Broadband multiwavelength properties of the archetypal blazar 3C 279 during the 2017 Event Horizon Telescope campaign

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Context. The archetypal blazar 3C 279 has a prominent relativistic jet and strong broadband variability across the electromagnetic spectrum. In April 2017, the Event Horizon Telescope (EHT) observed 3C 279 with an unprecedented angular resolution of about 20 μas, accompanied by one of the most extensive quasi-simultaneous multiwavelength (MWL) campaigns ever conducted, spanning from radio to TeV γ -ray energies. Aims. Taking advantage of this comprehensive MWL dataset, we investigated the physical processes governing 3C 279, with a particular focus on the formation, collimation, and acceleration of its relativistic jet and on the origin of its high-energy emission, including the underlying particle-acceleration mechanisms. Methods. We analyzed individual observations and multiband light curves. We also constructed a new quasi-simultaneous spectral energy distribution covering frequencies from the radio band to very high-energy (VHE) γ rays. We further performed a phenomenological modeling using the turbulent extreme multi-zone (TEMZ) model to constrain the fundamental physical properties of the source. Results. The EHT observations reveal a clear flux increase in the innermost core between April 5 and 11, 2017. Over a broader time span, radio observations at longer wavelengths reveal concurrent enhancements in the core flux and polarization around mid-April, coinciding with the ejection of a superluminal knot moving at (25 ± 2) c . Record UV–optical flares with strong polarization variability occurred in late March, followed by high-energy γ -ray activity that declined before the end of the EHT observing period. During this time, the source remained in a low X-ray state and exhibited no detectable VHE emission. Conclusions. The results of the TEMZ modeling indicate that the broadband spectrum and variability of 3C 279 might be explained with a jet scenario in which turbulent plasma cells are compressed by a stationary conical shock. Nonetheless, alternative interpretations, such as magnetic reconnection or a moving shock-in-jet event, remain possible. This coordinated MWL campaign advances our understanding of the origin of the jet and γ -ray emission in the blazar 3C 279, and it also provides a comprehensive publicly available dataset that will serve as a valuable reference for future studies.

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