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Blazar spectral variability as explained by a twisted inhomogeneous jet


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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Figure 1: Observed optical light curve of CTA 102 in the last two observing seasons of the WEBT campaign.
Figure 2: Spectral energy distributions of CTA 102 and orientation of the emitting regions of the jet.
Figure 3: Multifrequency behaviour of the jet emission of CTA 102 in 2008–2017.
Figure 4: Schematic representation of the proposed inhomogeneous jet model.

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The Acknowledgements are listed in the Supplementary Information.

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Authors and Affiliations



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.

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

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Extended data figures and tables

Extended Data Figure 1 Observed light curves of CTA 102 in the optical BVRI and near-infrared JHK bands.

The curves are built with data from 39 telescopes (marked with different symbols and colours) in 28 observatories participating in the WEBT project. Measurement errors (1 s.d.) are smaller than the symbol size.

Extended Data Figure 2 Four episodes of noticeable and well-sampled intranight variability.

Magnifications of the R-band light curve of Fig. 1 during the phases of the 2016–2017 optical outburst with the most dramatic changes reveal very fast brightness variations. Error bars represent 1 s.d. measurement errors.

Extended Data Figure 3 Colour behaviour of CTA 102.

a, The R-band light curve; red dots represent the data points used to build the colour indices. b–c, The B − R colour index as a function of time and R-band magnitude. Error bars were obtained by summing in quadrature the 1-s.d. measurement errors of the corresponding B and R data. The dashed line indicates the average BR value. The parameter α is the spectral index of the F να law. The redder-when-brighter trend that characterizes faint source states (R > 15) turns into a slight bluer-when-brighter trend as the source flux increases.

Extended Data Figure 4 Optical spectra of CTA 102 in different brightness states.

Data are from the Steward (blue) and Roque de los Muchachos (TNG and NOT; black and red, respectively) observatories and have been corrected for Galactic extinction. The observing epochs are given on the right, expressed in JD − 2,450,000. The main broad emission lines (more visible in faint states) are indicated. As the flux increases, the source spectrum first softens (redder-when-brighter trend) and then gradually hardens (bluer-when-brighter).

Extended Data Figure 5 Results of time-series analysis on the optical fluxes.

a, Structure function of R-band flux densities, corrected for the long-term trend due to variable relativistic beaming (see Fig. 3). b, Autocorrelation function of the same corrected fluxes. τ is the time separation between points, expressed in 1-day bins. Filled blue and empty red symbols refer to bright (more beamed) and faint (less beamed) observed states, respectively, and show that variation timescales are halved when the Doppler factor doubles.

Extended Data Figure 6 Temporal behaviour of the polarization of CTA 102.

a, The jet optical flux densities. b, The jet polarization fraction Pjet. The horizontal dotted line indicates the average value over the whole period and crosses show the mean values in each observing season. Error bars represent 1 s.d. c, The electric vector polarization angle. The red solid line displays the trend of the viewing angle in the R band (rescaled to fit in the plot; see Fig. 3) and the vertical lines mark the most interesting events, which are discussed in the text.

Extended Data Figure 7 One-zone model fits to the SEDs of CTA 102.

The standard one-zone model40 has been used to fit three SEDs in intermediate-, high- and low-brightness states (see also Fig. 2). Once the physical parameters of the emitting zone are adjusted to reproduce the intermediate-brightness state, the other two model fits are obtained by changing only the Doppler factor to match the optical data. As a result, the millimetre-wavelength flux is largely over- or under-estimated. In all model fits, the thermal component (accretion disk and torus; black line and symbols) was added to the one-zone model synchrotron component.

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Raiteri, C., Villata, M., Acosta-Pulido, J. et al. Blazar spectral variability as explained by a twisted inhomogeneous jet. Nature 552, 374–377 (2017).

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