2025A&A...704A.208A

Context. A recent study on the dynamics of the quasi-stationary component (QSC) in the jet of BL Lacertae highlighted its significance in evaluating the physical properties of relativistic transverse waves in the parsec-scale jet. Motivated by this finding, we selected a different type of blazar, the flat-spectrum radio quasar (FSRQ) 3C 279, which hosts a QSC at an angular median distance of 0.35 mas from the radio core, as has been revealed by 27 years of VLBA monitoring data at 15 GHz.
Aims. We investigate the positional scatter and dynamics of a QSC in the 3C 279 jet, aiming to detect the presence of a relativistic transverse wave and estimate its characteristics.
Methods. We employed an analytical statistical method to estimate the mean intrinsic speed of the QSC, while moving average and refinement methods were used to smooth its trajectory.
Results. Analysis of the QSC position scatter shows that the jet axis changes its direction by about 21° over 27 years and jet mean intrinsic full opening angle is ≈0.30° ±0.03°. The apparent displacement vectors of the QSC exhibit strong asymmetry and anisotropy in the direction of the jet, indicating pronounced anisotropic displacements of the core along the jet axis. We estimated the mean intrinsic speed of the QSC to be superluminal, $ \overline{\beta_{\mathrm{s}}} \approx 10 $ in units of the speed of light, which, within the framework of the seagull-on-wave model, is interpreted as evidence of a relativistic transverse wave propagating through the QSC. Analysis of the reversing trajectory of the QSC enables the classification and characterization of reversal patterns, which, in turn, allows us to determine key transverse wave parameters such as the frequency, amplitude, inclination angle, and magnetic energy of the wave.
Conclusions. Analysing the variations in the QSC position and dynamics provides a powerful diagnostic for determining changes in the jet cone angle and direction, for detecting relativistic transverse waves in the jets of two types of blazars, BL Lacertae and the FSRQ 3C 279, and for characterising the properties of these transverse waves.