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Filamentary structures as the origin of blazar jet radio variability


Supermassive black holes at the centre of active galactic nuclei power some of the most luminous objects in the Universe. Typically, very-long-baseline interferometric observations of blazars have revealed only funnel-like morphologies with little information on the internal structure of the ejected plasma or have lacked the dynamic range to reconstruct the extended jet emission. Here we present microarcsecond-scale angular resolution images of the blazar 3C 279 obtained at 22 GHz with the space very-long-baseline interferometry mission RadioAstron, which allowed us to resolve the jet transversely and reveal several filaments produced by plasma instabilities in a kinetically dominated flow. The polarimetric properties derived from our high-angular-resolution and broad-dynamic-range images are consistent with the presence of a helical magnetic field threaded to the jet. We infer a clockwise rotation as seen in the direction of flow motion with an intrinsic helix pitch angle of ~45° and a Lorentz factor of ~13 at the time of observation. We also propose a model to explain blazar jet radio variability in which emission features travelling down the jet may manifest as a result of differential Doppler boosting within the filaments, as opposed to the standard shock-in-jet model. Characterizing such variability is particularly important given the relevance of blazar physics from cosmic particle acceleration to standard candles in cosmology.

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Fig. 1: The filamentary structure of the jet in 3C 279 revealed by RadioAstron.
Fig. 2: Analysis of the recovered filamentary structure.
Fig. 3: The proposed model for the internal jet structure in 3C 279.

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Data availability

The pre-processed dataset used for imaging is available via Github at

Code availability

The software packages used to calibrate, image and analyse the data are available at the following websites: AIPS,; ParselTongue (, DIFMAP (, SMILI (, eht-imaging ( and lmfit (


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We thank L. Hermosa for useful comments on the manuscript. The work at the IAA-CSIC was supported in part by the Spanish Ministerio de Economía y Competitividad (grant numbers AYA2016-80889-P and PID2019-108995GB-C21), the Consejería de Economía, Conocimiento, Empresas y Universidad of the Junta de Andalucía (grant number P18-FR-1769), the Consejo Superior de Investigaciones Científicas (grant number 2019AEP112), the State Agency for Research of the Spanish MCIU through the ‘Center of Excellence Severo Ochoa’ award to the Instituto de Astrofísica de Andalucía (grant number SEV-2017-0709) and grant number CEX2021-001131-S funded by MCIN/AEI/10.13039/501100011033. J.M.M. and M.P. acknowledge support from the Spanish Ministerio de Ciencia through grant number PID2019-107427GB-C33 and from the Generalitat Valenciana through grant number PROMETEU/2019/071. J.M.M. acknowledges additional support from the Spanish Ministerio de Economía y Competitividad through grant number PGC2018-095984-B-l00. M.P. acknowledges additional support from the Spanish Ministerio de Ciencia through grant number PID2019-105510GB-C31. Y.Y.K. was supported by Russian Science Foundation grant number 21-12-00241. A.C. is an Einstein Fellow of the NASA Hubble Fellowship Program (grant number HST-HF2-51431.001-A), awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract number NAS5-26555. J.-Y.K. was supported in this research by the National Research Foundation of Korea (NRF) under a grant funded by the Korean government (Ministry of Science and ICT; grant number 2022R1C1C1005255). Y.M. acknowledges support from the National Natural Science Foundation of China (grant number 12273022) and the Shanghai pilot programme of international scientist for basic research (grant number 22JC1410600). T.S. was supported by the Academy of Finland projects 274477, 284495, 312496 and 315721. The RadioAstron project is led by the Astro Space Center of the Lebedev Physical Institute of the Russian Academy of Sciences and the Lavochkin Scientific and Production Association under a contract with the Russian Federal Space Agency, in collaboration with partner organizations in Russia and other countries. The European VLBI Network is a joint facility of independent European, African, Asian, and North American radio astronomy institutes. Scientific results from data presented in this publication are derived from the EVN project code GA030D. This research is partly based on observations with the 100 m telescope of the MPIfR at Effelsberg. This publication makes use of data obtained at Metsähovi Radio Observatory, operated by Aalto University in Finland. Our special thanks go to the people supporting the observations at the telescopes during the data collection. This research is based on observations correlated at the Bonn Correlator, jointly operated by the Max-Planck-Institut für Radioastronomie, and the Federal Agency for Cartography and Geodesy. This study makes use of 43 GHz VLBA data from the VLBA-BU Blazar Monitoring Program (VLBA-BU-BLAZAR;, funded by NASA through Fermi Guest Investigator grant number 80NSSC20K1567.

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Authors and Affiliations



A.F., J.L.G. and G.Y.-Z. worked on the data calibration. A.F., J.L.G., G.Y.-Z., R.L., A.C., K.A., K.L.B, H.S., I.C. and E.T. worked on the image reconstruction and analysis. G.B. correlated the space VLBI data. J.M.M., M.P., A.F., J.L.G. and Y.M worked on the interpretation of the results. All authors contributed to the discussion of the results presented and commented on the manuscript.

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Correspondence to Antonio Fuentes or José L. Gómez.

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Extended data

Extended Data Fig. 1 Baseline coverage for our RadioAstron observations of 3C 279 in March 2014.

Rainbow-coloured and grey points indicate individual ground-ground baselines and space-ground baselines, respectively. Dashed circles indicate the baseline length in Earth’s diameter units (D) and the corresponding angular resolution.

Extended Data Fig. 2 Fitting of the polarimetric RadioAstron image to a selection of data products.

Two minute time-averaged data (black points) and image model (red points) self-calibrated visibility amplitudes and phases, closure phases, log closure amplitudes, and polarimetric visibility phases as a function of time. Error bars indicate ± 1σ uncertainty from thermal noise plus 1.5 % non-closing error uncertainties added in quadrature. All these examples include RadioAstron measurements.

Extended Data Fig. 3 Top 48 image reconstructions from the parameter survey conducted.

Each image includes the closure phase (cp) and log closure amplitude (lca) reduced χ2, the image regularizers used and their weight, and the total flux reconstructed.

Extended Data Fig. 4 Synthetic data tests.

In the top row we present the geometric models used to generate the synthetic data. In the middle and bottom rows we show, respectively, the images reconstructed from each data set when RadioAstron is included in the array and when only ground stations participate.

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Fuentes, A., Gómez, J.L., Martí, J.M. et al. Filamentary structures as the origin of blazar jet radio variability. Nat Astron 7, 1359–1367 (2023).

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