tokens: well-studied extragalactic radio source Cygnus A to demonstrate the applicability of our code. This is one of the nearest Fanaroff–Riley type I (FR II) galaxies and has a well-defined jet and counterjet with a jet inclination angle thought to be between 55 ○ and 85 ○ (Bartel et al. 1995 ), with a redshift of z = 0.0562 ± 0.000067 (Carilli Barthel 1996 ). There is observational morphological evidence to suggest that the jets are precessing ( K19 ). We visually identified 48 jet points (with an astrometric error of 0.4 arcsec from VLA data) from the 5-GHz map of Perley, Dreher Cowan ( 1984 ), as shown in Fig. 5 . Jet points are taken to be any discrete, distinct feature that appears to be part of the jet; this includes both compact knots and sub-regions of more extended bright parts of the jet. For longer sub-regions of the jet, we placed multiple points, spaced by at least the resolution of the image, in order to constrain the curvature of the jet in these regions. Figure 5. Locations of visible jet knots within Cygnus A used in the model. Note the curvature in both the jet and counterjet paths. The final points in both the jet and counterjet lie above the primary hotspots of the two lobes, as discussed in the text. The inner, straighter points of the jet used in Fig. 7 are identified as being within the indicated brackets. One important constraint on the application of the MCMC model to real-world data is the requirement for the precessing jet to stay within the lobes. Since the model does not take lobe structure into account beyond the terminal hotspot, the constraint was implemented by a joint prior on position angle and precession cone opening angle. This dual parameter calculates the two extreme edges of the cone for any given position angle, and keeps them within the lobes, which are approximately 25 ○ across at their widest point. Without this additional prior, we found that unphysical jet paths were permitted as part of the fit, broadening the credible interval on precession period. As with the tests described in earlier sections, the priors were flat within the permitted ranges. We initially fitted the model to all identified points (see Fig. 6 ), including the terminal hotspot, which in both lobes is taken to be the more compact or ‘primary’ hotspot; we do not include the larger, brighter secondary hotspot since the position of this is almost certainly dominated by post-shock hydrodynamics (Cox, Gull Scheuer 1991 ). Then, with respect to the potential for hydrodynamic influences to dominate the jet path within the lobes, we fitted the model only to the straight portion of the jet, terminating just inside the lobes (Fig. 7 ). The full number of regions produced a peak in the posterior distribution corresponding to a precession period of 1 Myr. Figure 6. Main: ‘Corner plot’ of one-dimensional and two-dimensional marginalized posterior probability distributions for the model parameters, given the observed jet path of Cygnus A, including
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