Young University, Provo, Utah 84602, USA

    • J. W. Moody
  38. Michael Adrian Observatorium, Astronomie Stiftung Trebur, 65468 Trebur, Germany

    • J. M. Ohlert
  39. University of Applied Sciences, Technische Hochschule Mittelhessen, 61169 Friedberg, Germany

    • J. M. Ohlert
  40. Command Module Observatory, Tempe, Arizona, USA

    • T. A. Polakis
  41. Nordic Optical Telescope, E-38700 Santa Cruz de La Palma, Spain

    • T. Pursimo
  42. Osservatorio Astronomico Sirio, I-70013 Castellana Grotte, Italy

    • N. Rizzi
  43. Department of Physics, University of Colorado Denver, Denver, Colorado 80217-3364 USA

    • A. C. Sadun
  44. Lowell Observatory, Flagstaff, Arizona, USA

    • B. A. Skiff
  45. Steward Observatory, University of Arizona, Tucson, Arizona, USA

    • P. S. Smith
  46. Instituto de Radio Astronomía Milimétrica, E-18012 Granada, Spain

    • C. Thum

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Contributions

C.M.R. and M.V. managed the WEBT observing campaign, analysed the data, developed the geometric interpretation and wrote the manuscript. J.A.A.-P., A.A.A., M.I.C., N.C.-S., N.V.E., A.D.P., A.G., C.L., F.P., C.P., F.J.R.-L. and G.R.-C. performed near-infrared and optical observations and the related data reduction. I.A., C.C., A.F., J.L.G. and S.N.M. performed photometric and polarimetric optical and radio observations and the related data reduction. E.B., J.E., C.E., T.S.G., D.H., S.G.J., M.J., E.N.K., V.M.L., E.G.L., L.V.L., M.P.M., A.P.M., R.M., A.A.M., J.W.M., D.A.M., S.S.S., Yu.V.T., I.S.T. and A.A.V. acquired and reduced optical photometric and polarimetric data. R.B., G.V.B., G.A.B., V.B., M.S.B., P.C., D.C., W.-P.C., G.D., Sh.A.E., H.J., B.J., K.K., O.M.K., S.O.K., C.S.L., K.M., B.McB., B.Mi., M.M., D.O.M., S.V.N., M.G.N., J.M.O., D.N.O., E.O., T.A.P., N.R., K.S., A.C.S., M.R.S., E.S., B.A.S., L.S.-M., I.A.S., A.S. and O.V. carried out optical observations and the related data reduction. M.A.G., A.L., J.T., C.T. and M.T. performed radio observations and the related data reduction. W.B. acquired and reduced optical spectra. T.P. made optical photometric and spectroscopic observations and the related data reduction. P.S.S. carried out optical photometric, polarimetric and spectroscopic observations and reduced the data. F.D. and all the above authors reviewed and contributed to the manuscript.

Competing interests

The authors declare no competing financial interests.

Corresponding authors

Correspondence to C. M. Raiteri or M. Villata.

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    Supplementary Information

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