The multiwavelength behaviour of BL Lacertae explained by a wiggling filamentary jet

Abstract Blazars are active galactic nuclei that stand out for their extreme flux variability at all frequencies, from the radio to the γ-ray band. Their emission is dominated by the non-thermal radiation coming from a relativistic jet pointing close to the line of sight, with consequent Doppler beaming. This in particular implies an apparent increase of the source brightness and a shortening of the variability time scales. Many authors have explained the flares observed in the blazar light curves in terms of intrinsic energetic processes such as shocks propagating in the jet, magnetic reconnection, and turbulence. Alternative interpretations take into consideration possible geometric effects, due to changes in the orientation of the jet, as in the case of the twisting jet model proposed in Raiteri et al. (2017, Nature 552, 374). These geometrical models are supported by several observations and numerical simulations. Recent space-VLBI radio images of the source 3C 279 suggest that blazar emission comes from rotating filamentary structures produced by plasma instabilities (Fuentes et al. 2023, Nature Astronomy, 7, 1359). Moreover, mm-VLBI imaging observations of BL Lacertae found evidence of a wiggling helical jet structure close to the core (Kim et al. 2023, Astron. & Astrophys. 680, L3), confirming earlier results (Cohen et al. 2015, Astrophys. J. 803, 3). In 2020–2021 BL Lacertae underwent an extraordinary activity phase leading to its historical optical maximum brightness (Raiteri et al. 2023, Mon. Not. R. Astron. Soc. 522, 102), providing a formidable tool to study blazar variability. In the present work, we explain the multiwavelength behaviour of BL Lacertae in terms of a twisting filamentary jet, whose better alignment with the line of sight produces the observed flares. The exceptionally well-sampled optical light curve by the Whole Earth Blazar Telescope (WEBT) is used together with radio data to reconstruct the motion of the jet emitting regions in time. The γ-ray data from the Fermi satellite confirm the geometrical picture and allow us to assess the synchrotron self-Compton nature of the γ-ray radiation. This means that γ-ray photons are produced by inverse-Compton scattering of optical photons off the same relativistic electrons that are responsible for the optical radiation through a synchrotron process. The flux behaviour of other blazars suggests that the twisting filamentary jet model can provide an explanation for the multiwavelength variability of the whole blazar class.